Transcriber Note
Text emphasis denoted by _Italics_ and =Bold.= Whole and fractional parts
of numbers as 123-4/5.
THE ANCIENT VOLCANOES OF GREAT BRITAIN
[Illustration]
THE
ANCIENT VOLCANOES
OF
GREAT BRITAIN
BY
SIR ARCHIBALD GEIKIE, F.R.S.
D.C.L. Oxf., D.Sc. Camb., Dubl.; LL.D. St. And. Edinb.
DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN AND
IRELAND; CORRESPONDENT OF THE INSTITUTE OF FRANCE; OF THE
ACADEMIES OF BERLIN, VIENNA, MUNICH, TURIN, BELGIUM,
STOCKHOLM, GÖTTINGEN, NEW YORK; OF THE IMPERIAL MINERALOGICAL
SOCIETY AND SOCIETY OF NATURALISTS ST. PETERSBURG; NATURAL
HISTORY SOCIETY, MOSCOW; SCIENTIFIC SOCIETY, CHRISTIANIA;
AMERICAN PHILOSOPHICAL SOCIETY; OF THE GEOLOGICAL
SOCIETIES OF LONDON, FRANCE, BELGIUM, STOCKHOLM, ETC.
WITH SEVEN MAPS AND NUMEROUS ILLUSTRATIONS
IN TWO VOLUMES
VOL. II
London
MACMILLAN AND CO., Limited
NEW YORK: THE MACMILLAN COMPANY
1897
_All rights reserved_
CONTENTS
CHAPTER XXIX
The Carboniferous Volcanoes of England
PAGE
The North of England: Dykes, Great Whin Sill--The Derbyshire
Toadstones--The Isle of Man--East Somerset--Devonshire 1
CHAPTER XXX
The Carboniferous Volcanoes of Ireland
King's County--The Limerick Basin--The Volcanic Breccias of
Doubtful Age in County Cork 37
BOOK VII
THE PERMIAN VOLCANOES
CHAPTER XXXI
The Permian Volcanoes of Scotland
Geographical Changes at the Close of the Carboniferous Period--Land
and Inland-Seas of Permian time--General Characteristics and
Nature of the Materials erupted--Structure of the several
Volcanic Districts: 1. Ayrshire, Nithsdale, Annandale; 2.
Basin of the Firth of Forth 53
CHAPTER XXXII
Permian Volcanoes of England
The Devonshire Centre--Eruptive Rocks of the Midland Coal-fields 94
BOOK VIII
THE VOLCANOES OF TERTIARY TIME
CHAPTER XXXIII
Vast lapse of time between the close of the Palæozoic and beginning
of the Tertiary Volcanic Eruptions--Prolonged Volcanic
Quiescence--Progress of Investigation among the Tertiary
Volcanic Series of Britain 107
CHAPTER XXXIV
The System of Dykes in the Tertiary Volcanic Series
Geographical Distribution--Two Types of Protrusion--Nature of
component Rocks--Hade--Breadth--Interruptions of Lateral
Continuity--Length--Persistence of Mineral Characters 118
CHAPTER XXXV
The System of Dykes--_continued_
Direction--Termination upward--Known vertical extension--Evidence
as to the movement of the Molten Rock in the
Fissures--Branches and Veins--Connection of Dykes with
Intrusive Sheets--Intersection of Dykes--Dykes of more than
one infilling--Contact metamorphism of the Dykes--Relation of
the Dykes to the Geological Structure of the Districts which
they traverse--Data for estimating the Geological Age of the
Dykes--Origin and History of the Dykes 145
CHAPTER XXXVI
The Plateaux
Nature and Arrangement of the Rocks: 1. Lavas.--Basalts, Dolerites,
Andesites--Structure of the Lavas in the Field--2. Fragmental
Rocks.--Agglomerates, Conglomerates, and Breccias--Tuffs and
their accompaniments 181
CHAPTER XXXVII
The Several Basalt-Plateaux and their Geological History--Antrim,
Mull, Morven and Ardnamurchan 199
CHAPTER XXXVIII
The Basalt-Plateau of the Parish of Small Isles--Rivers of the
Volcanic Period 215
CHAPTER XXXIX
The Basalt-Plateaux of Skye and of the Faroe Isles 249
CHAPTER XL
The Modern Volcanoes of Iceland as illustrative of the Tertiary
Volcanic History of North-Western Europe 260
CHAPTER XLI
The Eruptive Vents of the Basalt-Plateaux
Vents filled with Basalt or other Lava-form Rock--Vents filled
with Agglomerate 270
CHAPTER XLII
The Basic Sills of the Basalt-Plateaux 298
CHAPTER XLIII
The Bosses and Sheets of Gabbro
Petrography of the Rocks--Relations of the Gabbros to the other
members of the Volcanic series--Description of the Gabbro
districts--Skye 327
CHAPTER XLIV
The Bosses and Sheets of Gabbro in the Districts of Rum,
Ardnamurchan, Mull, St. Kilda and North-East Ireland.
History of the Gabbro Intrusions 349
CHAPTER XLV
The Acid Rocks
Their Petrography--Their Stratigraphical Position and its
Analogies in Central France 364
CHAPTER XLVI
Types of Structure in the Acid Rocks--Bosses 378
CHAPTER XLVII
The Acid Bosses of Mull, Small Isles, St. Kilda, Arran, and
the North-East of Ireland 395
CHAPTER XLVIII
The Acid Sills, Dykes and Veins 430
CHAPTER XLIX
The Subsidences and Dislocations of the Plateaux 447
CHAPTER L
Effects of Denudation 455
CHAPTER LI
Summary and General Deductions 466
LIST OF ILLUSTRATIONS
FIG. PAGE
176. Section from the great Limestone escarpment on the west to
the Millstone Grit hills east of Teesdale 4
177. Sections of the Carboniferous Limestone series of
Northumberland showing the variations in the position of the
Whin Sill. By Messrs. Topley and Lebour 6
178. View of two volcanic necks in the Carboniferous Limestone
series, at Grange Mill, five miles west of Matlock Bath,
from the north 14
179. Plan of necks and bedded tuff at Grange Mill, five miles
west of Matlock Bath 15
180. Section across the smaller volcanic neck and the stratified
tuff in Carboniferous Limestone, Grange Mill 15
181. Section of vesicular and amygdaloidal diabase resting on
Carboniferous Limestone, Peak Forest Limeworks, Great Rocks
Quarry 19
182. View of the superposition of Carboniferous Limestone upon
toadstone, Raven's Tor, Millersdale (length about 100 feet) 19
183. Section at lime-kiln, south of Viaduct, Millersdale Station 20
184. Limestones passing under stratified tuffs, Poyll Vaaish,
Isle of Man 24
185. Section of tuff, showing intercalations of black impure
chert, west of Closenychollagh Point, near Castletown, Isle
of Man 25
186. Section of intercalated dark limestone, shale and chert in
the tuff south of Poyll Vaaish Bay, Isle of Man 26
187. Section of part of a volcanic neck on shore to the
south-east of Poyll Vaaish Bay, Isle of Man 29
188. Section of successive discharges and disturbances within a
volcanic vent. Scarlet Point, Isle of Man 29
189. Section of dyke and sill in the tuffs west of Scarlet Point,
Isle of Man 30
190. Section on south side of vesicular sill west of Scarlet
Point 31
191. Bands of vesicles in the same sill 31
192. Croghan Hill, King's County, from S.S.W. 38
193. Section in quarry on roadside east of Limerick, close to
viaduct of the Limerick and Erris Railway 44
194. Section of the volcanic escarpment, east of Shehan's
Cross-roads, south of Limerick 45
195. View of Derk Hill, a volcanic neck on the south side of the
Limerick basin 47
196. Section across the Limerick volcanic basin 48
197. Section of a bed of volcanic breccia in the Carboniferous
Slate; White Bull Head, County Cork 50
198. Volcanic breccia invading and enclosing Carboniferous slate,
White Bull Head 50
199. General section across the Permian basin of Ayrshire 59
200. Section of lavas, east side of Mauchline Hill 60
201. Section of the top of the volcanic series near Eastside
Cottage, Carron Water, Nithsdale 60
202. Section of two outliers of the Permian volcanic series at
the foot of Windyhill Burn, Water of Ae, Dumfriesshire 61
203. The Green Hill, Waterside, Dalmellington, from the south; a
tuff-neck of Permian age 62
204. Patna Hill from the Doon Bridge, Ayrshire; a tuff-neck of
Permian age 63
205. Ground plans of Permian volcanic vents from the Ayrshire
Coal-field. On the scale of six inches to a mile 64
206. Section of sills traversing the Permian volcanic series.
River Ayr, Ballochmyle 66
207. Section showing the relations of the later rocks of Arthur
Seat 68
208. Section in brooks between Bonny town and Baldastard, Largo 70
209. View of Largo Law from the east 71
210. View of small neck in Calciferous Sandstones, on the shore,
three miles east from St. Andrews 72
211. Ground-plan of Permian volcanic vents 73
212. Small neck in Calciferous Sandstones a little east from the
"Rock and Spindle," two and a half miles east from St.
Andrews 74
213. Plan of volcanic necks at Kellie Law, East of Fife, on the
scale of three inches to one mile 75
214. Plan of the craters in Volcanello, Lipari Islands 75
215. Section of the strata at the edge of the volcanic vent on
the east side of Elie Harbour 76
216. Agglomerate of neck on shore at Ardross, two miles east from
Elie 77
217. Ground-plan of volcanic neck, Elie Harbour, showing circular
disposition of the stratification 80
218. Section across the great vent of Kincraig, Elie, on a true
scale, vertical and horizontal, of six inches to a mile 81
219. Dyke in volcanic neck, on the beach, St. Monans 82
220. Section of part of crater rim, Island of Volcano 83
221. Dyke rising through the agglomerate of a volcanic vent;
Kincraig, Elie 84
222. Radiating columnar dyke in the tuff of a volcanic vent. Rock
and Spindle, two and a half miles east from St. Andrews 86
223. View of part of the shore front of the great vent at
Kincraig, looking westward, with the columnar basalt in
front 88
224. Plan of volcanic neck on beach near St. Monans 89
225. Columnar basalt in the neck of Kincraig, Elie, seen from the
west 90
226. Section across Largo Law 91
227. Vein of "white-trap" cutting black carbonaceous shales, a
little west from St. Monans Church 92
228. Section at Belvedere, S. W. of Exeter 97
229. Diagram to show the unconformability and overlap of the
Permian rocks in the Crediton Valley 97
230. Section of the volcanic series at Kellerton, Devonshire 98
231. Section of agglomerate overlain with sandstone and andesite,
Posbury, Crediton 99
232. Diagrammatic section across Titterstone Clee Hill 102
233. Dyke on the south-east coast of the Island of Mull 119
234. Fissure left by the weathering out of a dyke 120
235. Plan of basalt-veins with selvages of black basalt-glass,
east side of Beinn Tighe, Isle of Eigg 126
236. Arrangement of lines of amygdales in a dyke, Strathmore,
Skye 130
237. Systems of joints in the dykes 132
238. Section of cylindrical vein or dyke, cutting the bedded
lavas, east side of Fuglö, Faroe Islands 133
239. Joint-structures in the central vitreous portion of the
Eskdale Dyke (B. N. Peach) 133
240. Microscopic structure of the vitreous part of the Eskdale
Dyke 136
241. Section along the line of the Cleveland Dyke at Cliff Ridge,
Guisbrough (G. Barrow), scale, 12 inches to 1 mile 147
242. Section along the course of the Cleveland Dyke, at the head
of Lonsdale, Yorkshire (G. Barrow, in the _Memoirs of the
Geol. Survey_, Geology of Cleveland, p. 61) 148
243. Section across the extreme upper limit of Cleveland Dyke, on
the scale of 20 feet to one inch (Mr. G. Barrow) 149
244. Upper limit of Cleveland Dyke in quarry near Cockfield
(after Mr. Teall) 149
245. Section along the course of the Cleveland Dyke across the
Cross Fell escarpment (scale of one inch to one mile) 150
246. Branching portion of the great dyke near Hawick (length
about one mile) 153
247. Branching dyke at foot of Glen Artney (length about four
miles) 153
248. Basic veins traversing Secondary limestone and sandstone on
the coast cliffs, Aidnamurchan 155
249. Section showing the connection of a Dyke with an Intrusive
Sheet, Point of Suisnish, Skye 156
250. Section to show the connection of a Dyke with an Intrusive
Sheet, Stirlingshire Coal-field 157
251. Intersection of dykes in bedded basalt, Calliach Point, Mull 158
252. Basalt veins traversing bedded dolerites, Kildonan, Eigg 159
253. Ground-plan of intersecting dykes in Lias limestone, Shore,
Harrabol, East of Broadford, Skye 159
254. Compound dyke, Market Stance, Broadford, Skye 162
255. Section of coal rendered columnar by intrusive basalt,
shore, Saltcoats, Ayrshire 164
256. Dolerite dyke with marginal bands of "white trap," in black
shale, Lower Lias, Pabba 166
257. Map of the chief dykes between Lochs Ridden and Striven (C.
T. Clough, Geological Survey, Sheet 29) 170
258. Basalt-veins traversing granophyre, St. Kilda 173
259. Section of scoriaceous and prismatic basalt, Camas
Tharbernish, north shore of Canna Island 187
260. Banded amygdaloidal basalt showing layers of elongated and
steeply inclined vesicles, Macleod's Maidens, Skye 191
261. Termination of basalt-beds, Carsaig, Mull 193
262. Breccia and blocks of mica-schist, quartzite, etc., lying
between bedded basalts, Isle of Mull 197
263. Section of Knocklayd, an outlier of the Antrim
basalt-plateau lying on Chalk 202
264. Diagram-Section of the Antrim Plateau 203
265. View Of Basalt escarpment, Giant's Causeway, with the
Amphitheatre and Chimneys. (From a photograph by Mr. R.
Welch) 207
266. Basalt-capping on the top of Ben Iadain, Morven 209
266_a_. View of the south side of Staffa, showing the bedded and
columnar structure of the basalt 210
267. View of Rum from the harbour of Canna 216
268. Section of the cliffs below Compass Hill, Isle of Canna 218
269. Lava cutting out conglomerate and shale. Shore below Canna
House 224
270. Section of shales and tuffs, with a coniferous stump lying
between two basalt-sheets, Cùl nam Marbh, Canna 225
271. Dùn Mòr, Sanday. (From a photograph by Miss Thom) 226
272. View of the Dùn Beag, Sanday, seen from the south. (From a
Photograph by Miss Thom) 230
273. View of Dùn Beag, Sanday, from the north. The island of Rum
in the distance. (From a Photograph by Miss Thom) 231
274. Section of eastern front of Dùn Beag 232
275. Enlarged section on the western side of Dùn Beag 233
276. Geological map of the Island of Eigg 235
277. Section of the geological structure of the Island of Eigg 236
278. View of the Scuir of Eigg from the east 237
279. Natural section at the cliff of Bideann Boidheach,
north-west end of the Scuir of Eigg 239
280. View of the Scuir of Eigg from the south 242
281. View of the Scuir of Eigg from the south-west of the Loch a
Bhealaich, showing the bedded character of the mass 243
282. Section at the base of the Scuir of Eigg (east end) 244
283. Terraced hills of basalt plateau (Macleod's Tables), Skye 250
284. "Macleod's Maidens" and part of basalt cliffs of Skye 251
285. Intercalated group of strata between Basalts, An Ceannaich,
western side of Skye 252
286. Escarpment of Plateau-basalts, Cliffs of Talisker, Skye 253
287. Section of the largest of Macleod's Maidens 254
288. Dying out of lava-beds, east side of Sandö, Faroe Isles 257
289. Lenticular lavas, western front of Hestö, Faroe Isles 257
290. Lenticular lavas, east side of Svinö, Faroe Isles 258
291. Section at Frodbonyp, Suderö, Faroe 258
292. Fissure (gjá) in a lava-field, Iceland. (From a photograph
by Dr. Tempest Anderson) 262
293. Cones on the great Laki fissure, Iceland. (From a photograph
by Dr. Tempest Anderson) 263
293_a_. Plan of small craters along the line of great Laki
fissure, Iceland. (After Mr. Helland, reduced) 264
294. Slemish, a volcanic neck or vent on the Antrim plateau, seen
from the north 272
295. Section of volcanic vent at Carnmony Hill (E. Hull) 272
296. Section of the east side of Scawt Hill, near Glenarm 273
297. Section of Neck of basalt, Bendoo, Ballintoy 273
298. Volcanic Neck of dolerite near Cushendall 274
299. Section of Volcanic Neck at 'S Airde Beinne, near Tobermory,
Mull 274
300. Interior of the Volcanic Neck of 'S Airde Beinne, near
Tobermory, Mull 275
301. Diagram to show the probable relation of the Neck at
Carrick-a-raide, Antrim, to an adjacent group of tuffs 277
302. Section of agglomerate Neck at Maclean's Nose, Ardnamurchan 279
303. Diagram to show the probable relations of the rocks on the
southern flank of Beinn Dearg Bheag 282
304. Section of Volcanic Vent and connected lavas and tuffs,
Scorr, Camas Garbh, Portree Bay, Skye 284
305. Section of the Volcanic Series at Ach na Hannait, south of
Portree, Skye 288
306. View of part of a Volcanic Neck at the eastern end of the
island of Canna. (From a photograph by Miss Thom) 289
307. Columnar Basalt invading agglomerate of Volcanic Vent,
Coroghon Mòr, Isle of Canna. (Height above 20 feet) 291
308. Columnar Basalt invading Volcanic conglomerate, north side
of Alman Islet, Canna 291
309. View of neck-like mass of breccia, Brochel, Raasay 292
310. View of Volcanic Neck piercing and overlain by the
Plateau-Basalts, Stromö, entrance of Vaagöfjord, Faroe
Islands. (From a photograph by Colonel Evans) 294
311. Section of the same Neck as that shown in Fig. 310 295
312. Volcanic Neck close to that shown in Figs. 310 and 311 296
313. Section of wall of another Neck of agglomerate in the same
group with those represented in Figs. 310, 311, and 312 296
314. View of "Segregation-Veins" in a dolerite sill, Portrush,
Antrim 300
315. View of Fair Head, from the east, showing the main upper
sill and a thinner sheet cropping out along the talus 301
316. View of Fair Head from the shore. (From a photograph by Mr.
R. Welch) 302
317. Section at Farragandoo cliff, west end of Fair Head, showing
the rapid splitting up and dying out of an Intrusive Sheet
slope 304
318. View of the Trotternish Coast, showing the position of the
band of Sills 305
319. Columnar Sill intrusive in Jurassic Strata east of
Kilmartin, Trotternish, Skye 306
320. View of the northern precipice (500 feet high) of the
largest of the Shiant Isles. (From a photograph by Colonel
Evans) 308
321. Section of thin Intrusive Sheets and Veins in carbonaceous
shales lying among the Plateau-basalts, cliffs north of
Ach na Hannait, between Portree Bay and Loch Sligachan 311
322. Upper part of Sill, Moonen Bay, Waternish, Skye, showing the
divergence of veins 313
323. Section of the base of the Basalt-plateau with sill and
dykes, Sound of Soa, Skye 314
324. Section of Dolerite Sill cut by another sill, both being
traversed by dykes, Rudh' an Iasgaich, western side of
Sleat, Skye 316
325. Section to show Bedded and Intrusive Sheets, Eigg 318
326. Ground plan of Sills at Ben Hiant, Ardnamurchan. 321
327. Section of two Sills in schistose grits, west end of Beinn
na h-Urchrach, Ardnamurchan 322
328. Sill traversing bedded Basalts, cliffs of Stromö, at
entrance of Vaagöfjord 323
329. View of the same Sill seen from the channel opposite the
island of Kolter 324
330. Granulitic and coarsely foliated Gabbro traversed by later
veins of felspathic Gabbro, Druim an Eidhne, Cuillin
Hills, Skye 331
331. Scuir na Gillean, Cuillin Hills, showing the characteristic
craggy forms of the Gabbro. (From a photograph by Mr.
Abraham, Keswick) 335
332. Section across Glen Brittle, to show the general relations
of the Bedded Basalts and the Gabbros 336
333. View of the crest of the Cuillin Hills, showing the
weathering of the Gabbro along its joints and of a
compound basic dyke which rises through it. (From a
photograph by Mr. Abraham, Keswick) 338
334. Section across the Coire Uaigneich, Skye 341
335. Banded and puckered gabbro, Druim an Eidhne, Glen Sligachan,
Skye 342
336. Banded structure in the Gabbro, from the ridge of Druim an
Eidhne, between Loch Coruisk and Glen Sligachan 343
337. Banded and doubly folded Gabbro, Druim an Eidhne, 10 feet
broad 345
338. Sketch of banded structure in the Gabbros of the hills at
the head of Loch Scavaig 347
339. Outline of the hills of the Island of Rum, sketched from
near the Isle of Eigg 350
340. View of Allival, Rum, sketched from the base of the
north-east side of the cone 352
341. Section of foliated Gabbros in the Tertiary volcanic series
of Allival, Rum 353
342. Altered Plateau-Basalts invaded by Gabbro, and with a Dyke
of prismatic Basalt cutting both rocks, north slope of
Ben Buy, Mull 357
343. Theoretical representation of the structure of one of the
Gabbro bosses of the Inner Hebrides 362
344. Section through the Puy de la Goutte and Puy de Chopine 374
345. View of the Huche Pointue and Huche Platte west of Le
Pertuis 376
346. View of Glamich, 2537 feet, Glen Sligachan. (From a
photograph by R. J. A. Berry, M.D., lent by the Scottish
Mountaineering Club) 380
347. Section across the north slope of Beinn an Dubhaich, Skye 383
348. Section from Beinn Dearg to Beinn an Dubhaich, Skye 385
349. Section at north end of Beinn na Cro, Skye 388
350. Ground-plan of basic dyke in Cambrian limestones truncated
by granophyre which encloses large blocks of the dyke,
Torrin, Skye 393
351. Section on south side of Beinn an Dubhaich, Skye, showing
the truncation of a basalt-dyke 394
352. View of the hills on the south side of the head of Loch na
Keal, showing the junction of the Granophyre and the
bedded basalts 396
353. Section on south side of Cruach Tòrr an Lochain, Mull 398
354. Section at head of Allt na Searmoin, Mull 398
355. Section on south side of Beinn Fhada, Mull 399
356. Section to south of Loch na Dàiridh, Mull 400
357. Section of junction of south side of Loch Ba' Granophyre
boss, with the bedded basalts, Mull 401
358. Mass of dark gabbro about two feet in diameter traversed by
pale veins of Granophyre, lying on north slope of Creag
na h-Iolaire, Mull 402
359. Section at Creag na h-Iolaire, Glen More, Mull, showing
basalts and gabbros resting on and pierced by Granophyre 402
360. Section on north side of Orval, Rum 404
361. Junction of Quartz-porphyry (Microgranite) and basic rocks,
south-east side of Orval, Rum 404
362. Junction of Granophyre and gabbro, north side of St. Kilda 410
363. Veins of Granophyre traversing gabbro and splitting up into
thin threads, north side of St. Kilda 411
364. Pale Granophyre injected into dark basalt, South Bay, St.
Kilda 412
365. Veins of Granophyre traversing a fine-grained gabbro and
scarcely entering a coarse-grained sheet, west side of
Rueval, St. Kilda 413
366. View of sills and veins of pale Granophyre traversing sheets
of gabbro, west side of St. Kilda. (From a photograph by
Colonel Evans) 414
367. Section of the sea-cliff below Conacher, St. Kilda, showing
basic dykes in Granophyre 417
368. Triple basic dyke, sea-cliff, east side of St. Kilda 417
369. Jointed structure of the Granite near the top of Goatfell,
Arran. (From a photograph by Mr. W. Douglas, lent by the
Scottish Mountaineering Club) 419
370. Intrusive Rhyolite in the lower basalt group of Antrim,
Templepatrick 427
371. Section across the southern slope of Carnearny Hill, Antrim 427
372. Section across the Granophyre Sills at Loch a' Mhullaich,
above Skulamus, Skye 433
373. Section to show the connection of a sill of Granophyre with
its probable funnel of supply, Raasay 436
374. Granophyre sill resting on Lower Lias shales with a dyke of
basalt passing laterally into a sill, Suisnish Point, Isle
of Raasay 436
375. Weathered surface of spherulitic Granophyre from dyke
in banded gabbros, Druim an Eidhne, Meall Dearg, Glen
Sligachan, Skye. Natural size 438
376. Plan of portion of the ridge north of Druim an Eidhne, Glen
Sligachan, Skye, showing three dykes issuing from a mass
of Granophyre 439
377. Weathered surface of spherulitic Granophyre, from dyke
in banded gabbros, Druim an Eidhne, Meall Dearg, Glen
Sligachan, Skye. Natural size 440
378. Plan of pale Granophyric dyke, with spherulitic and
flow-structure, cutting and enclosing dark gabbro, Druim
an Eidhne 441
379. Dyke (six to ten feet broad) proceeding from a large body of
Granophyre and traversing gabbro, from the same locality
as Figs. 375 and 377 442
380. Section of intruded veins of various acid rocks above River
Clachaig, Mull 443
381. Pitchstone vein traversing the bedded basalts, Rudh an
Tangairt, Eigg 445
382. Reversed fault on the eastern side of Svinö, Faroe Isles 454
383. Reversed fault on the north-east headland of Sandö, Faroe
Isle 454
MAPS
V. Map of the Permian volcanic districts of Scotland _To face p._ 106
VI. Map of the Tertiary volcanic region of the West
of Scotland _To face p._ 296
VII. Map of the Tertiary volcanic district of the
North-East of Ireland _To face p._ 446
CHAPTER XXIX
THE CARBONIFEROUS VOLCANOES OF ENGLAND
The North of England: Dykes, The Great Whin Sill--The Derbyshire
Toadstones--The Isle of Man--East Somerset--Devonshire
1. THE NORTH OF ENGLAND
The volcanic intercalations which diversify the Lower Carboniferous
formations of Southern Scotland extend but a short way across the
English Border, and although, over the moors and hills of the north of
Cumberland and Northumberland, the Carboniferous sandstones, limestones
and shales are well exposed, they present no continuation of either the
plateau or puy-eruptions which play so prominent a part in the geology
of Roxburghshire and Dumfriesshire. This deficiency is all the more
noticeable seeing that the Carboniferous system is exposed down to its
very base, in the deep dales of the North of England. Had any truly
interstratified volcanic material existed in the system there, it could
hardly fail to have been detected.
But while contemporaneous volcanic rocks are absent, the northern
English counties contain many intrusive masses of dolerite, diabase,
andesite or other eruptive rocks, which may be found traversing all
the subdivisions of the Carboniferous system. These eruptive materials
have taken two forms: in some cases they rise as Dykes, in others they
appear as Sills.
Dykes.--With regard to the dykes, some are probably much later than
the Carboniferous period, and consequently will be more appropriately
considered in Chapters xxxiv. and xxxv. The great Cleveland dyke,
for example, which runs across the Carboniferous, Permian, Triassic
and Jurassic formations, is probably referable to the Older Tertiary
volcanic period. One dyke known as the Hett Dyke, has been plausibly
claimed as possibly of Carboniferous age. It runs in a W.S.W. direction
from the Magnesian Limestone escarpment at Quarrington Hill, a few
miles to the east of Durham, through the great Coal-field, across the
Millstone Grit and Carboniferous Limestone, disappearing near Middleton
in Teesdale. Its total length is thus about 23 miles. It varies in
breadth from about 6 to about 15 feet, and appears to increase in
dimensions as it goes westward.[1]
[Footnote 1: Sedgwick, _Trans. Geol. Soc._ 2nd series, iii. part 1
(1826-28), p. 63; _Trans. Cambridge Phil. Soc._ ii. (1822), p. 21. Sir
J. Lowthian Bell, _Proc. Roy. Soc._ xxiii. (1875), p. 543.]
The age of this dyke cannot at present be satisfactorily fixed.
It must be later than the Coal-measures through which it rises.
Sedgwick long ago pointed out that though it reaches the escarpment
of the Magnesian Limestone, it does not cut it; yet it is found in
coal-mining to traverse the Coal-measures underlying the Limestone. He
was accordingly inclined to believe it to be of older date than the
Magnesian Limestone. At its western extremity it approaches close to
the Great Whin Sill of Teesdale, though no absolute connection between
the two has been established. Mr. Teall, however, has called attention
to the similarity between the microscopic structure of the rock forming
the Hett Dyke and that of the mass of the Whin Sill, and he is strongly
inclined to regard them as belonging to the same period of intrusion.[2]
[Footnote 2: _Quart. Journ. Geol. Soc._ xl. (1884), p. 230.]
It is especially worthy of remark that in the course of its nearly
rectilinear course across the Durham Coal-field, the Hett Dyke, where
it crosses the Wear, is flanked on the north at a distance of a little
more than two miles by a second parallel dyke of nearly identical
composition. Between the two dykes, during mining operations, a sill
about 20 feet thick has been met with, lying between two well-known
coal-seams at a depth of about 60 fathoms from the surface, and
extending over an area of at least 15 acres.[3] Microscopic examination
of this sill by Mr. Teall proved that the rock presents the closest
resemblance to that of the Hett Dyke.[4] In this case, it may be
regarded as probable that the two dykes and the intermediate sill form
one related series of intrusions, and the conjecture that the Hett Dyke
may be connected with the Whin Sill thus receives corroboration. The
age of the Whin Sill itself will be discussed a few pages further on.
[Footnote 3: Sir Lowthian Bell, _Proc. Roy. Soc._ xxiii. (1875), p.
544.]
[Footnote 4: _Quart. Journ. Geol. Soc._ xl. (1884), p. 230.]
Of the other dykes which may possibly be coeval with the Hett Dyke we
may specially note those which follow the same W.S.W. trend, for that
strike differs from the general W.N.W. direction of most of the dykes.
Two conspicuous examples of the south-westerly trend may be seen,
one near Morpeth, the other north of Bellingham. The former dyke, as
regards microscopic structure, is more nearly related to the majority
of the series in the North of England. But that north of Bellingham
(High Green) presents affinities both in structure and composition
with the Hett Dyke,[5] and may perhaps belong to the same period of
intrusion.
[Footnote 5: Mr. Teall, _op. cit._ p. 244. _Quart. Journ. Geol. Soc._
xxxix. (1884), p. 656, and _Proc. Geol. Assoc._ (1886). See also Prof.
Lebour, _Geology of Northumberland and Durham_, chap. xi.]
The Great Whin Sill.--The geologist who, after making himself
acquainted with the abundant sills among the Carboniferous rocks in the
centre of Scotland, finds his way into Northumberland, meets there with
geological features that have become familiar to him further north.
The sea-cliffs of Bamborough and Dunstanborough, the rocky islets of
Farne, the long lines of brown crag and green slope that strike inland
through the Kyloe Hills and wind across the cultivated lowlands and the
moorlands beyond, remind him at every turn of the scenery in the basin
of the Forth. But not until he has traced these ridges for many miles
southwards and found their component rocks to form there an almost
continuous sheet does he realize that nothing of the kind among the
Scottish Carboniferous rocks can be compared for extent to this display
in the North of England.[6]
[Footnote 6: The Whin Sill has been the subject of much discussion,
and a good deal of geological literature has been devoted to its
consideration. The writings of Trevelyan, Sedgwick, W. Hutton, Phillips
and Tate are especially deserving of recognition. The intrusive
character of the Sill, maintained by some of these writers, was finally
established by the mapping of the Geological Survey, and was discussed
and illustrated by Messrs. W. Topley and G. A. Lebour in a paper in
the 33rd volume of the _Quart. Journ. Geol. Soc._ (1877), in which
references to the earlier observers will be found. See also Prof.
Lebour's _Outlines of the Geology of Northumberland_, 2nd edit. (1886),
p. 92. The petrography of the Whin Sill is fully treated by Mr. Teall
in _Quart. Journ. Geol. Soc._ xl. (1884), p. 640, where a bibliography
of the subject is also given.]
From the furthest skerries of the Farne Islands southwards to Burton
Fell on the great Pennine escarpment, a distance in a straight line of
about 80 miles, this intrusive sheet may be traced in the Carboniferous
Limestone series (Map I.). There are intervals where its continuity
cannot be actually followed at the surface, but that it really runs
unbroken from one end to the other underground cannot be doubted by any
one who has examined the region. This singular feature in the geology
and scenery of the North of England is known locally as the Great
Whin Sill.[7] From the rocky islets and castle-crowned crags of the
coast-line it maintains its characteristic topography, structure and
composition throughout its long course in the interior. So regularly
parallel with the sedimentary strata does it appear to lie, that it was
formerly regarded by many observers as a true lava-sheet, poured out
upon the sea-floor over which the limestones and shales were laid down.
But its really intrusive character has now been clearly demonstrated.
Not a vestige of any tuff has been detected associated with it, nor
does it ever present the usual characters of a true lava-stream.[8] Its
internal structure and the wonderful uniformity in its character mark
it out as a typical intrusive sheet.
[Footnote 7: "Whin" is a common term in Scotland and the North of
England for any hard kind of stone, especially such as can be used for
making and mending roads. "Sill" denotes a flat course or bed of stone,
and was evidently applied to this intrusive sheet from its persistent
flat-bedded position and its prominence among the other gently inclined
strata among which it lies. It is from this example in the North of
England that the word "sill" has passed into geological literature.]
[Footnote 8: On the coast at Bamborough and the Harkess Rocks the
usual petrographical characters of the Whin Sill are exchanged for
those of fine-grained amygdaloidal diabases arranged in distinct
sheets, which in their upper parts are highly vesicular and show ropy
surfaces--peculiarities suggestive of true lava-streams. But according
to Professor Lebour the rocks are intrusive into limestone and shale
(_Geology of Northumberland and Durham_, p. 98). Mr. Teall has
expressed the suspicion that these rocks must have consolidated under
conditions somewhat different from those which characterized the normal
Whin Sill (_Quart. Journ. Geol. Soc._ xl. p. 643). They seem to be
the only parts of the sill which present features that might possibly
indicate superficial outflow.]
Among the manifestations of the subterranean intrusion of igneous rocks
in the British Isles the Great Whin Sill, next after the Dalradian
sills of Scotland, is the most extensive. Its striking continuity for
so great a distance, and the absence around it of any other trace of
igneous action, save a few dykes, place it in marked contrast to the
ordinary type of Carboniferous sills. The occasional gaps on its line
of outcrop in the northern part of its course do not really affect
our impression of the persistence of the sheet. They not improbably
indicate merely that in its protrusion it had a wavy irregular limit,
which in the progress of denudation has occasionally been not yet
reached. For mile after mile the sill has been mapped by the Geological
Survey in lines of crag across the moorlands, and as a conspicuous
band among the limestones and shales that form the steep front of the
Pennine escarpment, where it has long been known in the fine sections
exposed among the gullies by which that noble rock-face has been
furrowed.
[Illustration: Fig. 176.--Section from the great Limestone escarpment
on the west to the Millstone Grit hills east of Teesdale.
1. Silurian strata; 2. Carboniferous Limestone series; 3. The Great
Whin Sill, which gradually rises to higher stratigraphical position as
it goes westward; 4. Millstone Grit.]
Along its main outcrop, the sill dips gently eastwards below the
portion of the Carboniferous Limestone series which overlies it. But
so slight are the inclinations, so gentle the undulations of the rocks
in this part of the country, that far to the east of that outcrop
the sill has been laid bare by the streams which in the larger dales
have cut their way through the overlying cake of Carboniferous strata
down to the Silurian platform on which they rest (Fig. 176). Among
these inland revelations of the eastward continuation of the sill
under Carboniferous Limestone strata, the most striking and best known
are those which have been made by the River Tees, and of which the
famous waterfalls of the High Force and Cauldron Snout are the most
picturesque features. The distance of the remotest of these denuded
outcrops or "inliers" from the main escarpment is not less than 20
miles.
It is not possible to form an accurate estimate of the total
underground area of the Whin Sill. In the southern half of the
district, south of the line of the Roman Wall, where, the inclination
of the strata being generally low, the same stratigraphical horizons
are exposed by denudation far to the east of the main outcrops of
the rocks, we know that the sill must have a subterranean extent of
more than 400 square miles. Yet this is probably only a small part of
the total area over which the molten material was injected. In the
northern part of the district, the Carboniferous Limestone series is
not exposed over so broad a stretch of country, and denudation has not
there revealed the eastward extension of the sill. But there is no
reason to suppose the sheet to be less continuous and massive there.
We must remember also that the present escarpment has been produced
by denudation, and that the intrusive sheet must have once extended
westwards beyond its present limits at the surface. If, therefore, we
were to state broadly that the Great Whin Sill has been intruded into
the Carboniferous Limestone series over an area of 1000 square miles
we should probably be still below the truth.
The rock composing this vast intrusive sheet is a dolerite or diabase,
which maintains throughout its wide extent a remarkable uniformity of
petrographical characters. In this and other respects it illustrates
the typical features of sills. Thus it is coarsest in texture where
it is thickest, and somewhat finer in grain towards its upper and
lower surfaces than in the centre. Among the coarser varieties the
component crystals of augite are not infrequently an inch in length and
occur in irregular patches.[9] Occasional amygdaloidal portions are
observable, but these are not more marked than those to be found in
the "whin-dykes" of the same region.[10] The amygdaloidal and vesicular
fine-grained rocks of the Bamborough district may possibly be quite
distinct from the main body of the Whin Sill.
[Footnote 9: Sedgwick, _Cambridge Phil. Trans._ ii. p. 166. Mr. Teall,
_Quart. Journ. Geol. Soc._ xl. p. 643.]
[Footnote 10: Messrs. Topley and Lebour, _Quart. Journ. Geol. Soc._
xxxiii. p. 418.]
Under the microscope the rock is seen to consist essentially of the
usual minerals--plagioclase, augite and titaniferous magnetic iron-ore.
An ophitic intergrowth of the augite and felspar is observable,
likewise a certain quantity of micropegmatite which plays the part of
groundmass between the interstices of the lath-shaped felspars. Full
details of the characteristics of the component minerals and their
arrangement are given by Mr. Teall in the paper already cited.
The main body of the sill is a sheet which sometimes diminishes to
less than 20 feet in thickness and sometimes expands to 150 feet,
but averages from 80 to 100 feet. It occasionally divides, as near
Great Bavington, where it appears at the surface in two distinct
beds separated by an intervening group of limestones and shales.
Occasionally, as at Elf's Hill Quarry, it gives out branches which send
strings into the adjacent limestone.[11]
[Footnote 11: Messrs. Topley and Lebour, _op. cit._ p. 413.]
Although in most natural sections it seems to lie quite parallel with
the strata above and below, yet a number of examples of its actual
intrusion have been observed. When traced across the country, it is
found not to remain on a definite horizon, but to pass transgressively
across considerable thicknesses of strata. Its variations in this
respect are well shown in the accompanying table of comparative
sections constructed by Messrs. Topley and Lebour.[12] It will be
seen that while at Harlow Hill the sill is found overlying the Great
Limestone of Alston Moor, at Rugley, five miles off it lies about 1000
feet lower down, far below the position of the Tyne-bottom Limestone.
Still farther north, however, the sill west of Holy Island is said to
lie 800 feet above the Great Limestone and to come among the higher
beds of the Carboniferous Limestone series.[13]
[Footnote 12: _Op. cit._ plate xviii.]
[Footnote 13: _Op. cit._ p. 414.]
The Whin Sill appears generally to thicken in an easterly or
north-easterly direction. There are further indications that it was
intruded from east to west. Thus, at Shepherd's Gap, on the Great Roman
Wall, the dolerite, coming evidently from an easterly quarter, has
broken up and thrust itself beneath a bed of limestone. Again, when the
sill bifurcates the branches unite towards the east or north-east.[14]
The sill can be proved to thin away to the west from Teesdale to the
Pennine escarpment, and in Weardale the "Little Whin Sill" diminishes
from 20 feet, till in three miles it disappears.[15]
[Footnote 14: _Op. cit._ p. 415.]
[Footnote 15: _Op. cit._ p. 419.]
[Illustration: _Walker & Bontall sc._
Fig. 177.--Sections of the Carboniferous Limestone series of
Northumberland showing the variations in the position of the Whin Sill.
By Messrs. Topley and Lebour.]
The strata in contact with the Whin Sill, both above and below, have
been more or less altered. Sandstones have been least affected;
shales have suffered most, passing into a kind of porcellanite, with
development of garnet and other minerals.[16] Limestone often shows
only slight traces of change, though here and there it has become
crystalline.
[Footnote 16: Mr. Teall, _op. cit._ xxxix. (1884), p. 642, and authors
cited by him.]
No trace of any boss or neck has been detected in the whole region
which might be supposed to mark a funnel of ascent for the material of
the Whin Sill. The Hett Dyke and the High Green Dyke, already noticed,
may, however, have been possibly connected with the injection of this
great intrusive sheet. No other visible mass of igneous rock in the
region has been even plausibly conjectured to indicate a point or line
of emission for the sill.
It is certainly singular that in so wide a territory, where the whole
succession of strata has been so admirably laid bare by denudation in
thousands of natural sections, and where, moreover, much additional
information has been obtained from lead-mining as to the nature of
the rocks below ground, not a single vestige of tuff, agglomerate or
interstratified lava has been up to the present time recorded, unless
the Harkess rocks already alluded to can be so regarded.
Judging, however, from the analogy of the other districts of igneous
rocks in Britain, we can hardly resist the conclusion that the Great
Whin Sill is essentially a manifestation of volcanic action, that it
was connected with the uprise of basic lava in volcanic orifices, and
that the subterranean energy may quite probably have succeeded in
reaching the surface and ejecting there both lavas and tuffs.
It appears to be certain that any vents which existed cannot have lain
to the west of the present escarpment of the sill, for no trace of them
can be found there piercing the Carboniferous or older formations. They
must have lain somewhere to the east in the area now overspread with
Millstone Grit and Coal-measures, or still farther east in the tract
now concealed under the North Sea. The evidence of the sill itself, as
we have seen, corroborates this view of the probable situation of the
centre of disturbance.
The question of the geological age of the sill is one of considerable
difficulty, to which no confident answer can be given.[17] The injection
of the diabase must obviously be considerably later than the highest
strata through which it has risen; that is, it must be younger than
some of the higher members of the Carboniferous Limestone series. But
here our positive evidence fails.
[Footnote 17: See Messrs. Topley and Lebour, _op. cit._ p. 418.]
The Sill is traversed by the same faults which disrupt the surrounding
Carboniferous rocks. It is therefore of older date than these
dislocations. Its striking general parallelism with the shales and
limestones probably proves that it was intruded before the rocks were
much disturbed from their original horizontal position. But the manner
in which the intrusive rock has been thrust into and has involved the
shales and limestones seems to indicate that these strata had already
become consolidated and lay under the pressure of a great thickness of
superincumbent Carboniferous strata.
In the absence of all certainty on the subject it seems most natural
to place the Whin Sill provisionally among the Carboniferous volcanic
series with which petrographically and structurally it has so much
in common. In Scotland the puy-eruptions continued till the time of
the Coal-measures. If, before the close of the Carboniferous period,
volcanic vents were opened somewhere to the east of the coal-fields of
Northumberland and Durham, they might be accompanied with basic sills
injected into the Carboniferous Limestone series, which was then lying
still approximately horizontal under a thickness of from 3500 to 5000
feet of Carboniferous sedimentary deposits. These still undiscovered
volcanoes seem to have been endowed with even more energy than those
of Central and Southern Scotland, at least nowhere else among the
Carboniferous records of Britain is there such a colossal manifestation
of subterranean intrusion as the Great Whin Sill.
2. THE DERBYSHIRE TOADSTONES
In the absence of any certain evidence that the Whin Sill belongs to
the Carboniferous period, we must advance southward into the very heart
of England before any clear vestiges can be found of contemporaneous
volcanic eruptions among the members of the Carboniferous system.
After quitting the lavas and tuffs of Roxburghshire and their brief
continuations across the English border, we do not again meet with any
truly bedded volcanic rocks in that system until we reach the middle
of Derbyshire. In this picturesque district, famous for its lead-mines
and its mineral waters, a feebly developed but interesting group of
intercalated lavas, locally called "toadstones," has long been known.
There is thus a space of some 150 miles across which, though the
formations are there so fully developed and so abundantly trenched by
valleys from the top to the bottom of the system, no volcanic vents nor
any trace of Carboniferous volcanic ejections has yet been found. On
the other hand, after the district of the "toadstones" is passed, the
Carboniferous rocks are again destitute of any volcanic intercalations
across the centre and south-west of England and over Wales, until after
a space of about 150 miles they reappear in Somerset.
The volcanic group of Derbyshire thus stands out entirely isolated.
Lying in the Carboniferous Limestone, where that formation is typically
developed, it presents an admirable example of a thoroughly marine
phase of volcanic action (Map I.).
One of the most prominent features in the geology of the centre of
England is the broad anticlinal fold which brings up the lower portion
of the Carboniferous system to form the long ridge of the Pennine chain
that runs from Yorkshire to the Midland plain, and separates the
eastern from the western coal-fields. This fold widens southwards until
not only the Millstone Grit and Yoredale rocks, but the underlying
Mountain Limestone is laid bare. A broad limestone district is thus
exposed in the very heart of the country, ranging as a green fertile
undulating tableland, deeply cut by winding valleys, which expose
admirable sections of the strata, but nowhere reach the base of the
system. The total visible depth of the limestone series is computed to
be about 1500 feet; the Yoredale shales and limestones may be 500 feet
more; so that the calcareous formations in which the volcanic phenomena
are exhibited reach a thickness of at least 2000 feet.
It is not yet definitely known through what vertical extent of this
thickness of sedimentary material the volcanic platforms extend, but
where most fully developed they perhaps range through 1000 feet, lying
chiefly in the Carboniferous Limestone, but apparently in at least one
locality extending up into the lower division of the Yoredale group.
The area within which they can be studied corresponds nearly with that
in which the limestone forms the surface of the country, or a district
measuring about 20 miles from north to south, with an extreme breadth
of 10 miles in an east and west direction.
A special historical interest belongs to the Derbyshire
"toadstones."[18] They furnished Whitehurst with material for his
speculations, and were believed by him to be as truly igneous rocks
as the lava which flows from Hecla, Vesuvius or Etna. But he thought
that they had been introduced among the strata and "did not overflow
the surface of the earth, according to the usual operations of
volcanoes."[19]
[Footnote 18: This word has by some writers been supposed to be
corrupted from _tod-stein_, dead-stone, in allusion to the dying out of
the lead veins there; by others the name has been thought to be derived
from the peculiar green speckled aspect of much of the rock, resembling
the back of a toad.]
[Footnote 19: _An Enquiry into the Original State and Formation of the
Earth_, 1778, Appendix, pp. 149, _et seq._]
His views were published as far back as 1778, three years after
Hutton read the first outline of his theory of the earth and made
known his observations regarding the igneous origin of whinstones.[20]
The first detailed account of the Derbyshire eruptive rocks was that
given by Fairey,[21] which has served as the basis of all subsequent
descriptions. Conybeare, in particular, prepared a succinct narrative
from Fairey's more diffuse statements, and thus placed clearly
before geologists the nature and distribution of these volcanic
intercalations.[22] Subsequently the district was mapped by De la Beche
and the officers of the Geological Survey, and the areas occupied by
the several outcrops of igneous rock could then be readily seen.[23]
[Footnote 20: _Trans. Roy. Soc. Edin._ i. p. 275, _et seq._ Hutton
specially mentions the toadstone of Derbyshire as one of the rocks
produced by fusion, p. 277.]
[Footnote 21: _General View of the Agriculture and Minerals of
Derbyshire_ (1811).]
[Footnote 22: _Outlines of the Geology of England and Wales_ (1822), p.
448.]
[Footnote 23: See Sheets 71 N.W., 72 N.E., 81 N.E. and S.E. and 82 S.W.
of the Geological Survey of England and Wales.]
Though the "toadstones" were believed to form definite platforms among
the limestone strata, and thus to be capable of being used as reliable
horizons in the mineral fields of Derbyshire, they appear to have
been generally regarded as intrusive sheets like the Whin Sill of the
north. Thus De la Beche in his _Manual of Geology_, giving a summary
of what was known at the time regarding intercalated igneous rocks,
remarks with regard to the Derbyshire toadstones that they may from
all analogy be considered to have been injected among the limestones
which would be easily separated by the force of the intruded igneous
material.[24] But the same observer, after his experience among the
ancient volcanic rocks of Devonshire, came fully to recognize the
proofs of contemporaneous outflow among the Derbyshire toadstones. In
his subsequently published _Geological Observer_, he described the
toadstones as submarine lavas that had been poured out over the floor
of the sea in which the Carboniferous Limestone was deposited, and had
been afterwards covered up under fresh deposits of limestone.[25] It is
remarkable, however, that he specially comments on the absence, as he
believed, of any contemporaneously ejected ashes and lapilli, such as
occur in Devonshire. That true tuffs or volcanic ashes are associated
with the toadstones was noticed by Jukes in 1861,[26] and afterwards by
the Geological Survey.[27] Since that time geologists have generally
recognized these Derbyshire igneous rocks as truly contemporaneous
intercalations. But very little has recently been written on the
structure of the district, our information regarding it being still
based mainly on the early observations of Fairey and the mapping of the
Geological Survey.
[Footnote 24: _Manual_, 3rd edit. 1833, p. 462.]
[Footnote 25: _Geological Observer_ (1851), pp. 642-645.]
[Footnote 26: _Student's Manual of Geology_, 2nd edit. (1863), p. 523.
For a general _résumé_ of the proofs of contemporaneity furnished by
the toadstones, see "The Geology of North Derbyshire," by Messrs. A.
H. Green and A. Strahan (_Memoirs of the Geological Survey_, 2nd edit.
(1887), p. 123).]
[Footnote 27: In the first edition of the _Memoir on the Geology of
North Derbyshire_, published in 1859, the authors of which were Messrs.
A. H. Green, C. le Neve Foster and J. R. Dakyns.]
The subject, however, has now been resumed by Mr. H. Arnold Bemrose,
who in 1894, after a prolonged study of the petrography of the
rocks, communicated the results of his researches to the Geological
Society.[28] In his excellent paper, to which I shall immediately make
fuller reference, he mentions the localities at which lava-form and
fragmental rocks may be observed, but does not enter on the discussion
of the geological structure of the region or of the history of the
volcanic eruptions. Before the announcement of his paper, hearing that
I proposed to make for the first time a rapid traverse of the toadstone
district, for the purpose of acquainting myself with the rocks on the
ground, he kindly offered to conduct me over it. My chief object,
besides that of seeing the general nature of the volcanic phenomena of
the region, was to examine more particularly the areas of the volcanic
fragmental rocks, with the view of discovering whether among them some
remains might not be found of the actual vents of discharge. In this
search I was entirely successful. Aided by Mr. Bemrose's intimate
knowledge of the ground, I was enabled to visit in rapid succession
those tracts which seemed most likely to furnish the required evidence,
and in a few days was fortunate enough to obtain proofs of six or seven
distinct vents, ranging from the extreme northern to the furthest
southern boundary of the volcanic district. Mr. Bemrose has undertaken
to continue the investigation, and will, I trust, work out the detailed
stratigraphy of the Carboniferous Limestone so as eventually to furnish
an exhaustive narrative of the whole volcanic history of Derbyshire.
Meanwhile no adequate account of the area can be given. But I will here
state all the essential facts which up to the present time have been
ascertained.
[Footnote 28: _Quart. Journ. Geol. Soc._ vol. l. (1894), p. 603.]
1. THE ROCKS ERUPTED.--Mr. Allport has described the microscopic
character of some of the toadstones,[29] and further details have
been supplied by Mr. Teall.[30] The fullest account of the subject,
however, is that given by Mr. Bemrose in the paper above referred to.
This observer distinguishes the lava-form from the fragmental rocks,
and gives the minute characters of each series. He does not, however,
separate true interstratified lavas from injected sills, nor the bedded
tuffs from the coarse agglomerates which fill up the vents. These
distinctions are obviously required in order that the true nature and
sequence of the materials in the volcanic eruptions may be traced,
and that the phenomena exhibited in Derbyshire may be brought into
comparison with those found in other Carboniferous districts. But
to establish them satisfactorily the whole region must be carefully
re-examined and even to some extent re-mapped.
[Footnote 29: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 529.]
[Footnote 30: _British Petrography_, p. 209.]
The lavas (including, in the meantime, sheets which there can be
little doubt are sills) show three main types of minute structure
and composition, which are discriminated by Mr. Bemrose as--(_a_)
Olivine-dolerites; these, the most abundant of the series, consist
of augite in grains, olivine in idiomorphic crystals, plagioclase
giving lath-shaped and tabular sections, and magnetite or ilmenite
in rods and grains; (_b_) Ophitic olivine-dolerites, consisting of
augite in ophitic plates forming the groundmass, in which are imbedded
idiomorphic olivine, plagioclase (often giving large lath-shaped
sections and magnetite or ilmenite); (_c_) Olivine-basalts; these rocks
are distinguished by containing crystals of augite and olivine in a
groundmass of small felspar-laths, granular augite and magnetite or
ilmenite, with very little interstitial matter. They have been noticed
only in two of the outcrops of toadstone.
The fragmental rocks have been shown by Mr. Bemrose to cover a much
more extensive space than had been previously supposed. He has found
them to be distinguished by an abundance of lapilli varying from minute
fragments up to pieces about the size of a pea, and composed of a
material that differs in structure from the dolerites and basalts with
which the tuffs are associated. These lapilli consist largely of a
glassy base more or less altered, which is generally finely vesicular
and encloses abundant skeleton crystals and crystallites. The tuffs
thus very closely resemble some of the Carboniferous basic tuffs of
Fife, already referred to (vol. i. p. 422), and like these they include
abundant blocks of dolerite and basalt.
2. GEOLOGICAL STRUCTURE OF THE TOADSTONE DISTRICT.--As the volcanic
rocks of Derbyshire lie among the Carboniferous Limestones of a broad
anticlinal dome, they are only exposed where these limestones have been
sufficiently denuded, and as the base of the limestones is nowhere laid
bare, the lowest parts of the volcanic series may be concealed. Over
the tract where the toadstones can be examined they appear as bands
regularly intercalated with the limestones, but varying in thickness in
the course of their outcrops. As they are prone to decay, they usually
form smooth grassy slopes between the limestone scarps, though isolated
blocks of the dull brown igneous rocks may often be seen protruding
from the surface. Now and then a harder bed of toadstone caps a hill,
and thus forms a prominent feature in the landscape, but as a rule
these igneous bands play no distinguishing part in the scenery, and are
indeed less conspicuous than the white escarpments of limestone which
overlie them.
It was the opinion of the older geologists that three distinct
platforms of toadstone extend without break throughout the district,
and subdivide the limestones into four portions. But this opinion
does not seem to have been based on good evidence either of sequence
or of continuity. Various facts were brought forward by the officers
of the Geological Survey to show that the supposed persistence of the
three platforms of toadstone did not really exist, but that these
sheets of igneous material are found at different spots on very
different horizons, and are of limited horizontal range.[31] So far as
my own limited observations go, they entirely corroborate this view.
There can be little doubt, I think, that the identity of certain
outcrops of toadstone has been assumed, and the assumption has been
carried throughout the district. The truth is that the number of
successive platforms on which igneous materials appear will never be
satisfactorily determined until the stratigraphy of the Derbyshire
Carboniferous Limestone is worked out in detail. When the successive
members of this great calcareous formation have been identified by
lithological and palæontological characters over the district, it will
be easy to allocate each outcrop of toadstone to its true geological
horizon. When this labour has been completed, it will probably be found
that instead of three, there have been many discharges of volcanic
material during the deposition of the limestone series; that these
have proceeded from numerous small vents, and that they are all of
comparatively restricted horizontal extent. Such a detailed examination
will also determine how far the toadstones include veritable sills, and
on what horizons these intrusive sheets have been injected.
[Footnote 31: _Geol. Surv. Mem. on North Derbyshire_, by Messrs. Green
and Strahan (1887), p. 104.]
In the meantime, we know that the lowest visible bands of toadstone
are underlain by several hundred feet of limestone, thus proving that
the earliest known volcanic explosions took place over the floor
of the Carboniferous Limestone sea, after at least 700 or 800 feet
of calcareous sediment had accumulated there. The latest traces of
volcanic activity are found in a part of the Yoredale group of shales
and limestones which form the uppermost member of the Carboniferous
Limestone of this region. But it is not quite clear whether the
vesicular diabase found there is interstratified or intrusive.
Certainly no contemporaneous tuffs have yet been found among the
Yoredale rocks, nor in any higher subdivision of the Carboniferous
system, though coarse agglomerates marking the position of vents do
traverse the Yoredale group at Kniveton.
It may be remarked that in the district over which the toadstones can
be seen, two areas are recognizable, in each of which the exposures of
the igneous rocks are numerous, while between them lies an intervening
tract wherein there is hardly any visible outcrop of these rocks. The
northern and much the more extensive area stretches from Castleton
to Sheldon, while the southern spreads from Winster to Kniveton.
This distribution not improbably points to the original position of
the vents, and indicates a northern more numerous group of volcanic
orifices, and a southern tract where the vents were fewer, or at least
spread their discharges over a more limited space.
3. THE VENTS.--It had always appeared to me singular that, in ground
so deeply trenched by valleys as the toadstone district of Derbyshire,
no trace had been recognized of any bosses or necks from which these
volcanic sheets might have been erupted. It is true that in mining
operations masses of toadstone had been penetrated to a considerable
depth without their bottom being reached, and the suggestion had been
made that in such cases a shaft may actually have been sunk on one
of the vents through which the toadstone came up.[32] One instance in
particular was cited where, at Black Hillock, on Tideswell Moor, close
to Peak Forest Village, a mass of toadstone was not cut through, though
pierced to a depth of 100 fathoms. In that neighbourhood, however,
several of the sheets of eruptive material are probably sills, and the
shaft at Black Hillock may have been sunk upon the pipe or vein that
supplied one or more of these intrusive sheets.
[Footnote 32: _Geol. Surv. Mem. on North Derbyshire_, p. 134.]
It was therefore with no little interest that I detected a series of
vents at four separate localities, viz. Castleton, Grange Mill, Hopton,
and Kniveton Wood. I have no doubt that a more extended search will
bring others to light. Those observed by me are all filled with coarse
agglomerate, the blocks in which are mostly composed of different
lavas, sometimes with the addition of blocks of limestone, while the
matrix consists mainly of lapilli of basic devitrified glass.
The most typical examples form a group of two, possibly three, vents
which rise into two isolated, smooth, grassy dome-shaped hills at
Grange Mill, five miles west from Matlock Bath.[33] In external form
and colour, these eminences present a contrast to the scarped slopes
of limestone around them. They at once recall the contours of many of
the volcanic necks in Central Scotland. On examination it is found that
the material composing them is a dull green agglomerate, the matrix
of which is a compact substance weathering spheroidally, and full of
small lapilli of minutely vesicular diabase. The larger stones consist,
for the most part, of various vesicular dolerites or diabases, together
with some pieces of limestone and occasionally large blocks of the
latter rock, altered into a saccharoid condition. Two dykes of dolerite
or basalt traverse the margin of the larger vent.
[Footnote 33: This is Mr. Bemrose's outcrop, No. 46, _op. cit._ p. 633.]
The steep sides of these agglomerate domes rise from the low ground
around them to a height of 100 to 180 feet, their summits being a
little more than 900 feet above the sea. The smaller neck is nearly
circular, and measures about 1000 feet in diameter. The larger mass
is less regular in shape, and is prolonged into such a bulge on the
south-east as to suggest that its prolongation in that direction may
really mark the position of a third and much smaller vent contiguous to
it. The longer diameter of the larger mass is 2300 and the shorter 1300
feet.
[Illustration: Fig. 178.--View of two volcanic necks in the
Carboniferous Limestone series, at Grange Mill, five miles west of
Matlock Bath, from the north.]
On the south and west sides, the surrounding limestone can be traced
up to within a few feet of the edge of the agglomerate, and its strata
are there found to be much jumbled and broken, while their texture is
rather more crystalline than usual, though not saccharoid. The two
necks are separated by a narrow valley in which no rock is visible.
Their opposite declivities meet at the bottom of this hollow, and are
so definitely marked off that, even in the absence of proof that they
are disjoined by intervening limestone, there can be little hesitation
in regarding each hill as marking a distinct vent. A wider valley
extends along the eastern base of the necks, and slopes upward on its
east side until it is crowned by a long escarpment of limestone, which
reaches a height of 1000 feet above the sea, or about 100 feet above
the valley from which it rises. Unfortunately, the bottom and slopes
of this depression are thickly covered with soil, but at one or two
places debris of fine tuff may be observed, and at the northern and
southern ends of the hollow well-bedded green and reddish tuff appears,
dipping gently below the limestone escarpment. This band of volcanic
detritus evidently underlies the limestone, and forms most of the
gentle slope on the east side of the valley. It may be from 70 to 100
feet thick. That it was discharged from one or both of the necks seems
tolerably clear. Its material resembles that forming the matrix of the
agglomerate. The general arrangement of the rocks at this interesting
locality is represented in Fig. 179, which is reduced from my survey on
the scale of six inches to a mile. A section across the smaller vent
would show the structure represented in Fig. 180.
[Illustration: Fig. 179.--Plan of necks and bedded tuff at Grange Mill,
five miles west of Matlock Bath.]
[Illustration: Fig. 180.--Section across the smaller volcanic neck and
the stratified tuff in Carboniferous Limestone, Grange Mill.
1. Limestone; 2. Stratified tuff intercalated among the limestones; 3.
Agglomerate.]
This group of vents lies in the southern of the two tracts of the
volcanic district. In the northern tract a mass of agglomerate pierces
the base of the limestone escarpment about a quarter of a mile west
from the entrance to the Peak Cavern at Castleton.[34] It is rudely
semicircular in area, stretching down the slope until its northern
extension is lost under the lower ground. The agglomerate is not well
exposed, but it can be seen to be a green, granular crumbling rock,
made up in great part of minutely vesicular lapilli, enclosing blocks
of various diabases two feet long or more. From the abrupt way in which
this agglomerate rises through the limestone, there can be little doubt
that it marks the position of one of the volcanic vents of the time.
As it stands on the extreme northern verge of the limestone area,
the ground further north being covered with the Yoredale rocks and
Millstone Grit, it is the most northerly of the whole volcanic district.
[Footnote 34: This is outcrop No. 1 of Mr. Bemrose's paper, p. 625.]
Along the southern margin of the limestone country a group of
agglomerate masses probably marks another chain of vents. These are
specially interesting, inasmuch as they abut on the Yoredale series,
and may thus be looked upon as among the latest of the volcanic
chimneys. One of them is seen at Hopton,[35] where along the side of the
road a good section is exposed of coarse tumultuous agglomerate, having
a dull green matrix, composed of green, brown, and black, minutely
cellular, basic, devitrified, glassy lapilli, showing under the
microscope abundant microlites and crystals or calcareous pseudomorphs
of olivine, augite, and felspar, and much magnetite dust. Through
this matrix are distributed blocks of slaggy basalt and dolerite. An
interesting feature of this mass is the occurrence in it of some veins,
two or three inches broad, of a compact black porphyritic basalt. I
did not trace the relations of this agglomerate to the stratified
rocks around it. But its internal structure and composition mark it
out as a true neck. It extends, according to the Geological Survey
map, for about half a mile along the edge of the limestone, and is
represented as being separated by two faults from the Yoredale series
immediately to the south. So long as the belief is entertained that
the toadstones are contemporaneous outflows of lava lying on certain
definite horizons, far below the summit of the limestones, the position
of the Hopton agglomerate is only explicable on the assumption of some
dislocation by which the Yoredale shales have been brought down against
it. But when we realize that the rock is an unstratified agglomerate,
probably marking the place of a volcanic vent, and therefore rising
transgressively through the surrounding strata, the necessity for a
fault is removed, or if a fault is inserted its existence should be
justified on other evidence than the relations of the igneous rock to
the surrounding strata.
[Footnote 35: _Geol. Surv. Mem. North Derbyshire_, p. 24. This is
outcrop No. 53 of Mr. Bemrose's paper, p. 635.]
Four miles to the south-west of Hopton, on the slope of the hill
at Kniveton Wood, another remarkable mass of agglomerate forms a
rounded ridge between the two forks of a small stream.[36] Its granular
matrix, like that of the other necks, consists of lapilli of minutely
vesicular basic glassy lava or pumice, and encloses large and small
rounded blocks of finely cellular basalt and pieces of limestone. The
rock is unstratified, and in all respects resembles that of ordinary
Carboniferous necks in Scotland. Its relations to the Yoredale rocks
are laid bare in the channels of the streamlets. There the shales and
thin limestones may be seen much broken and plicated, their curved
and fractured ends striking directly at the agglomerate. They may be
traced to within a yard of the agglomerate. On the Geological Survey
map the igneous rock is represented as bounded by two parallel faults.
But I hardly think that this explanation suffices to account for the
relations of the rocks and their remarkable boundary-line, which seems
to me to be undoubtedly the wall of a volcanic vent. To the east of
the streams, another mass of agglomerate may mark another neck, while
to the north, a third detached area of the same kind of rock, rising
among the limestones, may be regarded as likewise a distinct mass. At
this locality, therefore, there are two, possibly three, vents. One of
these, from the way in which it cuts across the Yoredale shales and
limestones, is to be assigned to a time later than the older part of
the Yoredale series, and thus, like the Hopton mass, it indicates that
in the south of the volcanic area eruptions did not cease with the
close of the deposition of the thick limestones, but were prolonged
even into the time of the Yoredale rocks.
[Footnote 36: Outcrop No. 56, p. 638 of Mr. Bemrose's paper.]
A further proof of the late age of these southern patches of volcanic
material is shown by two bands of vesicular toadstone in the Yoredale
series, a little south from the village of Kniveton. These rocks are
traced on the Survey Map, and are shown in a diagram in the Memoir,
where their position is sought to be explained by a system of parallel
faulting.[37] I was able to trace the actual contact of the western band
with the strata underneath it, and satisfied myself that there is no
fault at the junction. The igneous material is regularly bedded with
the Yoredale shales and limestones. Either, therefore, these bands
are intercalated lava-streams or intrusive sills. If mere vesicular
structure were enough to distinguish true outflowing lavas, then there
could be no doubt about these Kniveton rocks. But this structure is
found in so many Carboniferous sills, particularly in those thin sheets
which have been injected into coals and black shales, that its presence
is far from decisive. The vesicles in the Kniveton rocks are small and
pea-like, tolerably uniform in size and shape, and crowded together.
They are thus not at all like the irregular cavities in the ordinary
cellular and scoriaceous lavas of the toadstone series.
[Footnote 37: _Op. cit._ p. 87.]
Whether or not the question of their true relations be ever
satisfactorily settled, these Kniveton bands are certainly younger than
the lower portion of the Yoredale group. Their evidence thus agrees
with that of the southern agglomerates in showing that the volcanic
activity of this region was continued even after the thick calcareous
masses of the Carboniferous Limestone series had ceased to be deposited.
Besides the six necks to which I have referred, a rock in Ember Lane,
above Bonsall, probably belongs to another vent.[38] It is particularly
interesting from the great preponderance of limestone fragments in
it. The volcanic explosions at this locality broke up the already
solidified limestones on the floor of the Carboniferous Limestone sea,
and strewed them around, mingled with volcanic blocks and dust of the
prevailing type.
[Footnote 38: This is outcrop No. 39 of Mr. Bemrose's paper, p. 632.]
When the district has been more carefully searched, other centres of
eruption will no doubt be discovered. It may then be possible to depict
the distribution of the active vents, and to connect with them the
outflow of the bedded lavas. So far as I have been able to ascertain,
there are no necks of dolerite or basalt, though, as I have shown,
dykes or veins of molten rock are occasionally to be found in the
agglomerates of the necks.
4. THE LAVAS AND TUFFS.--I have referred to the opinion of De la Beche
that the toadstones of Derbyshire were poured out as lava-streams
without any accompanying fragmentary discharges, and to the correction
of this opinion by the subsequent observations of Jukes and of the
Geological Survey. But though the existence of interbedded tuffs
has long been known, it was not until Mr. Bemrose's more careful
scrutiny that the relative importance of the tuffs among the lavas was
first indicated. He has shown that a number of the bands mapped as
"toadstone" are tuffs, and he has discovered other bands of tuff which
have not yet been placed on any published map.
In examining the outcrops of the various toadstones of Derbyshire we
learn that some of them are lavas without tuffs, probably including a
number of bands, which are really sills; that others are formed of both
lavas and tuffs, and that a third type shows only bedded tuff. Each
of these developments will deserve separate description. But before
entering into details, we may take note of the varying thicknesses of
the different toadstones which have been determined by observation at
the surface or by measurement underneath in mining operations. In some
cases a distinct band of toadstone, separated by many feet or yards of
limestone from the next band, and therefore serving to mark a separate
volcanic discharge, may not exceed a yard or two in total thickness,
and from that minimum may swell out to 100 feet. The majority of the
bands probably range between 50 and 100 feet in thickness. In one
exceptional case at Snitterton, a mass of "blackstone" is said to have
been proved to be 240 feet thick, but this rock may not improbably have
been a sill.[39] The true contemporaneous intercalations seem to be
generally less than 100 feet in thickness.
[Footnote 39: A difference is made by the mining community between
"toadstone" and what is called "blackstone." The former name appears
to be restricted to the amygdaloidal green and generally more or less
decayed lavas; the latter, so far as I can learn, is applied to the
dark, more solid and crystalline rocks. If this distinction be well
founded the one name may perhaps serve to mark the open cellular lavas,
the other the more compact, dark, and heavy intrusive sheets.]
(_a_) Lavas without Tuffs.--Examples occur of sheets of toadstone
which consist entirely of contemporaneously ejected diabase, basalt
or dolerite. This rock is then dull green or brown in colour, more or
less earthy in texture, and irregularly amygdaloidal. The vesicles are
extremely varied in size, form and distribution, sometimes expanding
until the rock becomes a slaggy mass. A central more solid portion
between a scoriaceous bottom and top may sometimes be observed, as
at the Great Rocks Quarry, Peak Forest Limeworks (Fig. 181). In this,
as in other examples, a remarkably hummocky and uneven surface of
limestone lies below the igneous band, the calcareous rock presenting
knobs and ridges, separated by cauldron-shaped cavities and clefts,
some of which are several yards deep. These inequalities are filled
in and covered over with a soft yellow and brown clay, varying up to
three or four feet thickness, and passing upwards into the more solid
toadstone. There can hardly be any doubt that this singularly uneven
limestone surface is due to the solvent action of water lying between
the limestone and the somewhat impervious toadstone above, and that the
clay represents partly the insoluble residue of the calcareous rock,
but chiefly the result of the action of the infiltrating water on the
bottom of the igneous band.[40]
[Footnote 40: _Geological Survey Memoir on North Derbyshire_, p. 20 and
footnote.]
[Illustration:
Fig. 181.--Section of vesicular and amygdaloidal diabase resting on
Carboniferous limestone, Peak Forest Limeworks, Great Rocks Quarry.
1. Limestone with a surface dissolved into cauldron-like
hollows; 2. Rotten yellow and brown clay resulting from
decomposition of toadstone and white clay from the solution of the
limestone--sometimes three or four feet thick; 3. Toadstone, a
diabase with highly slaggy base.
]
[Illustration: Fig. 182.--View of the superposition of Carboniferous
limestone upon toadstone, Raven's Tor, Millersdale (length about 100
feet).
1. Toadstone; 2. Limestone; _f_, Fault.]
Junctions of the upper surfaces of the lava-sheets with the overlying
limestone show that the igneous material sometimes assumed hummocky
forms, which the calcareous deposits gradually overspread and
covered.[41] A good example of this kind may be observed by the roadside
at the foot of Raven's Tor, Millersdale. As shown in the subjoined
figure, the limestone has here been worn into a cave, the floor of
which is formed by the toadstone. The latter rock, of the usual dull
green, slaggy and amygdaloidal character, is covered immediately by the
limestone, but I did not observe any fragments of the toadstone, nor
any trace of ashy materials in the overlying calcareous strata. This
section shows that after the outflow of the lava, the sedimentation of
the limestone was quietly resumed, and the igneous interruption was
entirely buried.
[Footnote 41: Compare De la Beche, _Geological Observer_, pp. 559, 560,
and _North Derbyshire Memoir_, p. 123.]
In some cases there is evidence of more than one outflow of lava in the
same band of toadstone. Jukes believed that each band "was the result,
not of one simultaneous ejection of igneous matter, but of several,
proceeding from different foci uniting together to form one band," and
he found that near Buxton, two solid beds of toadstone could be seen
to have proceeded from opposite quarters towards each other without
overlapping.[42]
[Footnote 42: _Student's Manual of Geology_, 2d edit. (1862), p. 523.]
In Millersdale the authors of the _Geological Survey Memoir on North
Derbyshire_ observed that a band of toadstone about 100 feet thick
showed six distinct divisions, which they were disposed to regard as
marking so many separate beds.[43] In Tideswell Dale, on the west side
of the valley, immediately to the south of the old toadstone quarry,
two bands of toadstone are seen to be separated by a few yards of
limestone.
[Footnote 43: _Op. cit._ p. 19.]
(_b_) Lavas with Tuffs.--It will probably be found that in many, if
not in most cases, the outflow of lava was preceded, accompanied or
followed by fragmental discharges. As far back as 1861, Jukes noticed
that a toadstone band, about 50 feet thick, near Buxton consisted of
two solid beds of lava "with beds of purple and green ash, greatly
decomposed into clay, both above and below each bed and between the
two."[44]
[Footnote 44: _Op. cit._ p. 523.]
[Illustration:
Fig. 183.--Section at lime-kiln, south of Viaduct, Millersdale
Station.
]
An interesting section, showing this intercalation of the two kinds
of material is exposed at the lime-kilns beyond the southern end of
the railway viaduct at Millersdale Station. Over a mass of solid blue
limestone (1 in Fig. 183) lies a band of bright yellow and brown
clay (2), varying from six inches to two feet in thickness. This may
be compared with the clay found above the limestone at Peak Forest
(Fig. 181). But it is probably a layer of highly decomposed tuff.
It is succeeded by a thin band of greenish limestone (3) containing
an admixture of fine volcanic detritus, and partially cut out by an
irregular bed, four to eight feet thick, of a highly slaggy, greenish,
decomposing, spheroidal and amygdaloidal diabase (4). This unmistakable
lava-sheet is followed by a bed of green granular tuff (5), which
in some places reaches a thickness of three feet, but rapidly dies
out. Over a space several yards in breadth, the succeeding strata are
concealed, and the next visible rock is a dark, compact dolerite which
weathers spheroidally (6).
(_c_) Tuffs without Lavas.--Mr. Bemrose has shown that some of the
bands of toadstone consist entirely of bedded tuff. In these cases, so
far as the present visible outcrops allow us to judge, no outflow of
lava accompanied the eruption of fragmentary materials. But that the
ejection of these materials was not the result of a sudden spasmodic
explosion, but of a continued series of discharges varying in duration
and intensity, is indicated by the well-bedded character of the tuff
and the alternation of finer and coarser layers. Large blocks of
lava, two feet or more in diameter, may mark some of the more vigorous
paroxysms of the vents, while the usual fine granular nature of the
tuff may point to the prevailing uniformity and less violent character
of the eruptions. Bands of tuff 70 feet or more in thickness, without
the intercalation of any limestone or other non-volcanic intercalation,
point to episodes of such continued volcanic activity that the ordinary
sedimentation of the sea-bottom was interrupted, or at least masked, by
the abundant fall of dust and stones.
One of the best exposures of such intercalations of bedded tuffs was
pointed out to me by Mr. Bemrose, immediately to the east of the
village of Litton. The matrix is crowded with the usual minutely
vesicular glassy lapilli, and encloses fragments of diabase of all
sizes, up to blocks more than a foot in diameter. The rock is well
stratified, and the layers of coarse and fine detritus pass beneath
a group of limestone beds. The actual junction is concealed under
the roadway, but only two or three feet of rock cannot be seen. The
lowest visible layer of limestone is nodular and contains decayed
bluish fragments which may be volcanic lapilli. Immediately above the
lower limestones the calcareous bands become richly fossiliferous.
Some of their layers consist mainly of large bunches of coral; others
are crowded with cup-corals, or are made up mainly of crinoids with
abundant brachiopods, polyzoa, lamellibranchs, gasteropods and
occasional fish-teeth. This remarkable profusion of marine life is
interesting inasmuch as it succeeds immediately the band of volcanic
ash.
Another well-marked zone of tuff, with no traceable accompaniment of
lava, has already been referred to as connected with the Grangemill
vents. In this case also, the limestone that lies directly upon the
volcanic material is rather impure and nodular in character. The tuff
itself is well bedded, perhaps from 70 to 100 feet thick and dips
underneath an overlying series of marine limestones.
I did not observe thin partings of tuff and disseminated volcanic
lapilli among the limestones, such as are so marked in the Lower
Carboniferous formations of West Lothian, and in the Limerick basin,
to be described in the following chapter. But a diligent search might
discover examples of them, and thus prove that, besides the more
prolonged and continuous eruptions that produced the thick bands of
tuff, there were occasional feeble and intermittent explosions during
the accumulation of the thick sheets of limestone. Some of the layers
of "red clay" observed in shafts sunk for mining purposes may perhaps
represent such spasmodic discharges of fine fragmental material.
5. THE SILLS.--No attempt has yet been made to determine whether and
to what extent the toadstone bands include true intrusive sheets. My
own brief examination of the ground does not warrant me in making
any positive statement on this subject. I can hardly doubt, however,
that some, perhaps not a few, of the toadstone bands are really
sills. In the accounts of these rocks contained in the mining records
a distinction, as already remarked, appears to have been generally
drawn between "toadstone" and "blackstone." The latter term is applied
to the black, fresh, more coarsely crystalline, and generally
non-amygdaloidal rocks, which, so far as I have been able to examine
them, have the general external and many of the internal characters of
the Carboniferous sills of Central Scotland. At Snitterton near Matlock
one of these "blackstones," as already mentioned, is said to have been
found to be 240 feet thick.[45]
[Footnote 45: _North Derbyshire Memoir_, p. 23.]
It is stated that the toadstones, though subject to great variations
in thickness, are never seen to cut across the limestones.[46] But
I suspect that proofs of intrusion and transgression will be found
when diligently sought for. It appeared to me that the dark, compact,
crystalline dolerite, which was formerly quarried in the middle of
Tideswell Dale, may be separated from the vesicular toadstone of
that valley, which is undoubtedly a true lava-flow, and that it does
not always occupy the same horizon there, being sometimes below and
sometimes above the amygdaloid. Where it rests on a band of red clay
the latter rock has been made columnar to a depth of nine feet.[47]
Alteration of this kind is very rare among the Carboniferous bedded
lavas, but is by no means infrequent in the case of sills. But the
most important proof of alteration which I have myself observed occurs
at Dale Farm near the village of Peak Forest, where the limestone
above a coarsely crystalline dolerite has been converted into a white
saccharoid marble for about two yards from the junction.
[Footnote 46: _Op. cit._ p. 123.]
[Footnote 47: J. M. Mello, _Quart. Journ. Geol. Soc._ vol. xxvi. (1871),
p. 701.]
3. THE ISLE OF MAN
Rising from the middle of the Irish Sea, within sight of each of the
three kingdoms, with a history and associations so distinct, yet so
intimately linked with those of the rest of Britain, this interesting
island presents in its geological structure features that connect it
alike with England, Scotland and Ireland, while at the same time it
retains a marked individuality in regard to some of the rocks that
form its framework. Its great central ridge of grits and slates, which
still rises 2000 feet above the sea in the summit of Snaefell, must
have formed a tract of dry land in Carboniferous time, until it sank
under sea-level, and was buried beneath the Carboniferous and later
formations. Along the southern margin of this ancient land, a relic of
the floor of the Carboniferous sea has been preserved in a small basin
of Carboniferous Limestone which covers about seven or eight square
miles. This remnant has a special interest in geological history, for
it has preserved the records of a series of volcanic eruptions which
took place contemporaneously with the deposition of the Carboniferous
Limestone.
The geology of the Isle of Man was sketched in outline by J. F.
Berger,[48] J. Macculloch,[49] and J. S. Henslow,[50] and was afterwards
more fully illustrated by J. G. Cumming.[51] To the last-named observer
we owe the recognition of true intercalated volcanic rocks among
the calcareous formations of the southern end of the island. These
rocks have subsequently been studied in greater detail by a number of
geologists. An excellent general account of them was published in 1874
by Mr. John Horne, of the Geological Survey.[52] A few years later some
further observations on them were prepared by J. Clifton Ward.[53] More
recently their petrography has been studied by Messrs. E. Dickson, P.
Holland and F. Rutley,[54] and in more detail by Mr. B. Hobson.[55] To
some of the observations of these writers reference will be made in
the succeeding pages. During the progress of the Geological Survey in
the Isle of Man, the rocks in question have been mapped in detail by
Mr. A. Strahan and Mr. G. W. Lamplugh, and I have had an opportunity
of examining the coast-sections with the last-named geologist. The
following description of these sections is taken mainly from my field
note-book. The full details will appear in the official _Memoirs_.
[Footnote 48: _Trans. Geol. Soc._ 1st ser. vol. ii. (1814), p. 29.]
[Footnote 49: _Western Islands of Scotland_ (1819), vol. ii. p. 571.]
[Footnote 50: _Trans. Geol. Soc._ 1st ser. vol. v. (1821), p. 482.]
[Footnote 51: _The Isle of Man_ (1848), chap. x.]
[Footnote 52: _Trans. Geol. Soc. Edin._ ii. (1874), p. 332.]
[Footnote 53: _Geol. Mag._ 1880, p. 4.]
[Footnote 54: _Proc. Liverpool Geol. Soc._ vol. vi. (1888-89), p. 123.]
[Footnote 55: _Quart. Journ. Geol. Soc._ xlvii. (1891), p. 432. This
paper was reprinted with additions and corrections in _Yn Lioar
Manninagh_, Douglas, Isle of Man, vol. i. No. 10, April 1892.]
It may be remarked at the outset that the last outcrop of the
plateau-lavas of the Solway basin occurs only 60 miles from the south
end of the Isle of Man, at the foot of the hills of Galloway, the blue
outline of which can be seen from that island. The distance from the
Manx volcanoes to the nearest of the puys of Liddesdale is about 100
miles. Though the fragment which has been left of the ejections is too
small to warrant any confident parallelism, there appears to be reason
to believe that, alike in geological age and in manner of activity, the
Manx volcanoes may be classed with the type of the puys.
The Carboniferous strata of the Isle of Man lie in a small trough at
the south end of the island. The lowest members of the series consist
of red conglomerates and sandstones, which pass upward into dark
limestones full of the characteristic fossils of the Carboniferous
Limestone. As the bottom of the basin is on the whole inclined
seawards, the highest strata occur along the extreme southern coast. It
is there that the volcanic rocks are displayed. They occupy a narrow
strip less than two miles in length, which is almost entirely confined
to the range of cliffs and the ledges of the foreshore. Yet though
thus extremely limited in area, they have been so admirably dissected
along the coast, that they furnish a singularly ample body of evidence
bearing on the history of Carboniferous volcanic action.
Unfortunately the bottom of the volcanic group is nowhere visible.
At the east or lower end of the series, exposed on the shore, an
agglomerate with its dykes appears to truncate the Castletown
Limestones. No trace of any tuff has been noticed among these lower
limestones. We may infer that the volcanic activity began after they
were deposited. The highest accessible portions of the volcanic group,
as Mr. Horne showed, are clearly exposed on the coast at Poyll Vaaish,
intercalated in and overlying the dark limestones of that locality
(Fig. 184), which have been assigned, from their fossil contents, to
the upper part of the Carboniferous Limestone series.[56] The Manx
volcanoes may therefore be regarded as having probably been in eruption
during the later portion of the Carboniferous Limestone period.
[Footnote 56: R. Etheridge jun., in Mr. Horne's paper above cited.]
[Illustration: Fig. 184.--Limestones passing under stratified tuffs,
Poyll Vaaish, Isle of Man.]
Owing to irregularities of inclination, the thickness of the volcanic
group can only be approximately estimated. It is probably not less
than 200 or 300 feet. But as merely the edge of the group lies on the
land, the volcanic rocks may reach a considerably greater extent and
thickness under the sea.
The volcanic materials consist mainly of bedded tuffs, but include
also several necks of agglomerate and a number of dykes and sills. So
far as I have observed, they comprise no true lava-streams.[57] These
Manx tuffs present many of the familiar features of those belonging
to the puy-eruptions of Central Scotland, but with some peculiarities
worthy of attention. They are on the whole distinctly bedded, and as
their inclination is generally in a westerly direction, an ascending
order can be traced in them from the eastern end of the section to the
highest parts of the group associated with the Poyll Vaaish limestones.
Their colour is the usual dull yellowish-green, varying slightly in
tint with changes in the texture of the materials, the palest bands
consisting of the finest dust or volcanic mud. Great differences in the
size of their fragmentary constituents may be observed in successive
beds, coarse and fine bands rapidly alternating, with no admixture
of non-volcanic sediment, though occasional layers of fine ash or
mudstone, showing distinct current-bedding, may be noticed.
[Footnote 57: The occurrence of intercalated lavas has been described
in this series, but, as I shall show in the sequel, they are probably
intrusive masses.]
Pauses in the succession of eruptions are marked by the intercalation
of seams of limestone or groups of limestone, shale and black impure
chert. Such interstratifications are sometimes curiously local and
interrupted. They may be observed to die out rapidly, thereby allowing
the tuff above and below them to unite into one continuous mass. They
seem to have been accumulated in hollows of the tuff during somewhat
prolonged intervals of volcanic quiescence, and to have been suddenly
brought to an end by a renewal of the eruptions. There are some four or
five such intercalated groups of calcareous strata in the thick series
of tuffs, and we may regard them as marking the chief pauses in the
continuity or energy of the volcanic explosions.
An attentive examination of these interpolated sedimentary deposits
affords some interesting information as to the submarine conditions in
which the eruptions took place. The intercalations, sometimes 12 feet
or more in thickness, consist mainly of dark limestones, enclosing
the usual Carboniferous Limestone fossils; black shales, sometimes
showing very fragmentary and much macerated remains of ferns and other
land-plants; and black impure argillaceous chert or flint, arranged
in bands interposed between the other strata, and also in detached
lumps and strings. The dark flaggy limestones and black shales may
be paralleled lithologically with those of Castletown and Poyll
Vaaish. Indeed, there seems to be little doubt that they represent
the contemporaneous type of marine sediment that was gathering on
the sea-floor outside the volcanic area, and which during intervals
of quiescence or feeble eruptivity spread more or less continuously
into that area. The thick mass of tuff must thus have been strictly
contemporaneous with a group of calcareous muddy and siliceous deposits
which gathered over the bottom beyond the limits of the showers of
ashes.
[Illustration: Fig. 185.--Section of tuff, showing intercalations of
black impure chert, west of Closenychollagh Point, near Castletown,
Isle of Man.]
One of the most singular features of these sedimentary intercalations
is the occurrence of the black cherty material. It may generally be
observed best developed at the bottom and top of each group of included
strata. Looking at the lumps of this substance scattered through the
adjoining tuffs, we might at first take them for ejected fragments, and
such no doubt may have been the derivation of some of them. But further
examination will show that, as a rule, they are of a concretionary
nature, and were formed _in situ_ contemporaneously with or subsequent
to the deposition of the tuffs. The accompanying section (Fig. 185)
represents the manner in which the chert is distributed through two
or three square yards of tuff overlying one of the calcareous groups.
The material has been segregated not only into lumps, but into veins
and bands, which, though on the whole parallel with the general
stratification-planes of the deposits, sometimes run irregularly in
tongues or strings across these planes, as shown in Fig. 186, where the
dark chert band which overlies the limestones and shales sends a tongue
upwards for several inches into the overlying tuff.
That these interstratified calcareous and muddy strata were laid down
in water of some considerable depth may be inferred from their general
lithological characters. The dark carbonaceous aspect of the limestones
points to the probable intermingling of much decayed vegetation with
the remains of the calcareous organisms of which these strata chiefly
consist. The thin unimportant bands or partings of dark shale show that
only the finest muddy sediment reached the quiet depths in which the
strata were deposited, while the macerated fern-fragments suggest a
long flotation and ultimate entombment of terrestrial vegetation borne
seawards from some neighbouring land.
[Illustration:
Fig. 186.--Section of intercalated dark limestone, shale and chert
in the tuff south of Poyll Vaaish Bay, Isle of Man.
1. Limestones and shales; 2. Chert; 3. Tuff.]
The cherty bands and nodules, like the flints of the chalk, bear their
testimony to the quiet character of the sedimentation in rather deep
water beyond the limits within which the sediment from the land was
mainly accumulated on the sea-bottom. The origin of these siliceous
parts of the series of deposits has still to be investigated. Whether
or not they are to be referred to organic causes like chalk-flints, and
the radiolarian cherts of the Lower Silurian system, they furnish a
fresh example of the remarkable association of such siliceous material
with volcanic phenomena, which has now been observed in many widely
separated areas all over the world.
If we next turn to the stratification of the tuffs, we obtain further
evidence of undisturbed conditions of deposition on the sea-floor.
The bedding of these volcanic masses, though distinct, appears for
the most part to be due rather to the eruption and settlement of
alternately finer and coarser detritus than to any marked drifting
and rearrangement of these materials by current-action into different
layers. Throughout the series of tuffs, indeed, there is, on the whole,
a notable absence of any structure suggestive of strong currents or
of wave-action in the dispersal and reassortment of the volcanic
detritus. The ashes and stones were discharged in such a way as to
gather irregularly over the sea-floor into ridges and hollows. There
does not seem to have been sufficient movement in the bottom water
to level down these inequalities of surface, for we find that they
remained long enough to allow twelve feet or more of calcareous and
siliceous ooze to gather in the hollows, while the intervening ridges
still stood uneffaced until buried under the next fall of ashes.
At rare intervals some transient current or deeper wave may have
reached the bottom and spread out the volcanic detritus lying there.
Such exceptional disturbances of the still water are not improbably
indicated by occasional well-defined stratification, and even by
distinct false-bedding, in certain finer layers of tuff.
The materials of the tuffs are remarkably uniform in character and
conspicuously volcanic in origin. With the exception of occasional
blocks of limestone, which range up to masses several feet, and
occasionally several yards, in diameter, the dust, lapilli and included
stones consist entirely of fragmentary basic lava, so persistent in
its lithological features that we may regard its slightly different
varieties as merely marking different conditions of the same rock. The
accumulation of pumiceous ash in this southern coast of the Isle of Man
is one of the most remarkable in Britain. As Mr. Hobson has well shown,
the matrix of this tuff consists of irregular lapilli, representing
what may have been various conditions of solidification in one original
volcanic magma. This magma he has described as an "augite-porphyrite"
or olivine-basalt. Some of the lapilli, as he noted, consist of a
pumice "crowded with vesicles which occupy more space than the solid
part"; others show nearly as many vesicles, but the glass is made brown
by the number of its fine dust-like inclusions; a third type presents
the cells and cell-walls in nearly equal proportions. The same observer
found that where the substance is most cellular the vesicles, fairly
uniform in size, measure about a tenth of a millimetre in longest
diameter.
An interesting feature of the tuffs is the abundant occurrence of loose
felspar crystals throughout the whole group up to the highest visible
strata. These crystals, sometimes nearly an inch in length, appear
conspicuously as white spots on weathered surfaces of the rock. They
are so much decayed, however, that it is difficult to extract them
entire. On the most cursory inspection they are observed to enclose
blebs of a greenish substance like the material that fills up the
vesicles in the pumiceous fragments and in the pieces of cellular lava.
I have not ascertained the original source of these scattered felspars.
In one of the dykes on the north side of the agglomerate at Scarlet
Point, as was pointed out by Mr. Hobson, large crystals of plagioclase
occur in the melaphyre, but the felspars in the tuffs and agglomerates
differ so much from these that we cannot suppose them to have come from
the explosion of such a rock. I failed to detect any other mineral in
detached crystals in the tuffs, but a more diligent search might reveal
such, and afford some grounds for speculating on the probable nature
of the magma from the explosion of which the scattered crystals were
derived. It is at least certain that this magma must have included a
large proportion of plagioclase crystals.
Between the lapilli and the minute pumice-dust that constitute the
matrix of this tuff much calcite may be detected. Though this mineral
may have been partly derived from the decay of the felspar in the
lava-fragments, I believe that it is mainly to be attributed to the
intermingling of fine calcareous ooze with the ash accumulated on the
sea-floor. A more remarkable association of the same kind will be
described in later pages from King's County in Ireland. That abundant
calcareous organisms peopled the sea in which the Manx Carboniferous
volcanoes were active is shown by the contemporaneously deposited
limestones. The tuffs themselves are occasionally fossiliferous.
Species of _Spirifer_, _Productus_ and other brachiopods, together
with broken stems of encrinites, may be found in them, and doubtless
the diffused calcite, though now crystalline, as in the limestones, and
showing no organic structure, owes its presence to the detritus of once
living organisms.
The stones imbedded in the tuff consist almost exclusively of slightly
different varieties of the same pale, always vesicular rock, and
sometimes pass into a coarse slag. They vary up to six feet or more
in length. In many cases, they appear to have been derived from the
disruption of already solidified lava, for their vesicles are not
elongated or arranged with reference to the form of the block, but have
been broken across and appear in section on the outer surface. In other
instances, however, the cavities are large and irregular in the centre
of the block, while on the outside they are smaller and are drawn out
round the rudely spherical shape of the mass, as in true volcanic bombs.
The limestone fragments enclosed in the tuff include pieces of the
dark carbonaceous and of the pale encrinal varieties. In no case did I
observe any sensible alteration of these fragments. They seem to have
been derived from material disrupted and ejected during the opening of
successive vents, and not to have been exposed for any considerable
time to the metamorphic influence of volcanic heat and vapours.
Narrow though the strip of volcanic material is along the south coast
of the Isle of Man, it has fortunately preserved for us some of the
vents from which the tuffs were ejected. A group of these vents, three
or four in number, may be traced along the shore in a general W.N.W.
and E.S.E. line from Scarlet Point for rather more than a mile. Their
margins are in some places exceedingly well defined. The most striking
example of this feature occurs in the most westerly vent, where a neck
of remarkably coarse volcanic agglomerate rises vertically through
well-bedded, westerly-dipping tuff (Fig. 187). In other portions of
their boundaries no sharp line can be drawn between the material
filling the vent and that of the surrounding tuffs. Hence it is
difficult to define precisely the form and size of the vents. I am
inclined to believe from this indefiniteness of outline, and from the
remarkable structure of the dykes, to which I shall afterwards refer,
that the presently visible parts of these necks must lie close to the
mouths of the original vents, if indeed they do not actually contain
parts of the craters and of their surrounding walls.
The materials that have filled up the eruptive vents consist
chiefly of agglomerate, but partly also of intrusive portions of
vesicular lava. The agglomerate is composed of similar materials to
the tuffs. Its matrix shows the same extraordinarily abundant fine
greenish-grey basic pumiceous lapilli, with the same kind of plentiful
loose felspar-crystals. The large blocks of lava, too, resemble in
composition and structure those of the bedded tuffs, but greatly exceed
them in size and abundance.
Besides the fragments of vesicular lava, there occur also occasional
blocks of limestone. Some of these are several yards in length.
Messrs. Strahan and Lamplugh have mapped a large mass of limestone
at the Scarlet vent, which, so far as can be observed, lies in the
agglomerate--a large cake of white limestone with pebbles of quartz,
which has probably been broken off from some underlying bed and carried
up in the chimney of the volcano.
As a rule the agglomerate is a tumultuous, unstratified mass. But in
many places it shows lines of bedding and, as already stated, passes
outward into ordinary bedded tuff, the number and size of the ejected
blocks rapidly diminishing. Where this transition occurs we seem to
see a remnant of the base of the actual volcanic cone. Thus, in the
most westerly vent already cited, while the wall of the vent has been
laid bare on the side next the sea, so that the agglomerate on the
beach descends vertically through the surrounding bedded tuffs, on the
western side the cliffs have preserved a portion of the material that
accumulated outside the orifice (Fig. 187). In this section we observe
that the coarse agglomerate which fills up the main part of the vent
has been left with a hummocky, uneven surface, and that a subsequent
and perhaps feebler eruption of finer material has covered over these
inequalities, and has extended to the left above the fine tuffs through
which the agglomerate has been drilled.
[Illustration: Fig. 187.--Section of part of a volcanic neck on shore
to the south-east of Poyll Vaaish Bay, Isle of Man.]
[Illustration: Fig. 188.--Section of successive discharges and
disturbances within a volcanic vent. Scarlet Point, Isle of Man.]
Again, in the largest of the vents, that near Scarlet Point, still
clearer proof of successive eruptions and dislocations within a
volcanic chimney may be noticed. At one point the accompanying section
(Fig. 188) has been laid bare by the waves. The oldest accumulation is
a fine green granular tuff (_a_), rudely and faintly arranged in layers
inclined at high angles, like the fine materials in many of the vents
of the basin of the Firth of Forth. This peculiar stratification, due
not to the assortment of materials in water, but to the deposition of
coarser and finer detritus by successive explosions, and to subsequent
slipping or tilting, is a characteristic feature of the detritus which
has filled up ancient volcanic funnels. A later explosion from some
adjacent part of the same vent has given rise to the discharge of a
coarse agglomerate (_b_), which with blocks sometimes six feet long,
overspreads the earlier material. A third detrital accumulation in the
same vent, consisting of a firm brecciated tuff (_c_) with much calcite
in its matrix, has been brought down by a slip (_f_) which cuts across
both of the previous deposits. A broad dyke (_d_) of vesicular diabase
(augite-porphyry) traverses the vent, and is probably later than any of
the other rocks in the section.
I will conclude this account of the Manx Carboniferous volcanic rocks
with a brief reference to the intrusive masses which form a prominent
feature of the coast-line. From the picturesque headland of Scarlet
Point the broad dyke which forms that promontory may be traced for
some distance westwards. Several other parallel dykes run in the
same direction which, it will be observed, is also that of the chain
of vents. It might be said that the vents are, as it were, strung
together by a line of dykes. These eruptive masses traverse both the
agglomerates and the bedded tuffs. They probably belong, therefore,
to a comparatively late part of the volcanic history. That they are
truly intrusive and not lava-flows is, I think, clearly shown by their
vertical walls which descend through the surrounding rocks, and by
the greater closeness of their texture, as well as the diminution in
the size of their vesicles along the contact surfaces. But it must be
admitted that in their remarkably developed vesicular structure they
look more like streams of lava than ordinary dykes.
It is this structure which gives to these dykes their peculiar
interest. Bands of vesicles, from an inch or less to several inches in
breadth, run along the dykes parallel to the outer walls. Unlike the
familiar rows of little amygdaloidal cells in ordinary basalt dykes,
such as those of the Tertiary series in Scotland, these vesicles,
though small and pea-like in the narrower bands towards the margins of
the dykes, became so large, numerous, and irregular in the broader and
more central bands, that the rock passes there into a rough slag.
[Illustration: Fig. 189.--Section of dyke and sill in the tuffs west of
Scarlet Point, Isle of Man.]
While the intrusive material has for the most part risen in the form
of dykes, in one part of the coast-section, a little to the west
of Scarlet Point, it has been injected as a sill among the bedded
tuffs.[58] A section taken at this locality gives the structure
represented in Fig. 189. On the north side of the great dyke, the
strata of tuff which dip under it, roll over and support an outlying
sheet of the same material. The slaggy structure of parts of this sill
give it some resemblance to a true lava-flow. But it is the same
structure which can be seen in the dykes, while the closer grain along
the contact-surface further connects it with these intrusions.
[Footnote 58: It is this sheet which has been described as a
lava-stream.]
[Illustration:
Fig. 190.--Section on south side of vesicular sill west of Scarlet
Point.
]
[Illustration: Fig. 191.--Bands of vesicles in the same sill.]
There is, however, a peculiarity about the development of the
vesicular structure in this sill which I have not observed anywhere
else. If we examine the southern side of the crag near its eastern
end we observe that the successive bands of vesicles are arranged
in the same direction as the surface of contact with the underlying
tuffs, precisely as they are ranged in dykes parallel to the bounding
walls. So far the structure is quite normal. But, moving a few yards
westwards, we find that the bands begin to curve, and, instead of
following the contact surface, strike it first obliquely and then
at right angles, until we have the structure shown in Fig. 191. The
bands here vary from less than an inch to more than a foot in breadth,
and where broadest assume a slaggy texture. I sought in vain for any
evidence of subsequent disturbance such as might have truncated these
parallel rows of vesicles and pushed the rock bodily over the tuffs.
The perfect parallelism of the bands with the surface of the tuff at
the east end, and the absence of all trace of a thrust-plane at the
base of the sill, seem to show that, though the rows of vesicles were
undoubtedly at first arranged parallel to the surfaces between which
the intrusion took place, the mass, before completely consolidating and
coming to rest, was ruptured, and a portion of it was driven onwards at
right angles to its previous line of movement.
A consideration of the singularly slag-like structure of the injected
masses in the tuffs and agglomerates leads to the conclusion that
though what we now see of these rocks did not actually flow out at
the sea-bottom in streams of lava, it was intruded so close to the
surface that the imprisoned vapours had opportunity to expand, as in
superficial outflows.[59] This inference is in accord with that derived
from an examination of the necks, wherein we find evidence of the
probable survival of parts of the actual craters and volcanic cones.
[Footnote 59: As illustrative of the occurrence of the vesicular
structure in superficial intrusions, I may again cite the dyke which
cuts the ash of the outer crater-wall of the Puy de Pariou in Auvergne.
The andesite of this dyke is in places as vesicular as the lava-stream
with which it was doubtless connected, but the vesicles have been
flattened and drawn out parallel to the walls of the dyke. In this
instance it is quite certain that there could never have been any great
depth of detrital material above the fissure into which the material of
the dyke was injected (see vol. i. p. 66).]
As the records of the earliest eruptions during the Carboniferous
Limestone period in the district of the Isle of Man are concealed,
so also those of the last of the series lie under the sea. Where the
highest visible tuffs overlie the Poyll Vaaish limestones they show
no change in the nature of the materials ejected, or in the energy of
eruption. They lie so abruptly on the dark calcareous deposits as to
show that a considerable pause in volcanic activity was followed by a
violent explosion. The same abundant grey-green pumice, the same kind
of loose crystals of felspar, the same type of lava-blocks and bombs
as had characterized the foregoing eruptions remained as marked at the
end. But the further volcanic records cannot be perused, and we are
left to speculate whether the coast-sections reveal almost the whole
chronicle, or if they merely lay before us the early chapters of a
great volcanic history of which the main records lie buried under the
waves of the Irish Sea.
4. EAST SOMERSET
Various limited outcrops of igneous rocks have long been known to
occur in the eastern part of Somerset. The largest of these lies in
the midst of the Old Red Sandstone, on the crest of the axis of the
Mendip Hills, between Downhead and Beacon Hill. Smaller patches occur
in the Carboniferous Limestone near Wrington Warren, on the north
side of Middle Hope, on Worle Hill and at Uphill. These rocks have
been mapped as intrusive, though some of them have been described as
conglomeratic or as volcanic breccias. While some of the masses are
probably intrusive, others appear to be truly contemporaneous with
the deposition of the Carboniferous Limestone. The highly vesicular
basalt of Middle Hope looks much more like a superficial lava than
an intrusion. Mr. Aveline gave a section showing three alternations
of limestone and "igneous rock" at Middle Hope. A recent examination
of that coast-line by Mr. A. Strahan shows that there are undoubted
tuffs interstratified with the calcareous strata. There is thus
proof that one or more small volcanic vents were in eruption on the
floor of the Carboniferous Limestone sea in the neighbourhood of
Weston-super-Mare.[60]
[Footnote 60: See _Geological Survey Memoir_ "On East Somerset," by H.
B. Woodward, 1876, and authorities there cited. Mr. Aveline's section
above referred to will be found on p. 22.]
5. DEVONSHIRE
The change from the typical Old Red Sandstone of South Wales
to the Devonian system of Devonshire, to which I have already
referred, is hardly more striking than the contrast between the
Carboniferous formations of these two areas.[61] The well-marked
threefold subdivisions of Carboniferous Limestone, Millstone Grit and
Coal-measures, so persistent throughout Britain, and nowhere more
typically developed than in South Wales, are replaced in a distance of
less than forty miles by the peculiar "Culm-measures" of Devonshire--a
series of black shales, grey sandstones and thin limestones and
lenticular seams of impure coal (culm), which are not only singularly
unlike in original characters to the ordinary Carboniferous formations,
but have been made still more unlike by the extensive and severe
cleavage to which the Palæozoic rocks of Devon and Cornwall have been
subjected. That these Culm-measures are truly Carboniferous is made
abundantly clear by their fossil contents, though it has not yet been
possible to determine how far they include representatives of the great
stratigraphical subdivisions in other parts of the country.
[Footnote 61: In the centre of England numerous outlying areas of
igneous rocks are found in the Carboniferous Limestone, Millstone Grit
and Coal-measures. These will be considered by themselves in Chap.
xxxii.]
It is to De la Beche that geology owes the first intimation of the
occurrence of interstratified igneous rocks in the Carboniferous
series of Devonshire. As far back as the year 1834, in his singularly
suggestive treatise, _Researches in Theoretical Geology_, this eminent
geologist expressed his opinion that not only were the "trappean" bands
regularly intercalated in the sedimentary series and continuously
traceable with the general stratification, but that they occurred at
various localities in such a manner as to raise the suspicion that
these points may mark some of the centres of eruption. He particularly
cited the example of Brent Tor as a remarkable volcanic-looking hill,
composed in part of a conglomerate "having every appearance of volcanic
cinders."[62]
[Footnote 62: _Op. Cit._ p. 384.]
In his subsequently published _Report on the Geology of Cornwall,
Devonshire and West Somerset_, De la Beche dwelt in more detail on
the results of his study of these rocks, which he had traced out on
the ground and expressed upon the maps of the Ordnance Geological
Survey.[63] Hardly any additions have since been made to our knowledge
of the field-relations of the rocks. It is to the maps and Report of
De la Beche that we must turn for nearly all the published information
on the subject. I shall therefore give here a summary of what can be
gathered from these publications.
[Footnote 63: Sheets 22, 23, 24, 25, 30, 31, 32 and 33.]
In tracing the limits of the Culm-measures, De la Beche found that
no well-defined line could be drawn between these strata and the
"grauwacke" or Devonian formations underneath. The Carboniferous
series lies in a great trough, of which the axis runs nearly east and
west, so that the lowest members of the series rise along the northern
and southern margins. But De la Beche was struck with one remarkable
contrast between the two opposite sides of the trough--a contrast
which marks the Devonian as well as the Carboniferous formations of
this region. On the south side an abundant and persistent group of
intercalated bands of igneous, or as he called them, "trappean,"
materials can be followed along the whole line of boundary, while
no such group occurs on the north side. He found these bands to be
lenticular, traceable sometimes for a number of miles, then dying out
and reappearing on the same or other horizons. He mapped them the
whole way from Boscastle on the west to near Exeter on the east, and
found that though the individual sheets might be short, the trappean
zone was continuous as far as the southern margin of the Carboniferous
series could be seen, except where it had been broken through by the
great granitic mass of Dartmoor. He ascertained that the intercalated
trappean rocks are not confined to the Culm-measures, but occur also in
the contiguous portions of the "grauwacke" or Devonian system.
But further, he clearly recognized that the bands of igneous material
which he mapped included both "greenstones," together with other
varieties of massive eruptive rocks, and also volcanic ash or tuff,
though he did not attempt to separate these out upon the maps, but
contented himself with representing them all under the same colour.
He admitted that some doubt might be entertained as to the age of the
greenstones, for some of them might be intrusive and therefore later
than the sedimentary deposits between which they lie. But he contended
that there could be no uncertainty with regard to the trappean ash
or tuff, which being regularly interstratified in the Carboniferous
series, must be contemporaneous with it. He pointed out that many of
the greenstones, as well as fragments in the conglomerates or ashes,
were highly vesicular and must originally have been in the condition of
pumice.
As an illustration of the centres of eruption from which these
materials were ejected, De la Beche drew special attention once
more to the conspicuous eminence of Brent Tor and the rocks in its
neighbourhood. His remarks on this subject are well worthy of being
quoted--"The idea that in the vicinity of Brent Tor a volcano has
been in action, producing effects similar to those produced by active
volcanoes, forcibly presents itself. That this volcano projected ashes,
which, falling into adjacent water, became interstratified with the
mud, silt and sand there depositing, seems probable. That greenstones
and other solid trappean rocks constituted the lavas of that period
and locality, here and there intermingled with the ash, appears also a
reasonable hypothesis. Upon the whole there seems as good evidence as
could be expected that to the north and north-west of Tavistock, ash,
cinders and liquid melted rocks were ejected and became intermingled
with mud, silt and sand during this ancient geological epoch,
corresponding with the phenomena exhibited in connection with volcanoes
of the present day, more particularly when they adjoin or are situated
in the sea, or other waters where ejected ashes, cinders and lava can
be intermingled with ordinary mud, silt and sand."[64]
[Footnote 64: _Op. cit._ p. 122.]
It remains for some future observer to fill up the outlines thus
sketched by De la Beche, by tracing the respective areas of lavas and
tuffs, distinguishing the various petrographical types, separating the
intrusive from the interstratified sheets, identifying the necks and
bosses that may mark centres of eruption, and expressing these various
details upon maps on a sufficiently large scale.
A serious difficulty in this research arises from the effect of the
profound alteration which has been produced on the igneous rocks by
the cleavage of the region. Many of the "greenstones" have been so
cleaved as to become slaty or almost schistose. De la Beche recognized
this change and wrote of the "schistose trappean ash." A result of this
metamorphism has been to impart to rocks originally massive the same
fissile structure as the adjacent slates possess; and in this condition
it is often hardly possible to distinguish between "greenstone" and
fine-grained "ash." There can indeed be little doubt that among these
Carboniferous volcanic rocks, as we have seen to be the case with those
of the Devonian system in the same region, many lavas or sills have
been mapped as tuffs.
The chief additions to our knowledge of the Carboniferous volcanic
group of Devonshire since the time of De la Beche have been made by
Mr. F. Rutley, Mr. W. A. Ussher and General M'Mahon. Mr. Rutley[65] has
endeavoured to trace the respective areas occupied by the different
varieties of volcanic rocks in the district around Brent Tor, near
Tavistock, and to show the probable connection of the successive bands
of lavas and tuffs with a central vent of discharge situated at that
hill. He believes that these bands occur on four different horizons in
the sedimentary series. He has studied the microscopic structure of
the rocks, which in his view include "amphibolites, gabbros, basalts,
pitchstones and schistose ashes, or clastic rocks of a doubtful
nature."[66]
[Footnote 65: "The Eruptive Rocks of Brent Tor and its Neighbourhood,"
_Mem. Geol. Surv._ 1878. "On the Schistose Volcanic Rocks occurring on
the west of Dartmoor, with some Notes on the Structure of the Brent Tor
Volcano," _Quart. Journ. Geol. Soc._ xxxvi. (1880), p. 286.]
[Footnote 66: "The Eruptive Rocks of Brent Tor," p. 45.]
Mr. Ussher has re-mapped the tract of Culm-measures on the east side
of the Dartmoor granite, besides visiting some of the other areas
outside of the granite mass. While confirming the general accuracy
of De la Beche's survey, he has been able to improve the mapping
by inserting more detail, separating especially the tuffs from
the "greenstones." The latter have been found by him to be mostly
dolerites, some of which, from their parallelism the bands of tuff,
may be in his opinion contemporaneous lavas, though the majority of
them are evidently intrusive. The tuffs are regularly interstratified
among the Culm-measures, their most important band in this district
having an average breadth of about 100 yards, and being traceable for
at least two miles, possibly considerably further.[67] In going over
this tract with Mr. Ussher I was led to regard many of the sheets of
diabase (dolerite) or gabbro as true sills and bosses. Most of them
occur as short lenticular or oval patches tolerably numerous, but not
traceable for more than a short distance, though a connection may
often exist which cannot be detected by the scanty evidence on the
surface. One sheet which has been followed by Mr. Ussher from Combe
to beyond Ashton, a distance of nearly two miles, presents in the
centre a somewhat coarsely crystalline texture which rapidly gives way
to a much closer grain, and the rock then becomes highly vesicular.
It is overlain with dark Culm-shales and bands of fine shaly tuff,
passing upward into a granular tuff. Some layers of this tuff assume
a finely foliated appearance by the development of pale leek-green
folia, which show slickensided surfaces parallel with the bedding. The
rock then presents one of the usual appearances of schalstein. This
structure seems obviously due to mechanical movement along the planes
of stratification.
[Footnote 67: "The British Culm-measures," _Proc. Somerset Archæol. and
Nat. His. Soc._ xxxviii. (1892), p. 161.]
Bands of black chert and cherty shale are interpolated among the tuffs,
which also contain here and there nodular lumps of similar black impure
earthy chert--an interesting association like that alluded to as
occurring in the Carboniferous volcanic series of the Isle of Man, and
like the occurrence of the radiolarian cherts with the Lower Silurian
volcanic series already described.[68]
[Footnote 68: Cherts containing numerous species of radiolaria have
recently been found by Dr. Hinde and Mr. Howard Fox to form an
important part of the Lower Culm-measures of Devonshire, _Quart. Journ.
Geol. Soc._ vol. li. (1895), p. 609.]
The volcanic belt in the valley of the Teign can be followed for
about two miles. It is undoubtedly interstratified among the dark
Culm-measures, which are distinctly seen dipping under and overlying it.
General M'Mahon has recently shown what may be done by careful and
detailed examination of the ground broadly sketched in by De la Beche.
He chose for study a strip of "greenstone" shown on the Geological
Survey Map to extend for about three and a half miles along the
north-west margin of the Dartmoor granite. He has found that what is
represented under one wash of colour on that map includes both tuffs
and lavas. The tuffs, in spite of the alteration which they appear
to have undergone from the proximity of the great granite mass, are
found by microscopic investigation to be made up of fine volcanic
dust containing minute lapilli of various lavas. Sometimes as many as
six or seven different kinds of lava may be represented in the same
microscopic slide. These include felsitic or rhyolitic and trachytic
rocks together with fragments of dark glassy lava full of magnetite
dust. With the tuffs are intercalated sheets of felsite and trachyte.
In the same district coarse volcanic agglomerate occur, made up of
blocks of different lavas and pieces of different sedimentary rocks.[69]
[Footnote 69: _Quart. Journ. Geol. Soc._ vol. l. (1894), p. 338.]
These observations are of special interest, inasmuch as they point to
the eruption of a much more acid series of volcanic lavas and tuffs
than had previously been known to exist in the Culm-measures. Until the
ground has been more accurately mapped, it is impossible to say whether
these rocks are older or younger than those that lie around Brent Tor,
a few miles to the south-west. General M'Mahon has noted the presence
of more basic eruptive rocks in the same district. He specially cites
the occurrence of mica-diorite, of basaltic lavas altered into a
serpentinous mass, and of a dolerite which may possibly mark the actual
vent of the old Brent Tor volcano. His observations on the influence
of the Dartmoor granite in inducing new mineral rearrangements in the
igneous rocks of the Culm-measure series are full of interest.
CHAPTER XXX
THE CARBONIFEROUS VOLCANOES OF IRELAND
King's County--The Limerick Basin--The Volcanic Breccias of
Doubtful Age in County Cork.
Although the Carboniferous system spreads over by far the larger
part of the surface of Ireland, and is laid bare in many thousands
of natural and artificial sections, it displays undoubtedly
contemporaneous igneous rocks, so far as at present known, at only one
locality--the region around Limerick. A second district, however, lies
in King's County, where some vents occur which may be of Carboniferous
age, and of which a description will be given in the following
pages. That the relics of volcanic action should be so few, while
the exposures of the Carboniferous formations are so numerous and so
completely disclose the geological history of the whole system, must be
regarded as good evidence that while volcanoes abounded and continued
long active in Scotland and in parts of the Centre and South-west
of England, they hardly appeared at all in Ireland. It is worthy of
remark, also, that the Irish eruptions belong to the time of the
Carboniferous Limestone--a period distinguished by volcanic activity in
Scotland and England--that the nature of the materials erupted bears a
close resemblance to that of the lavas and tuffs of the sister island,
and that the manner of their eruption finds a close counterpart in the
Puy-eruptions, already described.
1. KING'S COUNTY
In the progress of the Geological Survey several small tracts of
"greenstone ash" and "greenstone" were mapped within an area of a few
square miles lying to the north of Philipstown. These igneous rocks
were shown to form Croghan Hill, which, rising into a conical eminence
769 feet above the sea, and some 450 feet above the general level of
the great limestone plain around it, forms the only conspicuous feature
in the landscape for many miles. In the maps and their accompanying
Explanations, the "greenstones" are treated as intrusive masses, but
the "greenstone ash" or breccia appears to have been regarded as
interstratified in the Carboniferous Limestone, though the admission
is made that "from the scanty exposures of the rocks and the total
absence of any connected section, it has been found impossible to
arrive at any definite conclusion as to the relations existing between
these traps and ashes with regard to each other or to the surrounding
limestone."[70]
[Footnote 70: See Sheets 109 and 110 of the Geological Survey of Ireland
and Explanation to accompany Sheets 98, 99, 108 and 109, by F. J. Foote
and J. O'Kelly (1865), pp. 7-18.]
In the course of a brief visit to this locality I did not succeed in
obtaining any certain proof of the age of the igneous rocks, but I
found their structures to be more varied and interesting than would be
inferred from the way in which they have been mapped, and I came to
the conclusion that the strong balance of probability was in favour of
regarding them as of the age of the Carboniferous Limestone.
[Illustration: Fig. 192.--Croghan Hill, King's County, from S.S.W.]
The first and most important fact to be announced regarding the
district is that it includes a group of volcanic necks which rise
through the Carboniferous Limestones. The chief of these forms Croghan
Hill. It is nearly circular in ground-plan, and measures about 4000
feet in diameter from the limestone on one side to that on the other.
It rises with steep grassy slopes out of the plain, the naked rock
projecting here and there in crags and low cliffs. Its general outward
resemblance to the Carboniferous necks of Scotland strikes the eye of
the geologist as he approaches it (Fig. 192).
But Croghan Hill, though the chief, is not the only vent of the
district. It forms the centre round which a group of subsidiary vents
has been opened. These form smaller and lower eminences, the most
distant being one and a half miles E.S.E. from the summit of Croghan
Hill, and measuring approximately 1200 feet in its longest and 800 feet
in its shortest diameter.
That the igneous materials of these necks really break through the
limestones may be clearly seen in several sections. Thus by the
roadside at Gorteen, on the south-western side of Croghan Hill, the
limestones have been thrown into a highly inclined position, dipping
towards the east at 60° or more, and their truncated ends abut against
the side of the neck. Again, on the eastern side of the same hill the
limestones have been much disturbed close to the margin of the neck,
sometimes dipping towards the volcanic centre, and sometimes striking
at it. Among these strata a small neck of breccia, of which only a
few square yards are visible, rises close to the edge of the bog that
covers the adjacent part of the great plain.
The material which chiefly forms these necks is one of the most
remarkable breccias anywhere to be found in the volcanic records
of the British Isles. The first feature noticeable in it is the
pumiceous character of its component fragments. These consist of a
pale bluish-grey basic pumice, and are generally about the size of
a hazel-nut, but descend to mere microscopic dust, while sometimes
exceeding a foot in length. They are angular, subangular and rounded.
Occasionally they stand out as hollow shells on weathered surfaces, and
in one instance I noted that the vesicles were flattened and drawn out
parallel to the surfaces of the shell, as if deformed by gyration, like
a true bomb.
The breccia remains singularly uniform in character throughout all
the necks. Its basic pumice presents much resemblance to that so
characteristic of the Carboniferous necks of Scotland, Derbyshire and
the Isle of Man. The abundant vesicles are generally spherical, and as
they have been filled with calcite or chlorite, they look like small
seeds scattered through a grey paste. Though I broke hundreds of the
lapilli, I did not notice among them any volcanic rock other than this
pumice. I am not aware of any other neck so homogeneously filled up
with one type of pyroclastic material, and certainly there is no other
example known in the British Isles of so large and uniform a mass of
fragmentary pumice.
Limestone fragments are not uncommon in this breccia. They resemble
the strata around the vents. Pieces of the adjacent cherts may also be
observed. In one or two cases, the limestone fragments were found by
me to have an exceptionally crystalline texture, which may possibly
indicate a certain degree of marmarosis, but on the whole there is
little trace of alteration.
The fragments of pumice in the breccia are bound together by a cement
of calcite. In fact the rock is, so to speak, saturated with calcareous
material, which, besides filling up the interstices between the
lapilli, has permeated the pumice and filled up such of its vesicles as
are not occupied by some chloritic infiltration.
I did not observe unmistakable evidence that any part of the breccia is
stratified and intercalated among the limestones, nor any vestige of
ashy material in these limestones. But it is possible that traces of
such interstratification may occur in the low ground to the north-west
of Croghan Hill, which I did not examine.
In only two places did I notice even a semblance of the intercalation
of limestone in the breccia. One of these is at Gorteen, where a
band of limestone strata a few feet thick is underlain and overlain
by breccia. But though the superposition of the layers of finely
stratified dark limestone and chert on the breccia is well seen and
thoroughly defined, no lapilli or ashy material are to be seen in
the limestone. Detached pieces of similar limestone and chert occur
in the breccia. The band of stratified rock, if _in situ_, may be a
tongue projecting from the wall into the body of the neck, like some
instances already cited from Scotland, but more probably it is really a
large included mass lying within the vent itself. The breccia here as
elsewhere is entirely without any trace of stratification. The second
locality occurs at the most easterly neck north of Coole House, where
the limestones, rapidly undulating, seem at last to plunge below the
breccia, which shows a series of parallel divisional planes suggestive
of bedding. But these may be only joint-structures, for there is no
stratification of the component materials of the rock.
In the necks, and also through the limestone surrounding them, masses
of eruptive rock have been intruded as irregular bosses and veins.
The material of these intrusions presents little variety, and, so far
as I could note, gives no indication of the successive protrusion
of progressively different lava. It varies from a deep blue-black
fine-grained basalt to a dolerite where the plagioclase is distinct.
Some portions, however, are more basic and pass into limburgite.
Externally there is nothing worthy of special remark in these rocks
unless it be their prevalent amygdaloidal structure. The amygdales,
generally of calcite, vary from small pea-like forms in the basalts
up to kernels half an inch long or more in the dolerites. From a
microscopic examination Mr. Watts found that some of the basalts
have a base of felspar and augite rich in brown mica, and that their
porphyritic felspars enclose idiomorphic crystals of augite.
Perhaps the most noticeable feature in these later parts of the
volcanic series is the occurrence in them at one locality in Croghan
Demesne of lumps of a highly crystalline material quite distinct from
the surrounding rock. These enclosures vary from an inch or two to a
foot or more in diameter. They must be regarded as blocks which have
been carried up in the ascent of the basic lava. Their composition
has been ascertained by Mr. Watts from microscopic examination to
be somewhat singular. One specimen "contains relics of garnets,
surrounded by rings of kelyphite, imbedded in a mosaic of felspar,
with a mineral which may possibly be idocrase." Another specimen from
the same locality (south-east from Gorteen) "contains the relics of
garnets preserved as kelyphite, set in a matrix of quartz-grains, much
strained, and containing a profusion of crystals of greenish-yellow or
red sillimanite. This appears to be a metamorphic rock, and may be a
fragment of some sediment enclosed in the igneous rocks."[71]
[Footnote 71: _Guide to the Collections of Rocks and Fossils belonging
to the Geological Survey, in the Museum of Science and Art, Dublin_
(1895), pp. 38, 39.]
As regards the history of volcanic action in Britain one of the
chief points of interest connected with these Irish breccias and
lavas relates to their geological age. As no proof has been produced
that any portion of them is contemporaneously interstratified in the
Carboniferous Limestone which surrounds them, we cannot definitely
affirm that the volcanic eruptions which they record took place during
the accumulation of that formation. The vents must, of course, be
later than that portion of the limestone which they pierce. But the
evidence seems to me to be on the whole most favourable to the view
that they are of Carboniferous Limestone age, for the following
reasons:--
1. The breccias of Croghan Hill do not present a resemblance to any of
those belonging to the Tertiary volcanic series in Antrim or the Inner
Hebrides. The possibility of their being of Tertiary age may therefore
be dismissed from consideration.
2. There are no known Permian volcanic rocks in Ireland. Nor does the
Croghan Hill breccia show any resemblance to the ordinary material of
the breccias in the Permian necks of Scotland. It is thus not likely to
be of Permian age.
3. The peculiar basic pumice of these Croghan Hill vents has many
points in common with the palagonite fragments so abundant among
the volcanic breccias and tuffs of Carboniferous age in Scotland,
Derbyshire, and the Isle of Man, and which occurs also among the
Carboniferous tuffs of the Limerick basin. It differs from the
general type of the material in its pale colour, in its uniformity
of character, in its calcareous cement, and above all in its vast
preponderance over all the other materials in the breccia.
4. The saturation of the Croghan Hill breccia with calcite is a
singular feature in the composition of the rock. Had the vents
been opened long subsequent to the deposition of the Carboniferous
Limestone, it is difficult to understand how this calcite could
have been introduced. Mere percolation of meteoric water from the
adjacent limestone does not seem adequate to account for the scale
and thoroughness of the permeation. But if the vents were opened on
the floor of the Carboniferous Limestone sea, it is intelligible that
much fine calcareous silt should have found its way down among the
interstices of the breccia and into the pores of the pumice which,
being caked together within the vent, did not all float away when the
sea gained access to the volcanic funnel. The effect of subsequent
percolation would doubtless be to carry the lime into still unfilled
crevices, and to impart to the cement a crystalline structure similar
to that which has been developed in the ordinary limestones.
2. THE LIMERICK BASIN
About 70 miles to the south-west of the area just described lies the
most compact, and, for its size, one of the most varied and complete,
of all the Carboniferous volcanic districts of Britain (Map I.). It
takes the form of an oval basin in the Carboniferous Limestone series
near the town of Limerick, about twelve miles long from east to west
and six miles broad from north to south. Round this basin the volcanic
rocks extend as a rim about a mile broad. A portion of a second or
inner rim, marking a second and higher volcanic group, partially
encloses a patch of Millstone Grit or Coal-measures, which lies in the
heart of the limestone basin. (See the section in Fig. 196.)
But it is evident that, as the denuded edges of the volcanic sheets
emerge at the surface all round the basin, the present area over
which these rocks extend must be considerably less than that which
they originally covered. Some indication of their greater extension
is supplied by outliers of the bedded lavas and tuffs, as well as by
bosses which doubtless indicate the position of some of the eruptive
vents. The distance between the furthest remaining patches is 24 miles.
The original tract over which the volcanic materials were spread cannot
have been less than 24 miles long by 10 miles broad. If we assume its
area to have been between 250 and 300 square miles we shall probably be
under the truth.
This volcanic centre made its appearance on the floor of the
Carboniferous Sea in the same district which had witnessed the
eruptions of Upper Old Red Sandstone time. The two visible vents that
crown the Knockfeerina and Ballinleeny anticlines (Chapter xxii.), are
only some ten miles distant, and there may be others of the same age
even under the Limerick basin. This district thus supplies another
instance of that recurrence of volcanic energy in the same area,
after a longer or shorter geological interval, which stands out as a
conspicuous feature in the history of volcanic action in Britain. That
a prolonged interval elapsed between the extinction of the Old Red
Sandstone volcanoes and the outbreak of their successors during the
accumulation of the Carboniferous limestone series, may be inferred
from the thickness of strata which separate their respective tuffs.
From the published sections of the Geological Survey there would appear
to be about 500 feet of Old Red Sandstone above the volcanic series of
that formation. Then comes the Lower Limestone shale, which is computed
to be about the same thickness. From the scarcity of observable dip
among the Lower Limestones and their variable inclination, it is not
easy to form any satisfactory estimate of the depth of this group up
to the base of the volcanic series. It may be as much as 800 feet,[72]
and if so there would thus intervene a mass of sedimentary material
nearly 2000 feet in thickness between the two volcanic platforms.
Throughout this thick accumulation of stratified deposits no trace
of contemporaneous volcanic activity has been detected. From the
descriptions published more than thirty years ago by Jukes and his
colleagues in the Geological Survey of Ireland, geologists learnt
how full and interesting are the proofs of great volcanic activity
contemporaneous with the deposition of the Carboniferous Limestone
series in the Limerick district.[73] Nowhere, indeed, is the evidence
more complete for the occurrence of a long succession of volcanic
eruptions during a definite period of geological time. The officers of
the Survey showed that two epochs of activity during the older part
of the Carboniferous period were each marked by a group of tuffs and
lavas, while the interval of quiescence between them is represented
by a thousand feet of limestone. The same observers likewise mapped
outside the volcanic ring a number of eruptive bosses, which they
regarded as probably marking some of the actual vents of that time.
[Footnote 72: This is the thickness given in the Explanation to Sheet
144 of the Geological Survey of Ireland, p. 8. A still greater
thickness is claimed in Explanation to Sheet 154, p. 8.]
[Footnote 73: See especially Explanations of Sheets 143, 144, 153 and
154, Geol. Surv. Ireland (1860, 1861). The geology of the district
had been previously noticed by earlier observers, to whose writings
reference is made on p. 26 of the Explanation of Sheet 144. See also
Jas. Apjohn, _Journ. Geol. Soc. Dublin_, vol. i. (1832), p. 24;
Prof. Hull, _Geol. Mag. for 1874_, p. 205. Jukes (_Student's Manual
of Geology_, 2nd edit. 1862, p. 325) gave subsequently an excellent
epitome of the volcanic history. The microscopic structure of some of
the Limerick volcanic rocks has been described by Mr. Allport, _Quart.
Journ. Geol. Soc._ vol. xxx. (1874), p. 552, and by Prof. Hull, _Geol.
Mag. for 1873_, p. 153. See also Mr. Watts' account of these rocks in
the _Guide to the Collections of Rocks and Fossils_ (Dublin, 1895), p.
93.]
The lower volcanic group, which forms a complete ring round the Upper
Limestones of the Limerick basin, is estimated to reach a thickness of
1000 feet in some parts of its course.[74] Its base appears to coincide
generally with the upward termination of the Lower Limestone group of
this district, though here and there small patches of volcanic rocks in
that group have been regarded as interstratified and contemporaneous
bands.[75] It consists of a series of lavas and tuffs, the alternations
and rapid incoming and dying out of which were well made out by the
Geological Survey.
[Footnote 74: Explanation of Sheet 144, p. 27.]
[Footnote 75: Some of them, however, have characters that rather seem to
place them with the intrusive materials of the district, and therefore
not necessarily earlier than the bedded lavas and tuffs. The boundary
line of the volcanic series is not consistently followed along the
same horizon on the Survey maps. Thus to the east of Caherconlish, a
strip of the Upper Limestone is inserted below the base of the tuffs
for a distance of about four miles. Unless a different horizon has
been in some places taken for the boundary between the two groups of
limestones, it would appear that the eruptions had not extended over
the north and north-east of the district until some time after the
deposition of the Upper Limestone had begun. The division between the
two limestone groups is taken at a set of chert-bands, but as these are
not constant it is sometimes difficult to draw a satisfactory line of
division.]
_Tuffs._--The base of the volcanic series is generally formed by a band
of tuff sometimes as much as 350 feet thick,[76] which may be traced
nearly continuously round the basin as well as in detached outliers
even as far as Carrigogunnel overlooking the alluvial plain of the
Shannon. The manner in which the bottom of this tuff is interstratified
with the limestone below it may be instructively examined in many
quarries around the town of Limerick. Striking evidence is there
supplied that the first eruptions were comparatively feeble and
spasmodic, and were separated by intervals of longer and shorter
duration, during which the limestone with its fragmentary organisms
was deposited, little or no volcanic detritus falling at that time.
Yet even in some of the limestones the microscope reveals fine broken
needles of felspar, representing doubtless the finest ejected dust.[77]
[Footnote 76: Explanation of Sheet 154, p. 21.]
[Footnote 77: For the details of the microscopic structure of the
Limerick volcanic rocks I am mainly indebted to the examination of them
made for me by my Survey colleague, Mr. W. W. Watts.]
As an illustration of the way in which the volcanic and organic
detritus alternated over the sea-floor, the following section from a
quarry in the townland of Loch Gur on the southern side of the basin is
here given:[78]--
[Footnote 78: Explanation of Sheet 154, pp. 21, 22.]
Cherty limestone more than 20 feet 0 in.
Decomposed green tuff 2 " 6 "
Bluish-green, calcareous laminated tuff 4 " 0 "
Limestone, slightly ashy 1 " 8 "
Green tuff 0 " 2 "
Fine-grained decomposed tuff 0 " 4 "
Green tuff, obliquely laminated 1 " 7 "
Fine laminated tuff 0 " 8 "
Green compact tuff 1 " 8 "
Obliquely laminated shaly tuff 0 " 10 "
Concretionary ashy limestone 1 " 4 "
Compact ashy limestone 2 " 0 "
Green shaly tuff, much weathered 0 " 5 "
Ashy limestone 0 " 7 "
Compact green tuff more than 4 " 0 "
--------------
41 feet 9 in.
The tuffs which in the southern part of the basin underlie the less
basic lavas differ in some respects from those which further north
are associated with the Upper Limestones. They are green, sometimes
dull purplish-red, finely granular rocks, made up in large part of
andesitic debris. They are full of loose felspar crystals, minute,
somewhat rounded and subangular lapilli of andesite or some less basic
lava, together with bits of grit and baked shale. Though generally
much decomposed, they are sometimes compact enough to be used for
building-stone. Under the microscope these tuffs are seen to abound in
andesite-lapilli, with a few pieces of felsitic rocks enclosed in an
opaque base, through which are scattered broken felspars and occasional
vesicular lapilli.
[Illustration: Fig. 193.--Section in quarry on roadside east of
Limerick close to viaduct of the Limerick and Erris Railway.
1. Limestone; 2. Calcareous tuff; 3. Ashy limestone or calcareous tuff.]
The tuffs around Limerick, interbedded with the Black (Upper)
Limestone, are distinguished by a scarcity of andesitic debris, by
their persistent dull greenish-grey colour, and more particularly
by the abundance of minute lapilli and larger fragments of an
epidote-green, finely vesicular, easily sectile basic pumice. Under
the microscope much of this material is found to be an altered basic
glass of the nature of palagonite. These tuffs are in evident relation
with the more basic lavas that accompany them. The manner in which they
alternate with the black limestone shows that the conditions for the
eruption of this more basic detritus continued to be very similar to
those that existed when the andesitic tuffs were ejected. As a good
illustration of this feature the accompanying section (Fig. 193) is
given from a quarry on the side of the high-road between Limerick and
Annacotty. The total depth of strata here represented is about 15 feet.
The black limestone at the bottom is a tolerably pure calcareous rock.
It is divided into bands by thin partings of a fine greenish calcareous
tuff, each marking a brief discharge of ashes from some neighbouring
vent. Half-way up the succession of strata, the ashy material rapidly
increases until it usurps the place of the limestone, though its
calcareous composition shows that the accumulation of calcareous
sediment had not been entirely suspended during the eruption of ash.
Among these tuffs I have noticed fragments of fine, dark, flinty
felsite, grit and other rocks. The stones are for the most part small,
but vary up to blocks occasionally a foot in diameter.
_Lavas._--The lavas occur in numerous sheets, sometimes separated by
thin partings or thicker beds of tuff and volcanic conglomerate. On the
northern rim of the basin Mr. G. H. Kinahan has described the volcanic
series east of Shehan's Cross-roads as composed of six zones of tuff,
each bed varying from about 50 to 250 feet in thickness, alternating
with as many sheets of lava ranging from 27 to 180 feet in thickness,
the total depth of tuff being estimated at nearly 500 feet and that
of the lavas at about 800 feet.[79] Some of these tuffs are coarse
conglomerates or agglomerates, with blocks of lava occasionally 10 feet
long.
[Footnote 79: Explanation of Sheet 144, p. 28.]
Some of the lavas in the lower volcanic group are andesites quite like
those of the plateau series in the Carboniferous system of Scotland.
Externally they appear as dull reddish-brown or purplish-red compact
rocks, with abundant porphyritic felspars scattered through the
fine-grained base. They are generally much decomposed, showing on a
fresh fracture pseudomorphs of chlorite, hæmatite and calcite after
some of the minerals, with abundant hæmatitic staining through the body
of the rock. Amygdaloidal structure is commonly developed.
These andesites, when examined microscopically, were found by Mr.
Watts to present the characteristic base of minute felspar-laths with
magnetite and enstatite, and with porphyritic crystals, often large, of
zoned plagioclase, as well as of ilmenite and hæmatite.
[Illustration: Fig. 194.--Section of the volcanic escarpment, east of
Shehan's Cross-roads, south of Limerick.
1. Limestone; 2 2. Tuffs; 3 3. Lavas.]
But besides the andesites there occur also, and, so far as I have
observed, in larger number, sheets of true basalt. This rock is
typically black, exceedingly close-grained in the central portion of
each sheet, but becoming highly slaggy and vesicular along the upper
and lower parts. Under the microscope it is found to contain granular
augite and magnetite, set in a more or less devitrified glass, with
microlites of felspar, porphyritic plagioclase, serpentinized olivine,
and some well-marked augite. These rocks form distinct escarpments
along the northern rim of the basin as in the foregoing section east
from Shehan's Cross-roads (Fig. 194). From the summit of this ridge,
which is about 600 feet above the sea, the eye looks northward over the
plain, across which low outliers of the volcanic series are scattered,
and southwards across the basin to the corresponding line of volcanic
heights forming the southern rim.
The upper volcanic group has been estimated by the officers of the
Geological Survey to lie about 1000 feet higher in the Carboniferous
system than the lower, the intervening strata consisting of the Upper
Limestone.[80] It is possible that the interval is greater in some parts
of the district than in others, and if so, the difference may be due
either to greater local accumulation of volcanic materials, or to local
prolongation of the eruptions into higher stratigraphical horizons. The
outcrop of the upper volcanic band forms about half of a ring round the
little cup of Millstone Grit or Coal-measures which lies within the
volcanic basin. On the north-west side of the cup the volcanic rocks
disappear. Hence the upper band has a much more restricted area than
the lower. But if the tuffs immediately around Limerick are assigned
to the upper group, its extent will be proportionately increased.
There can be little doubt, however, that neither in thickness nor in
superficial area did the lavas and tuffs of the second group equal
those of the first. The volcanic energy was gradually dying out.
[Footnote 80: Explanation of Sheet 154, p. 24.]
The lavas of the second period are characteristic dull, black, compact
basalts, like those of the first period, becoming here and there
strongly amygdaloidal, and being occasionally separated by slaggy or
conglomeratic partings. But they include also certain rocks wherein
the felspar diminishes in quantity, while augite and olivine become
conspicuous, together with a little enstatite. The augite occurs in
large porphyritic forms, as well as of medium size and in small prisms.
The olivine, as usual, is now in the condition of serpentine. These
rocks are more basic than the ordinary basalts, containing only 38·66
per cent of silica, and thus approaching the limburgites. With these
basic lavas are associated dull green tuffs and conglomerates, made
up largely of basalt-debris, together with abundant pieces of finely
vesicular basic pumice and lapilli of a palagonitic material.
The manner in which the lavas and tuffs have alternated with each
other, and also with the limestones, is well seen on Nicker Hill
above Pallas Grean.[81] The Survey sections show eight sheets of lava,
separated by six bands of tuff and eight intercalations of limestone,
the whole passing under the Coal-measures.
[Footnote 81: See Explanation of Sheet 144, p. 30, where a description
with detailed map and sections of this ground will be found.]
The upper volcanic group may be as much as 600 or 800 feet thick. It
appears to have been left, at the close of the eruptions, with a very
uneven surface, some portions being so low as to be overspread with the
Upper Limestones, other parts so high as not to be covered until the
Coal-measure shales and flagstones came to be deposited.[82]
[Footnote 82: Explanation of Sheet 154, pp. 24, 35.]
_Vents._--All round the edges of the Limerick basin, where the
escarpments of the volcanic groups, rising abruptly above the plain,
show that these rocks once extended beyond their present limits, the
progress of denudation has revealed a number of bosses which, as above
stated, Jukes and his associates looked upon as marking some of the
vents from which the lavas and tuffs were erupted. Especially striking
is the line of these vents along the southern margin. The rocks now
filling them present some unusual and rather anomalous features. They
are decidedly more acid than the lavas of the basin, some of them even
containing free quartz. Mr. Watts remarks that "though they have a good
deal in common with the trachytes, they are crystalline throughout.
They are red granite-looking rocks, which are made up chiefly of
stumpy idiomorphic prisms of felspar which is mainly orthoclase.
Some plagioclase also occurs, and the two felspars are imbedded in
interstitial quartz. A trace of hornblende or mica is frequently
present, and the rocks contain about 65 per cent of silica." These
characters are specially observable in the necks furthest removed from
the basin, which may possibly have been connected with the andesitic
outflows. Nearer to the basin the necks "contain about 60 per cent
of silica, seldom show any interstitial quartz, and stand between
trachytes and porphyrites, some perhaps being bostonites."[83]
[Footnote 83: _Guide to the Collections of Rocks, etc., Geol. Survey,
Ireland_, p. 93, Dublin 1895.]
[Illustration: Fig. 195.--View of Derk Hill, a volcanic neck on the
south side of the Limerick basin.]
A geologist, familiar with the Carboniferous and Permian necks of
Scotland, has no hesitation in confirming the surmise of Jukes and
his colleagues that the cones and domes around the Limerick basin
mark the sites of eruptive vents. On the south side of the basin, at
least nine such necks rise into view, partly from among the lavas and
tuffs, but chiefly through the limestones that emerge from below
these volcanic sheets. One of the most conspicuous of them, Derk Hill
(Fig. 195), rises to a height of 781 feet above the sea, and comes
through the bedded andesites, as represented in Fig. 196, which gives,
in diagrammatic form, the general structure of the Limerick volcanic
basin. Around the northern side of the basin a smaller number of necks
has been observed, consisting of similar acid rocks.
[Illustration: Fig. 196.--Section across the Limerick volcanic basin.
1. Lower limestone; 2. Lower series of lavas and tuffs; 3. Middle and
Upper Limestone; 4. Upper series of lavas and tuffs; 5 5. Two volcanic
necks; 6. Millstone Grit series.]
A few of the necks appear to be filled with volcanic agglomerate. Here
and there detached patches of fragmental volcanic material have been
shown on the Survey maps, and referred to in the Explanations, as
if they were outliers of the bedded tuffs; though in some cases the
coarseness of their materials and the want of any distinct bedding,
together with the absence of any indication of their relation to the
nearest limestones, have evidently offered considerable difficulty in
their mapping. One of the best examples occurs about two miles to the
south-east of the village of Oola. The boundaries of this patch, as put
on the map, are confessed to be "entirely speculative." It was only
seen on the side of the railway where it appeared as "a very coarse
brecciated purple ash."[84]
[Footnote 84: Explanation of Sheet 154, p. 25.]
On comparing the maps of the Limerick basin with those of the
Carboniferous districts of Scotland, the main difference will probably
be acknowledged to be the absence of any recognizable sills in the
Irish ground. That no sills actually occur, I am not prepared to
affirm. Indeed some of the more acid rocks, both outside the basin and
among the rocks of the older volcanic group, appeared to me during
my traverses of the ground to have much of the character of sills.
A more critical examination of the area would not improbably detect
some truly intrusive sheets which have hitherto been mapped among the
interstratified lavas. Some appear to exist among the surrounding Lower
Limestones.
An intrusive mass, like a sill or dyke, is represented on the
Geological Survey Map as traversing the Coal-measures in the inner
basin south of Ballybrood. But as the strata are on end along its
southern margin, it may possibly be only a portion of the upper
volcanic series which has been thrown into its present position by one
or more faults.[85]
[Footnote 85: Sheet 154 and Explanation to the same, p. 24.]
3. THE VOLCANIC BRECCIAS OF DOUBTFUL AGE IN COUNTY CORK
In the south-western headlands of Ireland, from Bear Island to
Dursey Island, various igneous rocks have been traced on the maps
of the Geological Survey. They have been described as consisting of
"greenstone," "felstone," and "ash" or "breccia," and as including
both interstratified and intrusive masses.[86] If contemporaneous with
the strata in which they occur, they would prove the existence of a
group of volcanic rocks in the Carboniferous slate, or lowest division
of the Carboniferous system. After an examination of the coast-line
I came to the conclusion that while there is undoubtedly evidence of
former volcanic activity in this part of Ireland, no proof has been
obtained that the eruptions occurred in the Carboniferous period. The
felsites and dolerites appeared to me to be all intrusive, the former
having certainly been injected before the terrestrial movements that
have disturbed the rocks, for some of them share very markedly in the
cleavage of the region. The dolerites and diabases, on the other hand,
so far as I observed, are not cleaved, and are thus probably of later
date.
[Footnote 86: See Sheets 197 and 198 of the Geological Survey of
Ireland, and the Explanation of these Sheets by Messrs. Jukes, Kinahan,
Wilson, and O'Kelly, 1860.]
The most interesting rocks are undoubtedly the "ash" and "breccia," for
they are obviously of volcanic as distinguished from plutonic origin.
On the coast north of White Bull Head, a bed of volcanic breccia may be
seen made up of rounded and angular fragments of different sandstones,
shales and limestones, with pieces of felsite and andesite wrapped up
in a dull-grey fine-grained sandy felspathic matrix. The rock weathers
with a rough or rugged surface, owing to the dropping out of the more
decomposable stones. This bed, about five feet thick, runs with the
bedding of the strata around it, and like these dips S.S.W. at an
angle of 70°. If no other evidence were obtainable, this breccia would
be naturally set down as a truly interstratified deposit of volcanic
detritus. A short distance from it, a second, rather thicker band of
similar material occurs, specially distinguished by its abundant worn
crystals of hornblende, sometimes three inches in diameter, as well as
large crystals of muscovite. These minerals are not unknown elsewhere
in volcanic agglomerates. The occurrence of lumps of augite in the
vents of Upper Old Red Sandstone age in Caithness has been already
alluded to, and a still larger series of ejected minerals will be
shown in a later chapter to characterize the younger necks of Central
Scotland.
In parts of its course, this second band appears to run so perfectly
parallel with the bedding of the strata between which it lies that the
observer would readily believe it to be a part of the same series of
deposits, and might therefore regard it as affording good evidence of
volcanic action contemporaneous with the formation of these deposits.
A transverse section of the bed, where thus apparently conformable, is
shown in Fig. 197.
[Illustration: Fig. 197.--Section of a bed of Volcanic Breccia in the
Carboniferous Slate; White Bull Head, County Cork.
1 1. Sandstones and shales; 2. Breccia.]
[Illustration: Fig. 198.--Volcanic Breccia invading and enclosing
Carboniferous Slate, White Bull Head.]
Further examination, however,reveals that this seemingly regular
sequence is entirely deceptive. At various points the breccia abruptly
truncates the sandstones, and involves large pieces of them, as shown
in Fig. 198 A. At other places, the lower side of the breccia, or what
would be its base if it were a regular bed, cuts out the strata and
sends veins into them (B). And the same structure is visible, on its
upper side, or what would be its top (C).
It is clear that these highly-inclined bands of breccia are not
contemporaneous with the deposition of the Carboniferous Slate, but
have been introduced into their position at some time subsequent
not only to the deposition, but to the disturbance and elevation of
the strata. The peninsula of White Bull Head is crossed by several
other similar bands. On Black Bull Head, also, together with abundant
felsitic and doleritic intrusions, a similar breccia or agglomerate
is to be seen. In some parts it is compact in texture with spheroidal
flinty lumps, and weathers somewhat like a nodular felsite. This
variety ends off rather abruptly to the north, but swells out
southward, and then runs out into a high, narrow headland, in which
it contains asbestos, as well as rounded crystals of hornblende. It
has here disrupted the shales and sandstones, and near the junction
is largely composed of fragments of them, the strata themselves being
jumbled, bent, and broken up.
The only semblance of a neck-like mass of this volcanic fragmental
material occurs on White Bull Head, where one of the bands expands
about the centre of the ridge, and is there full of large blocks of
grey sandstone. The breccia appears to have filled fissures which have
been opened between the bedding planes of the highly tilted strata,
giving rise to long narrow dyke-like intercalations. We have seen that
among the Carboniferous volcanic phenomena such dyke-like masses of
agglomerate occasionally present themselves in the vents both of the
plateaux and the puys.
In one or two places I noticed what may be traces of cleavage in
the breccia. The rock is not one that would yield easily to the
rearrangements required for the production of this structure, and the
doubtful cleavage may be deceptive. If we are justified in regarding
the introduction of this volcanic material as having necessarily taken
place after the tilting of the strata, we may not unreasonably infer
further that the eruptions could only have been effected at no great
distance from the surface. But the Carboniferous Slate in which these
agglomerates lie is the lowest member of the Carboniferous system. As
there is no known unconformability throughout this system in the south
of Ireland, the whole of the rest of the pile of Carboniferous strata,
amounting to a depth of several thousand feet, once probably extended
over this region. It must, therefore, have been not only after the
plication, but after extensive denudation of the formations that the
fissures were filled with agglomerate. These geological changes no
doubt occupied a vast period of time. While, therefore, no positive
evidence has yet been gathered to fix the age of these volcanic
eruptions of the south-west of Ireland, it is tolerably clear that they
cannot be assigned to the Carboniferous period, but must belong to some
later volcanic epoch. They may be of Permian age, perhaps even as late
as the Tertiary volcanic series.
BOOK VII
THE PERMIAN VOLCANOES
CHAPTER XXXI
THE PERMIAN VOLCANOES OF SCOTLAND
Geographical Changes at the Close of the Carboniferous
Period--Land- and Inland-Seas of Permian time--General
Characteristics and Nature of the Materials erupted--Structure of
the several Volcanic Districts: 1. Ayrshire, Nithsdale, Annandale;
2. Basin of the Firth of Forth.
The close of the Carboniferous portion of the geological record in
Britain is marked by another of those great gaps which so seriously
affect the continuity of geological history. No transitional formation,
such as in other countries marks the gradation from the Carboniferous
into the succeeding period, has been definitely recognized in this
country. The highest Carboniferous strata are here separated from all
younger deposits by an unconformability, indicating the lapse of vast
periods of time whereof, within the British area, no chronicle has been
preserved.
When we pass from the Carboniferous system to that which comes next
to it in order of time, we soon become sensible that great changes in
geography, betokening an immense interval, took place between them. The
prolonged subsidence during which the Coal-measures were accumulated,
not only carried down below sea-level all the tracts over which the
Carboniferous system was deposited, but possibly submerged the last
of the islets, which, like those of Charnwood Forest, had survived so
many geological changes. Eventually, however, and after what may have
been a vast period of quiescence, underground movements began anew, and
the tracts of Coal-measures were unequally ridged up into land. The
topography thus produced appears to have resulted in the formation of
a series of inland seas somewhat like those of the Old Red Sandstone,
but probably less in area and in depth. In these basins the water seems
to have been on the whole unfavourable to life, for the red sand and
mud deposited in them are generally unfossiliferous, though, when
the conditions became more suitable, calcareous or dolomitic sediment
accumulated on the bottom, to form what is now known as the "Magnesian
Limestone," and muddy sediment was deposited which is now the "Marl
Slate." In these less ferruginous strata, betokening a less noxious
condition of water, various marine organisms are met with.[87]
[Footnote 87: In some recent borings around Hartlepool the Magnesian
Limestone has been found to be interstratified with thick bands of
gypsum and anhydrite, and to be overlain by more than 250 feet of the
latter substance. Nothing could show more forcibly the exceedingly
saline and insalubrious character of the Permian lakes or inland seas.]
The vegetation of the land surrounding these basins was still
essentially Palæozoic in character. It presented a general resemblance
to that of Carboniferous time, but with some notable differences. The
jungles of _Sigillaria_ seem to have disappeared, while on the other
hand, conifers increased in number and variety. The sediments of the
water-basins have handed down only a scanty remnant of the animal
life of the time. Along the sandy shores walked various amphibians
which have left their footprints on the sand. A few genera of ganoid
fishes have been found in some of the shales, and a comparatively
poor assemblage of crinoids and molluscs has been obtained from the
Magnesian Limestone. To the geological period distinguished by these
geographical and biological characters the name of Permian is assigned.
In his survey of the progress of volcanic history in the area of
Britain, the geologist finds that the long period of quiescence
indicated by the deposition of the Coal-measures, and probably also
by the unconformability between the Coal-measures and the Permian
formations, was at length terminated by a renewed volcanic outbreak,
but on a singularly diminished scale and for a comparatively brief
period of time. Whether, had the Permo-Carboniferous strata which
connect the Coal-measures with the Permian formations on the Continent
been found in this country, they would have filled up the gap in the
geological record, and would have supplied any trace of contemporaneous
volcanic action, cannot even be surmised. All that we know is that,
after a vast interval, and during the deposition of the breccias and
red sandstones which unconformably overlie the Coal-measures, a few
scattered groups of little volcanoes appeared in the area of the
British Isles.
It is unfortunate that in those districts where these volcanic
relics have been preserved, the stratigraphical record is singularly
imperfect, and that on the eastern side of England, where this record
is tolerably complete, there are no intercalated volcanic rocks. The
latter occur in tracts where the strata are almost wholly destitute of
fossils, and where therefore no palæontological evidence is available
definitely to fix the geological age of the eruptions. Nevertheless
there is usually ample proof that the strata in question are much
later than the Coal-measures, while their geological position and
lithological characters link them with the undoubted Permian series of
the north-east of England. They may, however, belong to a comparatively
late part of the Permian period, if indeed some of them may not be
referable to the succeeding or Triassic period.
The comparatively feeble and short-lived volcanoes now to be described
are found in two regions wide apart from each other. The more important
of these lies in the south-west and centre of Scotland. A second
group rose in Devonshire. It is possible that a third group appeared
between these two regions, somewhere in the midlands. The evidence for
the history of each area will be given in a separate section in the
following pages.
i. GENERAL CHARACTERISTICS--NATURE OF MATERIALS ERUPTED
The chief district for the display of volcanic eruptions that may be
assigned to the Permian period lies in the centre of Ayrshire and the
valleys of the Nith and Annan. But, for reasons stated below, I shall
include within the same volcanic province a large part of the eastern
half of the basin of the Firth of Forth (see Map V.).
Unfortunately the interesting volcanic rocks now to be considered
have suffered severely from the effects of denudation. They have been
entirely removed from wide tracts over which they almost certainly
once extended. But this enormous waste has not been wholly without
compensations. The lavas and tuffs ejected at the surface, and once
widely spread over it, during the deposition of the red sandstones,
have been reduced to merely a few detached fragments. But, on the other
hand, their removal as a superficial covering has revealed the vents of
discharge to an extent unequalled in any older geological system, even
among the puys of the Carboniferous period. The Permian rocks, escaping
the effects of those great earth-movements which dislocated, plicated
and buried the older Palæozoic systems of deposits, still remain for
the most part approximately horizontal or only gently inclined. They
have thus been more liable to complete removal from wide tracts of
country than older formations which have been protected by having large
portions of their mass carried down by extensive faults and synclinal
folds, and by being buried under later sedimentary accumulations. We
ought not, therefore, to judge of the extent of the volcanic discharges
during Permian time merely from the small patches of lava and tuff
which have survived in one or two districts, but rather from the
number, size and distribution of the vents which the work of denudation
has laid bare.
The evidence for the geological age of the volcanic series now to be
described is less direct and obvious than most of that with which I
have been hitherto dealing. It consists of two kinds. (_a_) In the
first of these comes the series of lavas and tuffs just referred to
as regularly interstratified with the red sandstones, which, on the
grounds given in the next paragraph, it is agreed to regard as Permian.
(_b_) Connected with these rocks are necks which obviously served as
vents for the discharge of the volcanic materials. They pierce not only
the Coal-measures, but even parts of the overlying bedded lavas. So
far there is not much room for difference of opinion; but as we recede
northward from Ayrshire and Nithsdale, where the intercalation of
the volcanic series in the red sandstones is well displayed, we enter
extensive tracts where these interstratified rocks have disappeared and
only the necks remain. All that can be positively asserted regarding
the age of these necks is that they must be later than the rocks
which they pierce. But we may inferentially connect them with the
interstratified lavas and tuffs by showing that they can be followed
continuously outward from the latter as one prolonged group, having the
same distribution, structure and composition, and that here and there
they rise through the very highest part of the Coal-measures. It is
by reasoning of this kind that I include, as not improbably relics of
Permian volcanoes, a large number of vents scattered over the centre of
Scotland, in the East of Fife.
The red sandstones among which the volcanic series is intercalated
cover several detached areas in Ayrshire and Dumfriesshire.
Lithologically they present a close resemblance to the Penrith
sandstone and breccias of Cumberland, the Permian age of which is
generally admitted. They lie unconformably sometimes on Lower and Upper
Silurian rocks, sometimes on the lower parts of the Carboniferous
system, and sometimes on the red sandstones which form the highest
subdivision of that system. They are thus not only younger than the
latest Carboniferous strata, but are separated from them by the
interval represented by the unconformability. On these grounds they are
naturally looked upon as not older than the Permian period. The only
palæontological evidence yet obtained from them in Scotland is that
furnished by the well-known footprints of Annandale, which indicate the
existence of early forms of amphibians or reptiles during the time of
the deposition of the red sand. The precise zoological grade of these
animals, however, has never yet been determined, so that they furnish
little help towards fixing the stratigraphical position of the red
rocks in which the footprints occur.
The stratigraphical relations of the red sandstones of Ayrshire
and Nithsdale were discussed by Murchison, Binney and Harkness.[88]
These observers noticed certain igneous rocks near the base of the
sandstones, to which, however, as being supposed intrusive masses, they
did not attach importance. They regarded the volcanic tuffs of the
same district as ordinary breccias, which they classed with those of
Dumfries and Cumberland, though Binney noticed the resemblance of their
cementing paste to that of volcanic tuff, and in the end was doubtful
whether to regard the igneous rocks as intrusive or interstratified.
[Footnote 88: See Murchison's _Siluria_, 4th edit. p. 331; _Quart.
Journ. Geol. Soc._ vol. vii. (1851), p. 163, note; vol. xii. (1856), p.
267; Binney, _ibid._ vol. xii. (1856), p. 138; vol. xviii. (1862), p.
437; Harkness, _ibid._ vol. xii. (1856), p. 262.]
In the year 1862, on visiting the sections in the River Ayr, I
recognized the breccia as a true volcanic tuff. During the following
years, while mapping the district for the Geological Survey, I
established the existence of a series of contemporaneous lavas and
tuffs at the base of the Permian basin of Ayrshire, and of numerous
necks marking the vents from which these materials had been erupted.
An account of these observations was published in the year 1866.[89]
Since that time the progress of the Survey has extended the detailed
mapping into Nithsdale and Annandale, but without adding any new facts
of importance to the evidence furnished by the Ayrshire tract.[90]
[Footnote 89: _Geol. Mag. for 1866_, p. 243; and Murchison's _Siluria_,
4th edit. (1867), p. 332.]
[Footnote 90: The rocks are shown in Sheets 9, 14 and 15 of the
Geological Survey of Scotland, to which, and their accompanying
Explanations, reference is made. The Ayrshire basin was mapped by
me, the necks in the Dalmellington ground by Mr. James Geikie, the
Nithsdale area by Mr. R. L. Jack, Mr. H. Skae and myself.]
The materials erupted by the Scottish Permian volcanoes display a very
limited petrographical range, contrasting strongly in this respect with
the ejections of all the previous geological periods. They consist
of lavas generally more or less basic, and often much decayed at the
surface; and of agglomerates and tuffs derived from the explosion of
the same lavas.
The lavas are dull reddish or purplish-grey to brown or almost black
rocks; sometimes compact and porphyritic, but more usually strongly
amygdaloidal, the vesicles have been filled up with calcite, zeolites
or other infiltration. The porphyritic minerals are in large measure
dull red earthy pseudomorphs of hæmatite, in many cases after
olivine. These rocks have not yet been fully studied in regard to
their composition and microscopic structure. A few slides, prepared
from specimens collected in Ayrshire and Nithsdale, examined by Dr.
Hatch, were found to present remarkably basic characters. One from
Mauchline Hill is a picrite, composed chiefly of olivine and augite,
with a little striped felspar. Others from the Thornhill basin in
Dumfriesshire show an absence of olivine, and sometimes even of
augite. The rock of Morton Castle consists of large crystals of augite
and numerous grains of magnetite in a felspathic groundmass full of
magnetite. Around Thornhill are magnetite-felspar rocks, composed
sometimes of granular magnetite with interstitial felspar. Throughout
all the rocks there has been a prevalent oxidation of the magnetite,
with a consequent reddening of the masses.
The pyroclastic materials consist of unstratified agglomerates and
tuffs, generally found in necks, and of stratified tuffs, which more or
less mingled with non-volcanic material, especially red sandstone, are
intercalated among the bedded lavas or overlie them, and pass upward
into the ordinary Permian red sandstones.
The agglomerates, though sometimes coarse, never contain such large
blocks as are to be seen among the older Palæozoic volcanic groups.
Their composition bears reference to that of the bedded lavas
associated with them, pieces of the various basalts, andesites,
etc., which constitute these lavas being recognizable, together with
others, especially a green, finely-vesicular, palagonitic substance,
which has not been detected among the sheets of lava. In general the
agglomerates contain more matrix than blocks, and pass readily into
gravelly tuffs. A series of specimens collected by me from necks which
pierce the Dalmellington coal-field has been sliced and examined under
the microscope by Mr. Watts, who finds it to consist of basic tuffs, in
which the lapilli include various types of olivine-basalt, sometimes
glassy, sometimes palagonitic, and occasionally holocrystalline, also
pieces of grit, shale and limestone. In one case a crinoid joint
detached from its matrix was noticed. A specimen from Patna Hill
consists of "a clear irregularly cracked aggregate of carbonates and
quartz with hornblende, and its structure reminds one of that of
olivine. The hornblende is in small irregular patches surrounded by the
clear mineral, and is probably a replacement of a pyroxene, perhaps
diallage." If this stone was once an olivine nodule, the agglomerate
might in this respect be compared with some of the tuffs of the Eifel
so well known for their lumps of olivine.
The stratified tuffs are generally more or less gravelly deposits,
composed of lapilli varying in size from mere grains up to pea-like
fragments, but with numerous larger stones and occasional blocks of
still greater dimensions. They often pass into a tough dull compact
mudstone. In colour they are greenish or reddish. They have been
largely derived from the explosion of lavas generally similar to those
of which fragments occur in the agglomerates. They often contain
non-volcanic detritus, derived from the blowing up of the rocks through
which the vents were opened. Occasionally they include also various
minerals such as pyrope, black mica, sanidine, augite, and others which
appear to have been ejected as loose and often broken crystals. This
character is more fully described in regard to its occurrence among the
necks of the east part of Fife.
The intrusive rocks, probably referable to the same volcanic period,
consist chiefly of dolerites and basalts which occur as dykes, sills
and bosses, and are more particularly developed in the south-west of
Ayrshire.
ii. GEOLOGICAL STRUCTURE OF THE VOLCANIC DISTRICTS
1. Ayrshire, Nithsdale and Annandale
(1) _Interstratified Lavas and Tuffs._--It will be convenient to
consider first the volcanic chronicle as it has been preserved in
the south-west and south of Scotland, where the existence of Permian
volcanoes in Britain was first recognized. The volcanic rocks in the
middle of the Ayrshire coal-field rise from under a central basin of
red sandstone, which they completely enclose. Their outcrop at the
surface varies up to about a mile or rather more in breadth, and forms
a pear-shaped ring, measuring about nine miles across at its greatest
width (Map V.).[91]
[Footnote 91: Mr. Gunn has recently detected among the newest red
sandstones of Arran a small patch of volcanic rocks which may be of
this age. Mr. A. Macconochie has also found what may be traces of a
similar volcanic band below the Permian sandstones of Loch Ryan, in
Wigtonshire.]
This volcanic ring runs as a tract of higher ground encircling the
hollow in which the Permian red sandstones lie, and forming a marked
chain of heights above the Carboniferous country around. It is built up
of a succession of sheets of different lavas, with occasional partings
of tuff or volcanic breccia, which present their escarpments towards
the coal-field outside, and dip gently into the basin under the inner
trough of brick-red sandstones. Good sections of the rocks are exposed
in the ravines of the River Ayr, particularly at Ballochmyle, in the
Dippol Burn near Auchinleck House, and in the railway cutting near
Mossgiel.
That these are true lava-flows, and not intrusive sills, is
sufficiently obvious from their general outward lithological aspect,
some of them being essentially sheets of slag and scoriæ. Their
upper surfaces may be found with a fine indurated red sand wrapping
round the scoriform lumps and protuberances, and filling in the
rents and interspaces, as in the case of the Old Red Sandstone lavas
already referred to. As an example of these characteristics, I may
cite the section represented in Fig. 200. At the bottom lies a red
highly ferruginous and coarsely amygdaloidal basalt (_a_). Over it
comes a volcanic conglomerate three feet thick, made up of balls of
vesicular lava like that below, wrapped in a brick-red sandy matrix
(_b_). Lenticular bands of sandstone without blocks occur in the
conglomerate, and others lie in hollows of its upper surface (_c_).
This intercalation of detrital material is followed by another basic
lava (_d_), about six feet thick, highly amygdaloidal in its lower and
upper parts, more compact in the centre. The amygdales and joints are
largely filled with calcite. The slaggy bottom has caught up and now
encloses some of the red sand of the deposit below. Another lava from
three to six feet thick next appears (_e_), which is remarkable for its
slaggy structure, and is so decomposed that it crumbles away. Like the
others it is dull-red and ferruginous and full of calcite. It must have
been at the time of its outflow a sheet of rough slag that cracked into
open fissures. That it was poured out under water is again shown in the
same interesting way just referred to, by the red sand which has been
washed into the interspaces between the clinkers and has filled up the
fissures, in which it is stratified horizontally between the walls.
Above this band, and perhaps passing into it as its slaggy base, lies
another more compact lava (_f_) like the lower sheets.
[Illustration: Fig. 199.--General section across the Permian basin of
Ayrshire.
1. Highest group of the Coal-measures; 2. Volcanic tuffs and ashy
brick-red sandstones; 3. Lavas with interstratified tuffs and brick-red
sandstones; 4. Brick-red Permian sandstones; 5, 5. Necks of volcanic
agglomerate; 6. Boss of dolerite.]
Throughout the series of lavas, as indicated in the foregoing section,
traces of the pauses that elapsed between the separate outflows may
be seen in the form either of layers of red sandstone or of tuff and
volcanic breccia. Here and there, under the platform of bedded lavas,
the brick-red sandstone is full of fragments of slag and fine volcanic
dust. But the most abundant accumulation of such detritus is to be
seen at the top of the volcanic series, where it contains the records
of the closing phases of eruption. Thick beds of tuff and volcanic
breccia occur there, interleaved with seams of red sandstone, like the
chief mass of that rock, into which they gradually pass upward. Yet,
even among the sandstones above the main body of tuff, occasional nests
of volcanic lapilli, and even large bomb-like lumps of slag, point to
intermittent explosions before the volcanoes became finally extinct and
were buried under the thick mass of red Permian sandstone.
[Illustration: Fig. 200.--Section of lavas east side of Mauchline Hill.]
[Illustration: Fig. 201.--Section of the top of the volcanic series
near Eastside Cottage, Carron Water, Nithsdale.]
There is good reason to believe that both the volcanic sheets and
the red sandstones overlying them, instead of being restricted to an
area of only about 30 square miles, once stretched over the lowlands
of Ayrshire; and not only so, but that they ran down Nithsdale, and
extended into several of its tributary valleys, if indeed, they
were not continuous across into the valley of the Annan.[92] Traces
of the lavas and tuffs are to be found at intervals over the area
here indicated. The most important display of them, next to their
development in Ayrshire, occurs in the vale of the Nith at Thornhill,
whence they extend continuously up the floor of the Carron Valley
for six miles. They form here, as in Ayrshire, a band at the base of
the brick-red sandstones, and consist mainly of bedded lavas with
the basic characters above referred to. These lavas, however, are
followed here by a much thicker development of fragmental volcanic
materials. Abundant volcanic detritus is diffused through the overlying
sandstones, sometimes as a gravelly intermixture, sometimes in large
slaggy blocks or bombs, and sometimes in intercalated layers of tuff,
while an occasional sheet of one of the dull red lavas may also be
detected. The final dying-out of the volcanic energy in a series of
intermittent explosions, while the ordinary red sandy sediment was
accumulating, is here also admirably chronicled. As an illustration of
these features the accompanying section is given (Fig. 201). The last
of the lavas (_a_) presents an uneven surface against which the various
kinds of detritus have been laid down. First comes a coarse volcanic
breccia (_b_) made up of angular and subangular blocks of different
lavas imbedded in a matrix of red ashy sand. This deposit is succeeded
by a band of dull red tufaceous sandstone, evidently formed of ordinary
red sandy sediment, into which a quantity of volcanic dust and lapilli
fell at the time of its accumulation. Some of the ejected blocks which
lie inclosed in the finer sediment are upwards of a foot in length. A
more vigorous discharge of fragmental material is shown by the next bed
(_d_), which consists of a coarse nodular tuff, mingled with a little
red sandstone and crowded with blocks of the usual lavas. Beyond the
locality of this section these tuffs are found to pass up insensibly
into the ordinary Permian sandstone.
[Footnote 92: See _Memoirs of Geol. Surv. Scotland_, Sheet 15 (1871), p.
35; Sheet 9 (1877), p. 31.]
[Illustration: Fig. 202.--Section of two outliers of the Permian
volcanic series at the foot of Windyhill Burn, Water of Ae,
Dumfriesshire.]
But we can detect the edges of yet more distant streams of lava
emerging from under the red sandstones and breccias to the east of
the Nith. On the farther side of the Silurian ridge that forms the
eastern boundary of the Nith valley, above which it rises some 700
or 800 feet, there is preserved at the bottom of the valley of the
Capel Water, which flows into Annandale, another small outlier of a
similar volcanic band. Three miles to the south-east of it two little
fragments of the volcanic group lie on the sides of a small tributary
of the Water of Ae. Since these may serve as a good illustration of the
extent to which denudation has reduced the area of the Permian volcanic
series, a section of the locality is here given (Fig. 202). The general
foundation rocks of the country are the Silurian greywackes and shales
in highly inclined and contorted positions (_a_). Each outlier has, as
its basement material, a volcanic breccia (_bb_) in which, together
with the usual lava-fragments, are mingled pieces of the surrounding
Silurian strata. In the smaller outlier lying to the north-east, this
detrital layer is only about one foot thick. It is overlain by a slaggy
amygdaloid of the usual character (_cc_), which in the lower outlier is
covered with boulder clay (_d_). There can be little doubt that these
detached fragments were once united in a continuous sheet of lava which
filled the valley of the Water of Ae and that of its tributary. That
the lava stretched down the Ae valley for some distance is proved by
the occurrence of another outlier of it two miles below.
But there is still additional evidence for the wide extension of these
volcanic sheets. It appears to be certain that they stretch far to
the eastward, under the Permian sandstones of the Lochmaben basin of
Annandale, for breccias largely made up of pieces of the bedded lavas
are found close to the northern edge of the basin on the west side of
the River Annan. To this remarkable adherence of the lavas and tuffs
to the bottom of the Permian valleys I shall afterwards more specially
refer.
The thickness of the whole volcanic group cannot be very accurately
determined. It reaches a maximum in the Ayrshire basin, where, at its
greatest, it probably does not exceed 500 feet, but is generally much
less; while in the Nithsdale and Annandale ground the detached and
much denuded areas show a still thinner development.
[Illustration: Fig. 203.--The Green Hill, Waterside, Dalmellington,
from the south; a tuff-neck of Permian age.]
(2) _Vents._--One of the most interesting features in this
south-western district of Scotland is the admirable way in which the
volcanic vents of Permian time have been preserved. Their connection
with the lavas and tuffs can there be so clearly traced that they
serve as a guide in the interpretation of other groups of vents in
districts where no such connection now remains. In Ayrshire, the
lower part of the Permian volcanic band is pierced by several small
necks of agglomerate. There cannot, I think, be any doubt that these
necks mark the positions of some of the vents from which the later
eruptions took place. Immediately beyond them necks of precisely
similar character rise through the upper division of the Coal-measures.
There can be as little hesitation in placing these also among the
Permian vents. And thus step by step we are led away from the central
lavas, through groups of necks preserving still the same features,
external and internal, and rising indifferently through rocks of any
geological age from the Coal-measures backward. Thus, although if we
began the investigation at the outer limits of the chain of necks,
we might well hesitate as to their age, yet, when we can fix their
geological position in one central area, we are, I think, justified
in classing, as parts of one geologically synchronous series, all the
connected groups that retain the same general characteristics. It is to
denudation that we owe their having been laid bare to view; but at the
same time, denudation has removed the sheet of ejected materials which
may have originally connected most of these vents together.
In this regard, it is most instructive to follow the vents
south-eastwards from the Ayrshire basin into Nithsdale for a distance
of some eighteen miles. If we traced them down that valley to Sanquhar,
without meeting with any vestige of superficial outflows to mark their
stratigraphical position, we might possibly hesitate whether the age
of those which are so far removed from the evidence that would fix it
should not be left in doubt. But if we continued our traverse only a
few hundred yards farther, we should find some fragmentary outliers
of the Permian lavas capping the Upper Coal-measures; and if we merely
crossed from the Nith into the tributary valley of the Carron Water,
we should see preserved in that deep hollow a great series of Permian
lavas, tuffs and agglomerates. It is only by a happy accident that here
and there these superficial volcanic accumulations have not been swept
away. There was probably never any great thickness of them, but they no
doubt covered most, if not all, of the district within which the vents
are found.
The Permian necks are, on the whole, smaller than those of the
Carboniferous period. The largest of them in the Ayrshire and Nithsdale
region do not exceed 4000 feet in longest diameter; the great majority
are much less in size, while the smallest measure 20 yards, or even
less. Those of Fife, to be afterwards described, exhibit a wider range
of dimensions, and have the special advantage of being exposed in plan
along the shore.
[Illustration: Fig. 204.--Patna Hill from the Doon Bridge, Ayrshire; a
tuff-neck of Permian age.]
These necks, from their number and shapes, form a marked feature in the
scenery. They generally rise as prominent, rounded, dome-shaped, or
conical hills, which, as the rock comes close to the surface, remain
permanently covered with grass (Figs. 203 and 204). Such smooth green
puys are conspicuous in the heart of Ayrshire, and likewise further
south in the Dalmellington coal-field, where some of them are locally
known as "Green Hill," from their verdant slopes in contrast to the
browner vegetation of the poorer soil around them (Fig. 203).
As in those of older geological periods, the necks of this series
are, for the most part, irregularly circular or oval in ground-plan,
but sometimes, like those of the Carboniferous system, they take
curious oblong shapes, and occasionally look as if two vents had
coalesced (Fig. 205). Here and there also the material of the vents
has consolidated between the walls of a fissure or the planes of the
strata, so as to appear rather as a dyke than as a neck. Descending,
as usual, vertically through the rocks which they pierce, the necks
have the form of vertical columns of volcanic material, ending at the
surface in grassy rounded hillocks or hills.
In almost all cases, the necks of the Ayrshire region consist of a
gravelly tuff or agglomerate, reddish or greenish in colour, made up of
blocks of such lavas as form the bedded sheets, together with fragments
of the stratified rocks through which the chimneys have been blown out.
Thus, in some of the necks, pieces of black shale are abundant, as at
Patna. In other cases, there are proofs of the derivation of the stones
from much greater depths, as in the Green Hill of Waterside, where
fragments of fine greywacke are not infrequent, probably derived from
the Silurian formations which lie deep beneath the Carboniferous and
Old Red Sandstone series.
The fragmentary material of the necks is generally unstratified, but a
rude stratification may sometimes be noticed, the dip being irregularly
inward at high angles towards the middle of the vent. This structure,
best seen in the vents of the Fife coast, as will be shown in the
sequel, may be detected in some of the necks of the Dalmellington
district.
[Illustration: Fig. 205.--Ground plans of Permian volcanic vents from
the Ayrshire Coal-field. On the scale of six inches to a mile.
1. Neck half a mile north-west from Dalmellington; 2. Neck at
Auchengee, four miles north-east from Patna; 3. Neck at head of
Drumbowie Burn, five and a half miles due north from Dalmellington;
4. Patna Hill, 853 feet above sea-level (for outline of this hill
see the preceding Fig.); 5. Neck on Kiers Hill (1005 feet above the
sea), two miles south from Patna, with lava adhering to part of the
wall.
]
Occasionally some form of molten rock has risen in the funnel, and has
partially or wholly removed or concealed the agglomerate. This feature
is especially noticeable among the necks that pierce the Dalmellington
coal-field. Portions of basic lavas traverse the agglomerate or
intervene between it and the surrounding strata. These have probably in
most cases been forced up the wall of the funnel, while here and there
sills run outward from the necks into the surrounding Coal-measures.
Sometimes a thin sheet of lava, adhering to the wall of a funnel, may
be the remnant of a mass of rock that once filled up the orifice. In
one of the necks of the Muirkirk Coal-field, which was pierced by
a mine driven through it from side to side, fingers and sheets of
"white trap," or highly altered basalt, were found to run out from the
neck into the surrounding strata.[93] Dark heavy basalt, or some still
more basic rock, has here and there filled up a vent. As so many of
the necks rise through the coal-fields, opportunities are afforded of
studying the effects of volcanic action upon the coal-seams, which for
some distance from them have been destroyed.
[Footnote 93: Explanation of Sheet 23, Geol. Surv. Scotland, p. 39.]
Another feature, which can be recognized from the information obtained
in mining operations, is that, in the great majority of instances,
no connection is traceable between the positions of the vents and
such lines of dislocation as can be detected at the surface or in the
underground workings. Some vents, indeed, have evidently had their
positions determined by lines of fault, as, for instance, that of the
Green Hill below Dalmellington. Yet in the same neighbourhood a number
of other examples may be found where the volcanic funnels seem to have
avoided faults, though these exist close to them.
In this south-western district of Scotland upwards of sixty distinct
vents have been mapped in the course of the Geological Survey. They run
from the north of Ayrshire to the foot of the Southern Uplands, and
descend for some distance the vale of the Nith. The area over which
they are distributed measures roughly about forty miles from north-west
to south-east, and at its greatest breadth twenty miles from south-west
to north-east. Within this tract the vents are scattered somewhat
sporadically in groups, sometimes numbering twenty necks in a space of
sixteen square miles, as in the remarkable district of Dalmellington.
In considering their distribution we cannot but be impressed by the
striking manner in which these necks keep to the valleys and low
grounds. I have already alluded to this characteristic, as shown by the
volcanoes of the Old Red Sandstone and Carboniferous periods. But it
is displayed by the Permian volcanoes in a still more astonishing way.
Beginning at the northern end of the long chain of necks in the West
of Scotland, we find a row of them on the plains fronting the volcanic
plateau of the Ardrossan, Dunlop and Stewarton Hills. Thence we may
follow them, as single individuals or in small groups, across the broad
lowland of Ayrshire, southward to the very base of the great chain
of the Southern Uplands. There, a cluster of some two dozen of them
may be seen rising out of the Carboniferous rocks on the low grounds,
but they abruptly cease close to the base of the hills; not one has
been detected on the adjacent Silurian heights. Moreover, if we turn
into the valleys that lead away from the great Ayrshire plain to the
interior, we find necks of the same character in these depressions.
They ascend the valley of Muirkirk, and may be met with even at its
very head, near the base of the Hagshaw Hills. Again, on the floor
of the remarkable transverse valley trenched by the Nith across the
Southern Uplands, Permian necks pierce the Coal-measures, while the
outlying fragments of bedded lava show that these vents flooded the
bottom of that valley with molten rock. Turning out of Nithsdale into
the long narrow glen of the Carron Water, we observe its floor and
sides to be covered with the sheets of lava and tuff already noticed.
And so travelling onward from the vale of the Nith into that of the
Capel Water, thence into the Water of Ae and across into the great
strath of Annandale, we may detect, if not actual vents, at least the
beds of lava and layers of volcanic detritus that were ejected from
them.
All along these valleys, which were already valleys in Carboniferous
time, traces of the volcanic activity of this epoch may be detected.
But, so far as I am aware, in not a single case has any vent been
observed to have been opened on the high surrounding ridges. There has
obviously been a determining cause why the volcanic orifices should
have kept to the plains and the main valleys with their tributaries,
and should have avoided the hills which rise now to heights of 500 to
1000 feet or more above the bottoms of the valleys that traverse them.
It might be said that the valleys follow lines of fracture, and that
the vents have been opened along these lines. But my colleagues in the
Geological Survey, as well as myself, have failed, in most cases, to
find any evidence of such dislocations among the rocks that form the
surface of the country, while it is sometimes possible to prove that
they really do not exist there.
Though only a few scattered patches of the Permian bedded lavas
and tuffs have been preserved, enough is left to indicate that the
vents were active only in the early part of the period represented
by the Scottish Permian red sandstones, for it is entirely in the
lower part of these strata that volcanic rocks occur. The eruptions
gradually ceased, and the sheets of ejected material, probably also
the volcanic cones, were buried under at least several hundred feet of
red sandstone. Whether or not any portion of the erupted material was
for a time built up above the level of the water, there seems to be no
question that the vents were, on the whole, subaqueous.
[Illustration: Fig. 206.--Section of sills traversing the Permian
volcanic series. River Ayr, Ballochmyle.
_a_, Coal-measures; _b_ _b_, Basic lavas; _c_ _c_, Brick-red sandstones
with tuff; _d_, Red tuff and volcanic breccia; _e_ _e_, Dolerite sills.]
3. _Sills._--The phenomena of sills and dykes are less clearly
developed among the Permian volcanic rocks of the Ayrshire basin than
among those of older formations. In the section exposed in the course
of the River Ayr at Howford Bridge, a coarsely crystalline dolerite
which extends for nearly 300 yards up the stream, cuts the Permian
lavas, of which it encloses patches as well as pieces of sandstone.
At the contact, the rock becomes fine-grained (Fig. 206). Through the
coarsely crystalline material run long parallel "segregation veins" of
a paler, more acid substance, as among the Carboniferous sills. Similar
rocks are well seen in the Dippol Burn near Auchinleck House.
Passing outward into the Coal-measures, we encounter a much larger
display of similar intrusive sheets. The best district for the study
of these sills lies around Dalmellington. The Coal-measures are there
traversed by many intrusions, which have produced great destruction
among the coal-seams. Some of the rocks are extremely basic, including
a beautiful picrite like that of Inchcolm (Letham Hill, near
Waterside). The age of these sills must be later than the Coal-measures
into which they have been injected. Some of them are obviously
connected with the agglomerate-necks, and the whole or the greater
number should thus probably be assigned to the Permian period.[94]
The phenomena of intrusion presented by these rocks reproduce the
appearances already described in connection with the basic intrusive
sheets of Carboniferous age.
[Footnote 94: Explanation of Sheet 14, Geol. Surv. Scotland, p. 22.]
2. Basin of the Firth of Forth
The other district of Southern Scotland, where traces of volcanic
action later in age than the Coal-measures may be observed, lies in the
basin of the Firth of Forth (Map V.). They include no bedded lavas,
and only at one locality do any relics of a covering of stratified
tuffs overspread the Carboniferous formations. The evidence for the old
volcanoes consists almost entirely of necks of tuff, which mark the
position of vents of eruption.
(1) _Vents._--On the south side of the estuary of the Forth there is
only one neck which may be plausibly placed in this series. It forms
the upper part of Arthur Seat, at Edinburgh. This hill has already
been cited as consisting of two distinct portions. The lower, built up
of bedded tuffs, basalts and andesites, forms part of the Midlothian
volcanic plateau of Carboniferous time. The vent from which these
materials were ejected must lie at some little distance, and its site
has not been certainly ascertained. The upper part of the hill is
formed of a distinct group of rocks which has now to be described.
The geological structure of Arthur Seat has long been well known.
It served as a theme for discussion in the Neptunist and Plutonist
controversy, and was often referred to in the various mineralogical or
geognostical writings of the time. The first thorough examination of
it as a relic of ancient volcanic action was that of Charles Maclaren,
published in 1839.[95] This author clearly recognized the later age and
unconformable position of the coarse mass of agglomerate pierced by
the basalt of the apex, and pointed out the evidence of the upheaval
and denudation of the older volcanic series during a long interval of
repose before the latest eruptions took place. Subsequently Edward
Forbes suggested that the upper part of the hill might be of Tertiary
age.[96] Thereafter I mapped the ground in detail for the Geological
Survey, entirely confirming the observations of Maclaren.[97] In the end
it seemed to me that the interval between the two epochs of volcanic
activity might not be so great as Forbes had supposed; and after
tracing the Permian vents of Ayrshire, I came to the conclusion that
the younger unconformable agglomerate of Arthur Seat was not improbably
Permian.
[Footnote 95: _Geology of Fife and the Lothians_, p. 34. In a reprint of
this work, published in 1866, the venerable author briefly remarked in
a footnote that he no longer believed in the second period of volcanic
activity. This view was adopted in 1875 by Professor Judd, _Quart.
Journ. Geol. Soc._ xxxi. p. 131. For the reasons stated in the text I
believe Maclaren's original explanation of the structure of the hill to
be correct.]
[Footnote 96: Forbes never published his views regarding Arthur Seat,
but expounded them to his class, and explained them in diagrams, some
of which are preserved in the Edinburgh Museum of Science and Art, in
association with the specimens which he collected from the hill.]
[Footnote 97: Sheet 32, Geol. Survey of Scotland and descriptive Memoir.
See also _Rep. Brit. Assoc._ 1867, address Geol. Sect., and Murchison's
_Siluria_, 4th edit. p. 331.]
The older volcanic series of this hill has been broken through by the
agglomerate which occupies a true neck, and is abruptly marked off
from all the rocks older than itself. There is no trace of any of the
older lavas or tuffs thickening towards this vent. On the contrary
they are completely truncated by it, and their outcrops on the north
side reappear from under the agglomerate on the south side. Their
escarpments are wrapped round by the agglomerate which likewise fills
the head of the hollow that had been previously worn by denudation out
of the stratified deposits between the oldest lavas. There is thus a
violent unconformability between the later and the older volcanic rocks
of Arthur Seat.
The length of time indicated by this stratigraphical break must be
great. There is no known discordance in the Carboniferous system of
the Lothians, yet the Coal-measures, Millstone Grit, Carboniferous
Limestone series and much of the Calciferous Sandstones were stripped
from this hill before the eruption of the agglomerate. It will be shown
in the sequel that a nearly similar amount of denudation preceded some
of the probably Permian eruptions of Fife.
The agglomerate contains abundant fragments of the older volcanic
series. Its matrix is a dull red gravelly detritus, crowded with blocks
of all sizes up to a yard or more in diameter. It is pierced by a
column or plug of basalt, which sends veins into it, and rises to the
apex of the hill. A beautiful olivine-basalt forms the lateral mass of
the Lion's Haunch, which rests on the agglomerate.
[Illustration: Fig. 207.--Section showing the relations of the later
rocks of Arthur Seat.
1. Grey and reddish sandstones and shales (Calciferous Sandstones);
2. The lava of the Long Row: the oldest of the Carboniferous
volcanic series; 3. Tuffs of the Dry Dam; 4. Columnar basalts
overlying the tuffs; 5. Andesite lavas of the eastern half of
Arthur Seat; 6. Sill of Heriot Mount; 7. Sill of Salisbury Crags;
8. Sill of the Dasses. These complete the Lower Carboniferous
volcanic series (compare Fig. 112). 9. White sandstones and black
shales, upper division of the Calciferous Sandstones; 10. Younger
volcanic agglomerate resting on the denuded ends of the older
volcanic series; 11. Basalt of the summit sending veins into the
agglomerate; 12. Basalt of the Lion's Haunch.
]
In general characters the agglomerate of Arthur Seat resembles that of
some of the younger vents of Fife which pierce the Coal-measures and
are connected with tuffs that lie unconformably on the Carboniferous
Limestone. On these various grounds I think that it may be reasonably
assigned to the same geological period.
That a new vent should be opened, after the lapse of one or more
geological periods, on or near the site of more ancient volcanic
orifices is an incident of which, as we have seen, the geological
history of the British Isles furnishes a number of examples. It will
be remembered that little more than a mile to the south of Arthur Seat
lies the great vent of the Braid Hills, which in the time of the Lower
Old Red Sandstone gave forth such a huge pile of lavas and tuffs.
Volcanic energy thereafter entirely died away, and in this district
was succeeded by a prolonged period of quiescence, during which the
Lower Old Red Sandstone was upraised and extensively denuded, while
the Upper Old Red Sandstone was deposited. At length, in the immediate
neighbourhood, from one or more vents, the exact site of which is not
certainly known, the older lavas and tuffs of Arthur Seat, Calton
Hill and Craiglockhart Hill were erupted. Again, after another vast
interval, a new volcano appeared, and the agglomerate and younger
basalts of Arthur Seat were ejected from it. This is one of the most
striking examples in this country of the remarkable persistence of
volcanic energy in the same locality.
There is no evidence at Arthur Seat itself to fix the geological date
of the last volcanic activity of the hill. If the group of younger
rocks stood alone, with no other trace of post-Carboniferous eruptions
in the surrounding district, a plausible conjecture as to its age would
not be easily offered. But in reality it is not a solitary example
of such rocks; for within sight, on the opposite side of the Firth
of Forth, its counterparts may be seen. To the description of these
numerous and clearer illustrations I now proceed.
The East of Fife is remarkable for a large assemblage of volcanic
vents, which, unlike those in Ayrshire and Nithsdale, stand alone,
their superficial ejections having been removed by denudation, and no
connection being traceable between them and any Permian sandstones.
The vents filled up with agglomerate and pierced with plugs and veins
of basalt, rise through the Carboniferous rocks, but have left no
record for precisely defining their geological age. On the one hand,
it is quite certain that in this district volcanic eruptions took
place during the earlier half of the Carboniferous period. To the
north of Largo, and still more distinctly to the north-east of Leven,
sections occur to show the contemporaneous outpouring of volcanic rocks
during the time of the Carboniferous Limestone. The Leven section,
seen in a ravine a little to the north-east of the town, is specially
important. It presents a succession of red and green fine sandy tuffs,
interstratified with fire-clays and sandstones, and containing a zone
of basalt in the centre. These rocks lie not far from the top of the
Carboniferous Limestone series.
On the other hand, there is equally clear proof of far later eruptions.
From St. Andrews to Elie a chain of necks may be traced, having the
same general characters, and piercing alike the Calciferous Sandstones,
and the older part of the Carboniferous Limestone series. That these
vents must in many cases be long posterior to the rocks among which
they rise, is indicated by some curious and interesting kinds of
evidence. They are often replete with angular fragments of shale,
sandstone and limestone, of precisely the same mineral characters as
the surrounding strata, and containing the same organic remains in an
identical state of fossilization. It is clear that these strata must
have had very much their present lithological aspect before the vents
were opened through them. Again, the necks may often be observed to
rise among much contorted strata, as, for example, along the crest of a
sharp anticlinal arch, or across a synclinal basin. The Carboniferous
rocks must thus have been considerably plicated before the time of
the volcanic eruptions. In the next place, the vents often occur on
lines of dislocation without being affected thereby. They must be
posterior, however, not only to these dislocations, but also to much
subsequent denudation, inasmuch as their materials overspread the rocks
on each side of a fault without displacement. Hence we conclude with
confidence, that a great deal of volcanic activity in the East of Fife
must have been posterior to most, if not all, of the Carboniferous
period.
[Illustration: Fig. 208.--Section in brooks between Bonnytown and
Baldastard, Largo.
_a_, Sandstone shales and coals of Carboniferous Limestone series; _b_,
unconformable tuff.]
In the neighbourhood of Largo, further important evidence is presented,
confirming and extending this conclusion. The highest member of the
Upper Coal-measures, consisting of various red sandstones, with red
and purple clays, shales, thin coals and ironstones, is prolonged
from the Fife coal-field in a tongue which extends eastward beyond
the village of Lower Largo. It is well displayed on the shore, where
every bed may be followed in succession along the beach for a space
of nearly two miles. Two volcanic necks, presenting the same features
as those which pierce the older portions of the Carboniferous system
to the east, rise through these red rocks. We are thus carried not
only beyond the time of the Carboniferous Limestone, but beyond the
close of the very latest stage of the Carboniferous period in Central
Scotland. Connected with these and other vents farther north, there
is a large area of tuff which has been thrown out upon the faulted
and greatly denuded Carboniferous rocks. It may be traced passing
from the red Upper Coal-measures across the large fault which here
separates that formation from the Carboniferous Limestone, and
extending inland athwart different horizons of the latter series.
Outlying fragmentary cakes of it may be seen resting on the upturned
edges of the sandstones, shales and coal-seams, even at a distance
of some miles towards the north-west, proving that the fragmentary
materials discharged from the vents spread over a considerable area.
The accompanying section (Fig. 208) may serve as an illustration of the
relation between this sheet of bedded tuff and the underlying rocks.
Though interstratified volcanic rocks occur in the Carboniferous
system of the East of Fife, no connection has been traced between them
and any of the vents now referred to. While none of these vents can
be proved to be of Carboniferous age, it is of course possible that
such may be the true date of some of them. Others, nevertheless, and
probably much the largest number, judged from the data just given, may
be regarded as probably post-Carboniferous. Those which happen to rise
through the uppermost Coal-measures do not appear to be distinguishable
by any essential characters from those which pierce indifferently
the Carboniferous Limestone series and Calciferous Sandstones of the
East of Fife. They seem to be all one connected aggregate, resembling
each other alike in their external characters, internal structure and
component materials, and the limit of their age must be determined
by the geological horizon of the youngest formation which they
traverse. By this process of reasoning I reach the conclusion that
this remarkable series of old volcanoes in the East of Scotland not
improbably dates from the same time as that of Ayrshire and Nithsdale,
already described.
[Illustration: Fig. 209.--View of Largo Law from the east (the crag on
the left, at the base of the cone, is a portion of a basalt-stream. See
Fig. 226).]
Some idea of the importance and interest of the volcanic area of
Eastern Fife may be gathered from the fact that in a space of about
70 square miles no fewer than 60 necks may be counted, and others are
probably concealed below the drift-deposits which cover so much of the
interior of the country. The area of this remarkable display extends
from St. Andrews Bay and the Vale of the Eden southwards to the coast
of the Firth of Forth between Lundin Links and St. Monans. All over the
inland tract the necks form more or less marked eminences, of which the
largest are conspicuous landmarks from the southern side of the Firth.
But the distinguishing characteristic of the area is the display of the
necks along the coast, where, in a series of natural dissections, their
form, composition, internal structure and relations to the surrounding
rocks have been laid open in such clearness and variety as have been
met with in the volcanic records of no other geological period within
the compass of these islands. As this district thus possesses a
singular interest and value for the study of volcanic vents, I shall
enter in some detail into the description of the sections so admirably
laid bare.
[Illustration: Fig. 210.--View of small neck in Calciferous Sandstones,
on the shore, three miles east from St. Andrews.
(This illustration, likewise Figs. 212, 216, 219, 221, 222, 225 and 227
are from photographs taken for the Geological Survey by Mr. R. Lunn.)]
As in Ayrshire, the necks in the East of Fife generally rise as
isolated conical or dome-shaped hills, with smooth grassy slopes, but
where a dyke or boss of basalt occurs in them, it usually stands out
as a crag or knoll. Largo Law (Fig. 209) may be taken as a singularly
perfect example of the cone-shaped neck. This hill, however, comprises
more than one vent. The mass of tuff of which it consists probably
includes at least three distinct funnels of discharge, and surrounding
it there still remains a good deal of the fragmental material that
gathered around these vents and is now seen to lie unconformably upon
the Carboniferous formations (Fig. 208). There must be a total area
of not much less than four square miles over which tuff occupies the
surface of the ground.
While the Fife necks possess the great advantage of having been laid
bare by the sea, their frequent small size on the coast allows their
whole area to be examined. As illustrations of these little vents, two
plates are here given from the coast-line to the east of St. Andrews,
where a number of small necks of agglomerate have been planted among
the plicated Calciferous Sandstones. In Fig. 210 the abrupt truncation
of the sandstones by the volcanic rock is well shown. The strata on the
right have been broken through, and the sea has indented a small gully
along the wall of the old volcanic funnel. The sandstones in front,
however, still adhere firmly to the agglomerate, which rises above them
as a rugged mass of rock.
In Fig. 212 the edge of the vent can be traced partly in section and
partly in plan for about half of its circumference. On the right
hand, the actual wall of the funnel is visible where the false-bedded
sandstones are sharply cut off by the agglomerate. In front the strata
appear in plan on the beach, and their ledges can be seen to the left
striking at the margin of the neck.
[Illustration: Fig. 211.--Ground-plan of Permian volcanic vents.]
[Illustration: Fig. 212.--Small neck in Calciferous Sandstones a little
east from the "Rock and Spindle," two and a half miles east from St.
Andrews.]
The shape of the Fife vents is, as usual, generally circular or
oval; but is subject to considerable irregularity. The coast-section
between Largo and St. Monans exposes many ground-plans of them, and
permits their irregularities to be closely examined. The accompanying
figure (Fig. 211) exhibits some characteristic forms. Eccentricities
of outline no doubt arose from the irregular way in which the rocks
yielded to the forces of explosion during the piercing of a volcanic
orifice. This is often well shown by the veins and nests of tuff or
agglomerate which have been forced into the rents or sinuosities of
the orifices. In other cases, however, it is probable that, as among
the Ayrshire necks, and those of Carboniferous age already cited, what
appears now as one volcanic neck was the result of a shifting of the
actual funnel of discharge, so that the neck really represents several
closely adjacent vents. The case of Largo Law has been already noticed.
The necks at Kellie Law (Fig. 213) show clearly the same structure, the
Law itself (1) probably consisting of two contiguous vents, while a
third (2) forms a smaller cone immediately to the east. Such a slight
lateral displacement of the vent has been noticed at many Tertiary and
recent volcanic orifices. In the island or peninsula of Volcanello,
for example, three craters indicate successive shiftings of the vent,
the most perfect of them marking the latest and diminishing phase of
volcanic activity (Fig. 214, compare Fig. 29, vol. i., p. 70).
[Illustration: Fig. 213.--Plan of volcanic necks at Kellie Law, east of
Fife, on the scale of three inches to one mile.
1, Kellie Law (tuff); 2, Carnbee Law (tuff); 3, 4, 5, small tuff necks;
B B, basalt dykes and bosses; _c_ _c_, coal-seams; _l_, limestone; _f_,
fault. The arrows mark the dip of the strata through which the necks
have been drilled.]
[Illustration: Fig. 214.--Plan of the craters in Volcanello, Lipari
Islands.]
The Fife necks vary from only a few yards up to perhaps 4000 feet in
diameter. One of the smallest and most completely exposed occurs on the
shore at Newark Castle, near St. Monans. It measures only 60 yards in
length by about 37 yards in breadth. A ground-plan of it is given in
Fig 224. Still smaller is the neck at Buddo Ness, on the coast east of
St. Andrews, which measures only 20 yards across.
From the way in which the vents have been dissected by the sea along
the Fife coast, the geologist is enabled to study in minute detail
the effects of the volcanic operations upon the strata through which
the funnels have been drilled. Considerable variation may be observed
in the nature and amount of change. Sometimes the orifice has been
made without any noticeable alteration of the sandstones, shales and
limestones, which retain their dip and strike up to the very wall of
the chimney. Usually there is more or less jumbling and crushing of
the stratification, and often a considerable amount of induration. As
a typical example of these effects I give a section from the margin of
the neck of tuff on the east side of Elie Harbour (Fig. 215). Here the
sandstones and shales (_a_) have been doubled over and dragged down
against the tuff (_b_). They have likewise been hardened into a kind of
quartzite, and this alteration extends for about 20 to 30 feet from the
edge of the neck.
[Illustration: Fig. 215.--Section of the strata at the edge of the
volcanic vent on the east side of Elie Harbour.]
The material which has filled up the vents is almost entirely
fragmental, varying from a coarse agglomerate to a fine volcanic tuff.
Some minor necks have been completely or in great part filled with
angular debris of the ordinary rocks of the neighbourhood. In the
western neck on the Largo shore, for example, which rises through the
red rocks of the Upper Coal-measures, the material consists largely
of fragments of red sandstone, clay and shale. Between Elie and St.
Monans, some of the necks are filled almost wholly with debris of black
shale and encrinal limestone.
There does not appear to be any relation between the diameter of a
funnel and the size of the blocks that now fill it. Some of the larger
necks, for example, consist of comparatively fine tuff. The Buddo Ness,
on the other hand, though so small a vent, is packed with blocks of
shale six feet long, while the sandstone through which the orifice has
been drilled passes, as usual, into quartzite for several yards away
from the edge. As an example of the general aspect presented by one
of the coarse agglomerates in the necks of the Fife coast, a view is
given in Fig. 216 of a portion of the neck at Ardross, about two miles
east from Elie. This thoroughly volcanic accumulation is here shown to
consist of blocks of all sizes heaped together without any definite
arrangement.
[Illustration: Fig. 216.--Agglomerate of neck on shore at Ardross, two
miles east from Elie.]
Since the first stage in the history of the vents has been the
perforation of the solid crust by explosion, and the consequent
production of debris from the disrupted rocks, we may hope to detect
underneath the pile of thoroughly volcanic ejections, traces of the
first explosions. I have been much struck with the fact that in the
East of Fife such traces may frequently be found here and there within
the outer border of the vents. At Largo, and again between Elie and
St. Monans, it may be noticed that the mass of material adhering to
the wall of a neck, exposed in ground-plan upon the beach, often
consists largely, or even wholly, of debris of sandstone, shale and
limestone, while the central and chief mass is made up of green tuff or
agglomerate, with occasional pieces of the surrounding stratified rocks
scattered through it. It seems probable, therefore, that the sections
of these Fife necks, laid bare on the present shore, do not lie far
below the original crater-bottoms.
Some light might be expected to be thrown upon the phenomena in
an active volcanic chimney by the condition of the fragments of
recognizable sedimentary rocks imbedded in the ejected debris which has
filled up the orifice. But the assistance from this source is neither
so full nor so reliable as could be wished. In some of the Fife vents,
indeed, the fragments of shale, sandstone and other sedimentary strata
are so unchanged that they cannot on a fresh fracture be distinguished
from the adjacent parent strata. The _Spirifers_, _Lingulæ_, crinoids,
cyprid-cases, ganoid scales and other fossils are often as fresh and
perfect in the fragments of rock imbedded in tuff as they are in the
rock _in situ_. In some cases, however, distinct, and occasionally even
extreme, metamorphism may be detected, varying in intensity from mere
induration to the production of a crystalline texture. The amount of
alteration has depended not merely upon the heat of the volcanic vent,
but also in great measure upon the susceptibility of the fragments to
undergo change and the duration of their exposure to it.
Dr. Heddle has computed the temperature to which fragments of shale,
etc., in tuff-necks of the Fife coast have been subjected. He found
that the bituminous shales have lost all their illuminants, and of
organic matter have retained only some black carbonaceous particles;
that the encrinal limestones have become granular and crystalline;
that the sandstones present themselves as quartzite, and that black
carbonaceous clays show every stage of a passage into Lydian-stone. He
inferred from the slight depth to which the alteration has penetrated
the larger calcareous fragments, that the heat to which they were
exposed must have been but of short continuance. As the result of his
experiments, he concluded that the temperature at which the fragments
were finally ejected from the volcanic vents probably lay between 660°
and 900° Fahr.[98]
[Footnote 98: _Trans. Roy. Soc. Edin._ vol. xxviii. p. 487.]
It may be perhaps legitimate to infer that, while the fragments that
fell back into the volcanic funnel, or which were detached from
the sides of the vent, after having been exposed for some time to
intense heat under considerable pressure, would suffer more or less
metamorphism, those, on the other hand, which were discharged by the
æriform explosions from the cool upper crust, on the first outburst of
a vent, would not exhibit any trace of such a change. Where, therefore,
we meet with a neck full of fragments of unaltered stratified rocks, we
may suppose it to have been that of a short-lived volcano; where, on
the other hand, the fragments are few and much altered, they may mark
the site of a vent which continued longer active. The metamorphism
of the fragmentary contents of a volcanic funnel by the action of
ascending vapours has already been described in the case of one of the
vents of the Carboniferous plateaux (vol. i. p. 404).
One of the most curious and puzzling features in the contents of the
tuff necks of the Fife coast is the occurrence there of crystals and
fragments of minerals, often of considerable size, which do not bear
evidence of having-been formed _in situ_, but have undoubtedly been
ejected with the other detritus. Dr. Heddle has noticed the fact,
and has described some of the minerals which occur in this way. The
following list comprises the species which he and I have found:--
Hornblende, in rounded fragments of a glassy black cleavable variety.
Augite, sometimes in small crystals, elsewhere in rounded fragments of
an augitic glass.
Orthoclase (Sanidine), abundant in worn twin crystals in the tuffs of
the east of Fife.
Plagioclase.
Biotite.
Pyrope, in the tuffs (and more rarely in the basalts) of Elie.
Nigrine, common in some of the dykes, more rarely in the tuffs of Elie.
Saponite, Delessite and other decomposition products.
Semi-opal, one specimen found in the later (Permian?) agglomerate of
Arthur's Seat.
Asphalt, abundant at Kincraig, near Elie.
Fragments of wood, with structure well preserved, may be included here.
Dr. Heddle has described from the neck of tuff at Kinkell, near
St. Andrews, large twin crystals of a glassy orthoclase, which are
invariably much worn, and preserve only rudely the form of crystals.
He justly remarks that they have no connection with drusy cavity,
exfiltration vein, or with any other mineral, and look as if a portion
of their substance has been dissolved away. Internally, however,
they are quite fresh and brilliant in lustre, though sometimes much
fissured.[99]
[Footnote 99: _Trans. Roy. Soc. Edin._ vol. xxviii. p. 223.]
The tuffs at Elie are full of similar crystals. I obtained from one
of the necks east of that village a specimen which measures 4 inches
in length, 3-1/2 in breadth, and 2-1/4 in thickness, and weighs about
2 lbs. It is, however, a well-striated felspar. From the same tuff
I procured an orthoclase twin in the Carlsbad form. All the felspar
pieces, though fresh and brilliant internally, have the same rounded
and abraded external appearance.
The fragments of hornblende form a characteristic feature in several of
the Elie dykes (to be afterwards described), and in the neighbourhood
of these intrusive rocks occur more sparingly in the tuff. It is a
glossy-black cleavable mineral, in rounded pieces of all sizes, up to
that of a small egg. Dr. Heddle obtained a cleavage angle of 124° 19',
and found on analysis that the mineral was hornblende.[100]
[Footnote 100: _Op. cit._ xxviii. p. 522.]
Augite occurs sparingly in two forms among the rocks. I have obtained
small crystals from the red agglomerate on the south side of Arthur
Seat, recalling in their general appearance those of Somma. Lumps of
an augitic glass have been found by Dr. Heddle, sometimes as large as
a pigeon's egg, in two of the dykes at Elie, and in the tuff at the
Kinkell neck, near St. Andrews. He observed the same substance at
the Giant's Causeway, both in the basalt and scattered through one of
the interstratified beds of red bole. Much larger rounded masses of
a similar augitic glass, but with a distinct trace of cleavage, have
already been referred to as occurring in a volcanic vent of Upper Old
Red Sandstone age at John o' Groat's House.[101]
[Footnote 101: _Op. cit._ xxviii. pp. 481 _et seq._, and _ante_, vol. i.
p. 352.]
Biotite is not a rare mineral in some tuffs. It may be obtained in
Lower Carboniferous tuffs of Dunbar, in plates nearly an inch broad;
but the largest specimen I have obtained is one from the same Elie
vent which yielded the large felspar fragment. It measures 2-1/2 × 2
× 1/2 inches. These mica tables, like the other minerals, are abraded
specimens.
That these various minerals were ejected as fragments, and have not
been formed _in situ_, is the conclusion forced upon the observer
who examines carefully their mode of occurrence. Some of them were
carried up to the surface by liquid volcanic mud, and appear in dykes
of that material like plums in a cake. But even there they present the
same evidence of attrition. They assuredly have not been formed in
the dykes any more than in the surrounding tuff. In both cases they
are extraneous objects which have been accidentally involved in the
volcanic rocks. Dr. Heddle remarks that the occurrence of the worn
pieces of orthoclase in the tuff is an enigma to him. I have been as
unable to frame any satisfactory explanation of it.[102]
[Footnote 102: Occasionally the crystals can be matched in some lava-form
rock of the same volcanic area; but many of them have no such
counterparts. See vol. i. p. 62 and _note_.]
[Illustration:
Fig. 217.--Ground-plan of volcanic neck, Elie Harbour, showing
circular deposition of the stratification.
T, Tuff of the neck, the arrows showing its inward dip; B B, Dykes;
S, Sandstones and shales, through which the neck has been opened.
]
It might have been thought that within the throat of a volcano, if in
any circumstances, loose materials should have taken an indefinite
amorphous aggregation. And, as has been shown in the foregoing
chapters, this is usually the case where the materials are coarse and
the vent small. Oblong blocks are found stuck on end, while small and
large are all mixed confusedly together. But in numerous cases where
the tuff is more gravelly in texture, and sometimes even where it is
coarse, traces of stratification may be observed. Layers of coarse
and fine material succeed each other, as they are seen to do among
the ordinary interstratified tuffs. The stratification is usually
at high angles of inclination, often vertical. So distinctly do the
lines of deposit appear amid the confused and jumbled masses, that
an observer may be tempted to explain the problem by supposing the
tuff to belong not to a neck, but to an interbedded deposit which
has somehow been broken up by dislocations. That the stratification,
however, belongs to the original volcanic vents themselves is made
exceedingly clear by some of the coast-sections in the East of Fife.
On both sides of Elie, examples occur in which a distinct circular
disposition of the bedding can be traced corresponding to the general
form of the neck. The accompanying ground-plan (Fig. 217) represents
this structure as seen in the neck which forms the headland at Elie
harbour. Alternations of coarse and fine tuff with bands of coarse
agglomerate, dipping at angles of 60° and upwards, may be traced round
about half of the circle. The incomplete part may have been destroyed
by the formation of another contiguous neck immediately to the east. To
the west of Earlsferry another large, but also imperfect, circle may
be traced in one of the shore necks. A quarter of a mile farther west
rises the great cliff-line of Kincraig, where a large neck has been
cut open into a range of precipices 200 feet high, as well as into a
tide-washed platform more than half a mile long. The inward dip and
high angles of the tuff are admirably laid bare along that portion of
the coast-line. The section in which almost every bed can be seen, and
where, therefore, there is no need for hypothetical restoration, is as
shown in Fig. 218.
I have already referred to the frequently abundant pieces of stratified
tuff, found as ejected blocks in vents filled with tuff, and to the
derivation of these blocks from tuff originally deposited within the
crater. There can, I think, be little hesitation in regarding the
stratification of these Fife vents as a record of successive deposits
of volcanic detritus inside the vents. The general dip inwards from the
outer rim of the vent strikingly recalls that of some modern volcanoes.
By way of illustration, I give here a section of part of the outer rim
of the crater of the Island of Volcano, sketched by myself in the year
1870 while ascending the mountain from the north side (Fig. 220). The
crater wall at this point consists of two distinct parts--an older tuff
(_a_), which may have been in great measure cleared out of the crater
before the ejection of the newer tuff (_b_). The latter lies on the
outer slope of the cone at the usual angle of 30°. It folds over the
crest of the rim, and dips down to the flat tuff-covered crater bottom,
at an angle of 37°. These are its natural angles of repose.
[Illustration: Fig. 218.--Section across the great vent of Kincraig,
Elie, on a true scale, vertical and horizontal, of six inches to a mile.
1, Sandstones, shale, etc., of Lower Carboniferous age, plunging down
toward the neck T; B, columnar basalt, shown also in Figs. 223 and
225.]
[Illustration: Fig. 219.--Dyke in volcanic neck, on the beach, St.
Monans.]
[Illustration: Fig. 220.--Section of part of crater rim, Island of
Volcano.]
Applying modern analogies of this kind, I have been led to conclude
that the stratification so conspicuous in the tuff of the vents in the
east of Fife and in the Carboniferous series of the Lothians belongs
to the interior of the crater and the upper part of the volcanic
funnel.[103] These stratified tuffs, on this view of their origin, must
be regarded as remains of the beds of dust and stones which gathered
within the crater and volcanic orifice, and which, on the cessation of
volcanic action, sometimes remained in their original position, or were
dislocated and slipped down into the cavity beneath. That the tuffs
consolidated on slopes, perhaps quite as steep as those of Volcano,
is now and then indicated by an interesting structure. The larger
stones imbedded in the layers of tuff may be observed to have on their
fronts in one direction a small heap of coarse gravelly debris, while
fine tuff is heaped up against their opposite side. This arrangement
doubtless points to deposit on a slope of loose debris, from which the
larger blocks protruded so as to arrest the smaller stones, and allow
the fine dust to gather behind.
[Footnote 103: Further illustrations of this characteristic structure of
some vents will be found in the account of the Tertiary vents of the
Faroe Isles in Chapter xli. See also the remarks in the introductory
chapters, vol. i. p. 63.]
If the inference be correct, that the stratification here described
belongs to the old craters or the upper parts of the funnels, it
furnishes additional evidence of the wide interval of time that elapsed
between the deposition of the Carboniferous strata and the outbreak
of these vents. During that interval prolonged denudation reduced
the upturned Carboniferous Limestone series to nearly its present
form of surface, and any materials discharged from the vents over the
surrounding ground would obviously lie with a violent unconformability
on the rocks below.
The frequent great disturbance in the bedding of the tuff within the
vents may be connected with some kind of collapse, subsidence or
shrinkage of the materials in the funnel below. That a movement of this
nature did take place is shown by the remarkable bending down of the
strata round the margins of the vents, which has been already described.
The minor vents for the most part contain only fragmentary materials;
but those of larger size usually present masses of lava in some
characteristic forms. In not a few cases, the lava has risen in the
central pipe and has hardened there into a column of solid rock.
Subsequent denudation, by removing most of the cone, has left the top
of this thick column projecting as a round knoll upon the hill-top.
Arthur Seat presents a good example of this structure. Where the
denudation has not proceeded so far, we may still meet with a remnant
of the cake of lava which sometimes overflowed the bottom of a crater.
The summit of Largo Law affords indications of this arrangement, the
cone of tuff being there capped with basalt, evidently the product of
successive streams, which welling out irregularly covered the crater
bottom with hummocks and hollows (Fig. 226). The knolls are beautifully
columnar, and sometimes show a divergent arrangement of the prisms.
[Illustration: Fig. 221.--Dyke rising through the agglomerate of a
volcanic vent; Kincraig, Elie.]
But the most frequent form assumed by the lava in the necks is that
of veins or dykes running as wall-like bands through the tuff or
agglomerate. Many admirable examples may be cited from the shore
between Largo and St. Monans. Two illustrations of them are given in
Figs. 219 and 221. In Fig. 219 the dyke is about four feet broad, and
is seen to present the common transverse jointing as it pursues its
way through the tuff. White veins of calcite along its margin serve to
define its limits. Its position in reference to the general body of the
neck is shown in the ground-plan Fig. 224. The second instance (Fig.
221) is that of a dyke of basalt only one foot wide, which runs like a
wall up the agglomerate of the Kincraig neck near Elie. It is seen at
the bottom of the cliff projecting from the agglomerate; but higher up
it has decayed, leaving its fissure as a gaping chasm. Here the general
character of the pyroclastic material is well brought out. One or two
large blocks may be seen imbedded in it, the largest lying above where
the dyke bends away to the left.
The intruded masses vary in breadth from mere threadlike veins up to
dykes several yards in breadth, which sometimes expand into large
irregular lumps. They generally consist of some form of basalt; now
and then, as at Ruddon Point, near Elie, they are amygdaloidal; and it
may be observed among them, as among dykes in general, that where the
amygdaloidal texture is developed, it is apt to occur most markedly in
the central part of the vein, the amygdales running there in one or
more lines parallel with the general trend of the mass.
That the basalt of these veins and dykes was sometimes injected in an
extremely liquid condition is shown by its frequently exceedingly close
homogeneous texture. Within the neck on the shore to the west of Largo,
the basalt assumes in places an almost flinty character, which here and
there passes into a thin external varnish of basalt-glass. A farther
indication of the liquidity of the original rock seems to be furnished
by the great number of included extraneous fragments here and there to
be observed in the basalt.
But besides basalt, other materials may more rarely be detected
assuming the form of dykes or veins within the necks. Thus, at
the Largo neck just referred to, strings of an exceedingly horny
quartz-felsite accompany the basalt--a remarkable conjunction of
acid and basic rock within the same volcanic chimney. To the east
of Elie some dykes, which stand out prominently on the beach from a
platform worn by the sea in a neck, consist of an extremely compact
volcanic mudstone, stuck full of the worn twin crystals of orthoclase
and pieces of hornblende and biotite already noticed. So like is
this rock to one of the decomposing basalts that its true fragmental
nature may easily escape notice, and it might be classed confidently
as a somewhat decayed basalt. A considerable amount of a similar fine
compact mudstone is to be seen round the edges of some of the Elie
vents. This material must have been injected into open fissures, where
it solidified. There is further evidence of the presence of "mud-lava"
in some of the vents of East Fife, where these orifices contain a
remarkable compact volcanic sandstone, composed of the usual detritus,
but weathering into spheroidal crusts, so as externally to be readily
mistaken for some form of basalt.
[Illustration: Fig. 222.--Radiating columnar dyke in the tuff of a
volcanic vent, Rock and Spindle, two and a half miles east from St.
Andrews.]
A columnar arrangement may often be observed among the basalt dykes.
When the vein or dyke is vertical, the columns of course seem piled
in horizontal layers one above the other. The exposed side of the
dyke then reveals a wall of rock, seemingly built up of hexagonal
or polygonal, neatly fitting blocks of masonry, as in the Lower
Carboniferous vent of the Binn of Burntisland (Figs. 166, 168). An
inclination of the dyke from the vertical throws up the columns to
a proportional departure from the horizontal. Sometimes a beautiful
fan-shaped grouping of the prisms has taken place. Of this structure
the Rock and Spindle, near St. Andrews, presents a familiar example
(Fig. 222). Much more striking, however, though less known, is the
magnificent basalt mass of Kincraig, to the west of Elie, where the
columns sweep from summit to base of the cliff, a height of fully 150
feet, like the Orgues d'Expailly, near Le Puy in Auvergne. The general
position of this basalt in the vent is represented in the section (B,
Fig. 218). The curvature of the basalt is shown in Fig. 223, which is
taken from the Elie side looking westward, beyond the intrusions, to
the picturesque cliffs of tuff. The details of the cliff are given in
Fig. 225.
That many of the dykes served as lines of escape for the basalt to the
outer slopes of the cones is highly probable, though denudation has
usually destroyed the proofs of such an outflow. A distinct radiation
of the dykes from the centre of a neck is still sometimes traceable.
This structure is most marked on the south cone of Largo Law, where
a number of hard ribs of basalt project from the slopes of the hill.
Their general trend is such that if prolonged they would meet somewhere
in the centre of the cone. On the south-east side of the hill a minor
eminence, termed the Craig Rock, stands out prominently (Fig. 209).
It is oblong in shape, and, like the dykes, points towards the centre
of the cone. It consists of a compact columnar basalt, the columns
converging from the sides towards the top of the ridge. It looks like
the fragment of a lava-current which flowed down a gully on the outer
slope of the cone (B' in Fig. 226).
Veins of basalt are not confined to the necks, but may be seen running
across the surrounding rocks. The shore at St. Monans furnishes some
instructive examples of this character. As the veins thin away from
the main mass of basalt they become more close-grained and lighter in
colour, and when they enter dark shales or other carbonaceous rocks
they pass, as usual, into the white earthy clay-like "white-trap." The
influence of carbonaceous strata in thus altering basic dykes and sills
may be instructively studied along the shore of the East of Fife. A
good instance occurs near St. Monans Church (Fig. 227), where a vein of
"white-trap" traverses black shales which have been somewhat jumbled.
[Illustration: Fig. 223.--View of part of the shore front of the great
vent at Kincraig, looking westward, with the columnar basalt in front.]
In a modern volcano no opportunity is afforded of examining the contact
of the erupted material with the rocks through which the vent has
been opened. But in the basin of the Firth of Forth, within the area
now under description, a numerous series of coast-sections lays bare
this relation in the most satisfactory manner. The superincumbent
cones of tuff have been swept away, and we can examine, as it were,
the very roots of the old volcanoes. The margin of a neck or volcanic
vent is thus found to be almost always sharply defined. The rocks
through which the funnel has been drilled have been cut across, as if
a huge auger had been sunk through them. This is well displayed in
the beautifully perfect neck already cited at Newark Castle, near St.
Monans (Fig. 224). The strata through which this neck rises consist
of shales, sandstones, thin coal and encrinal limestones, dipping
in a westerly direction at angles ranging from 25° to 60°. At the
south end of the neck they are sharply truncated, as if by a fault.
Elsewhere they are much jumbled, slender vein-like portions of the tuff
being insinuated among the projecting strata. A large vertical bed
of sandstone, 24 yards long by 7 yards broad, stands up as a sinuous
reef on the east side of the vent (_s_). It is a portion of some of
the surrounding strata, but, so far as can be seen at the surface, is
entirely surrounded with agglomerate. Here and there the shales have
been excessively crumpled, and at the north end have been invaded by
a vein of basalt which, where it runs through them, assumes the usual
clay-like character. The strata have been blown out, and their place
has been occupied by a corresponding mass of volcanic agglomerate. But
their remaining truncated edges round the margin of the orifice have
undergone comparatively little alteration. In some places they have
been hardened, but their usual texture and structure remain unaffected.
[Illustration: Fig. 224.--Plan of volcanic neck on beach near St.
Monans.
T, Neck of tuff enclosing a mass of sandstone (_s_), and piercing
sandstones and shales With beds of limestone, (_l_ _l_), and a thin
seam of coal (_c_); B, Basalt "white-trap" dyke. The arrows show
the dip of the strata.
]
In a few examples, the progress of denudation has not advanced so far
that the cone cannot still be partially made out amidst its surrounding
masses of tuff. One of the most interesting of these is Largo Law, of
which an outline has been given in Fig. 209. The accompanying section
(Fig. 226) represents what appears to me to be the structure of this
hill. Each of the two now conjoined cones was probably in succession
the vent of the volcano. The southern and rather lower eminence, as
already mentioned, is traversed by rib-like dykes of basalt, which
point towards its top, where there is a bed of the same rock underlying
a capping of tuff. On its eastern declivity lies the basalt stream
already described (p. 87). The higher cone is surmounted by a cake of
basalt which, as I have above suggested, may have solidified at the
bottom of the latest crater. Of course all trace of the crater has
disappeared, but the general conical form of the volcanic mass remains.
Doubtless, still more of the old volcano would have been removed by
denudation but for the protection afforded to the tuff by the intrusion
of the basalt. The upper dotted lines in the figure are inserted merely
to indicate hypothetically how the cone may originally have stood.
On the west side the sheets of tuff which were thrown out over the
surrounding country have been almost entirely removed, but on the east
and south they still cover an extensive area. (See Fig. 208).
[Illustration: Fig. 225.--Columnar basalt in the neck of Kincraig,
Elie, seen from the west.]
[Illustration: Fig. 226.--Section across Largo Law.
_l_ _l_, Lower Carboniferous strata; _t_, tuff of cones; _t'_, tuff
of plain beyond the cones; B B, basalt ascending vents and sending
out veins: B', basalt which has probably flowed out at the surface.
The dotted lines are suggestive of the original outline of the hill.
]
(2) _Sills._--In the Clyde coal-field and in the basin of the Firth
of Forth, among the vast number of sills which there traverse the
Carboniferous formations, it is possible that some belong to the
Permian volcanic period (see vol. i. p. 474). Where the sheets have
been intruded along horizons that lie below the upper stratigraphical
limit of the puy eruptions, they may not unnaturally be held to belong
to these manifestations of volcanic energy, though it is obviously
quite conceivable that some of them may be of much later date. But
where they lie above the highest platforms of Carboniferous lavas and
tuffs, they may be assigned to a younger volcanic period. We know as
yet of only two such periods after the deposition of the Carboniferous
Limestone series in Scotland--Permian and older Tertiary. Unless,
therefore, these higher sills formed part of some other display
of subterranean activity which is not known to have culminated in
eruptions at the surface, they must be looked upon as probably either
Permian or Tertiary.
In the great coal-field of Stirlingshire and Lanarkshire, among the
large sills that break into the Millstone Grit and the Coal-measures,
one lies entirely in the Coal-measures, and covers about six square
miles of ground, stretching from near Caldercruix Station, a little
east of Airdrie, to near Kirk of Shotts, a distance of about four
miles. A group of smaller sheets, possibly connected with the larger
mass, runs for four miles further west to beyond New Monkland. Another
chain of sills, which may also be part of the same great intrusion,
extends from the Cant Hills, near the Kirk of Shotts, for more than
eight miles in a north-easterly direction. The largest mass in this
chain stretches from Blackridge, west of Bathgate, for upwards of three
miles, covering an area of about three square miles and terminating on
the north at the line of dislocation which has been followed by one
of the east and west dykes. Another large sill, which appears nearly
two miles further east on the north side of that dyke, lies on a lower
stratigraphical horizon, for it cuts the Carboniferous Limestone
series, and does not reach the top of the Millstone Grit. This sill is
cut through by two of the later dykes.
[Illustration: Fig. 227.--Vein of "white-trap" cutting black
carbonaceous shales, a little west from St. Monans Church.]
That these great intrusions took place later than the deposition of
the Coal-measures is obvious. There is no satisfactory evidence to
enable us to decide to which of the two post-Carboniferous volcanic
periods they may with most probability be assigned. As one of them is
distinctly cut by dykes that have been referred to the Tertiary series,
it might be plausibly argued that it at least is of pre-Tertiary date,
and therefore probably Permian. On the other hand, as will be shown in
a later chapter, some portion of the sills appears to be connected with
the younger or Tertiary dykes. This problem must for the present remain
unsolved.
In Ayrshire where, as already described, basic sills traverse the
Permian volcanic series, other large intrusive sheets are found around
the Permian basin. On the north side an important group of them,
passing through the Coal-measures into the Carboniferous Limestone
series, runs from Troon eastward for more than eight miles to beyond
Craigie. On the south side a much more extensive series may be traced
from the River Ayr southwards into the Dalmellington coal-field, and
thence north-eastwards in a wide semicircular sweep into the coal-field
of New Cumnock and Airds Moss. That some of these sills proceed from
the Permian necks has been definitely ascertained, and this fact has
been already alluded to in connection with the vents. I have little
doubt that the great majority, if not the whole, of these intrusive
sheets are to be referred to the Permian period.
Some of the sills must be later than a part of the Permian volcanic
eruptions, for they are found in at least three places intercalated in
the zone of lavas and tuffs. But no instance has been observed of their
traversing the basin of Permian sandstone which overlies that zone,
though a few dykes, possibly of Tertiary age, do cut this sandstone.
CHAPTER XXXII
PERMIAN VOLCANOES OF ENGLAND
The Devonshire Centre--Eruptive Rocks of the Midland
Coal-fields.
From the south of Scotland we need to pass to the extreme south-west of
England before we again encounter a group of volcanic rocks which may
be referred with some confidence to the Permian period. An interesting
group of lavas and tuffs has been preserved in some of the valleys over
a limited area in the east of Devonshire. The Midland coal-fields,
however, are traversed by a series of basic eruptive rocks which are
younger than the Coal-measures, and may possibly be Permian. Their mode
of occurrence, and the arguments regarding their geological age, will
be given in the present chapter.
1. DEVONSHIRE
The counties of Devon and Cornwall furnish one of the most striking
examples to be met with in Britain of the persistence of volcanic
action over a limited area through a long succession of geological
periods. The extensive eruptions in Devonian time were followed after
a long interval by a diminished series in the Carboniferous period.
But the subterranean energy was not then wholly exhausted, for it
showed itself on a feeble scale in at least one limited tract of the
same region during the Permian period. Thus throughout the later half
of Palæozoic time the extreme south-west of England continued to be a
theatre of volcanic action.
The geological age of the igneous rocks now to be referred to depends
upon the particular place in the geological record to which we assign
the remarkable breccias and sandstones with which they are associated.
By many geologists who have been unable to recognize any true break
in the red rocks from their base up to the bottom of the Lias, these
strata have been grouped as one great series referable to the "New
Red Sandstone" or Trias. This is the classification adopted on the
one-inch maps of the Geological Survey. On the other hand, various able
observers have pointed out the close resemblance of the coarse and fine
breccias at the bottom of the series to recognized Permian deposits
in the centre of England and to parts of the typical Rothliegende
of Germany. I need only refer to the strongly expressed views of
Murchison, in which, as he stated in his _Siluria_, he "entirely agreed
with Conybeare and Buckland, who, after a journey in Germany in 1816,
distinctly identified the Heavytree conglomerate, near Exeter, with
the Rothliegende of the Germans."[104] In the absence of any fossil
evidence, we have only lithological characters and sequence to guide
us, and though the known facts hardly warrant a very positive opinion,
my inclination is to regard these red Devonshire breccias as probably
Permian, and to follow Murchison in looking upon their associated
igneous masses as furnishing additional reason for assigning them to
that particular geological platform.[105]
[Footnote 104: _Siluria_, 4th edit. (1867), p. 333. See also Berger,
_Trans. Geol. Soc._ vol. i. (1811), pp. 98-102; Conybeare and Phillips,
_Geology of England and Wales_, p. 313, footnote; De la Beche, _Report
on the Geology of Cornwall, Devon and West Somerset_ (1839), chap.
vii. p. 193. Messrs. Hull and Irving (_Quart. Journ. Geol. Soc._ vol.
xlviii. 1892, pp. 60, 68) have more recently discussed the subject, and
follow the view of Murchison.]
[Footnote 105: Murchison cogently argued that as no signs of volcanic
activity were known in the Trias, but were abundant in the Permian
system, the Devonshire rocks might be regarded as appertaining to the
older series, _op. cit._ Triassic volcanic rocks, however, are now well
known on the Continent.]
No proper account has yet been written of the volcanic group which
I now propose to describe.[106] De la Beche was, I think, the first
to recognize the true volcanic nature of the rocks and their
contemporaneous interstratification in the red sandstone series.[107] As
traced by him on the Geological Survey maps, these rocks lie at or near
the base of the red sedimentary deposits, resting sometimes directly
on the Culm-measures, sometimes on an intervening layer of red strata.
He found them in three separate districts in the neighbourhood of
Exeter, the most northerly lying near Tiverton, the central extending
from Kellerton for a few miles up the Yeo Valley, beyond Crediton,
and the third stretching from the City of Exeter to Pen Hill, about
five miles to the south-west. He recognized the amygdaloids as slaggy
lavas, and saw that the volcanic breccias and tuffs are interleaved
with the sandstones. With regard to the probable vents from which these
materials were ejected, he thought that the chief centre of activity
lay at Kellerton Park, while in other localities he believed the bosses
of igneous rock "to descend in mass downwards, as if filling up some
crater or fissure through which these rocks had been vomited."[108] He
speaks also of "quartziferous porphyries" occurring among them, a
statement which, if petrographically accurate, would suggest the uprise
of a later more acid lava in some of the vents.
[Footnote 106: An outline of some of their characters will be found in a
paper by Mr. W. Vicary in _Trans. Devonshire Assoc._ 1865, vol. i. part
iv. p. 43.]
[Footnote 107: See his "Report" cited in the note above. De la Beche
quotes J. J. Conybeare as pointing out the intimate connection of these
igneous and stratified rocks (_Annals of Philosophy_, 2nd series, vol.
ii. (1821) p. 165); but this author wrote at the time of the Plutonist
and Neptunist controversy, and does not commit himself to any distinct
expression of opinion on the subject.]
[Footnote 108: Report, p. 201.]
More recently the ground has been revised by Mr. W. A. E. Ussher of the
Geological Survey, who has ascertained that the volcanic rocks appear
in many more places than those where they were noted on the older
maps, and likewise extend for some miles further to the north and west.
It now appears that in the central and chief district the lavas can
be followed westward from Spray Down near Kellerton to Greenslade
near North Tawton, a distance of about twenty-one miles. Their most
northerly outcrop is at Thorn above Loxbere in the Tiverton district,
and their most southerly visible portion passes under the Cretaceous
rocks of Pen Hill. The distance between these extreme points is
likewise about twenty-one miles. The whole display of volcanic
phenomena is comprised within an area of less than 400 square miles.
One of the most obvious features in this volcanic tract is the way in
which the erupted materials lie along the lines of hollow or valley in
which the red rocks were deposited. This is most distinctly exhibited
in the central district. Here a belt of breccias and sandstones,
varying from one to three and a half miles in breadth, runs for about
five and twenty miles westward in a depression of the Culm-measures.
At intervals, the lavas which lie near the base of the red rocks crop
out along the margin of the belt throughout most of its extent. But
they do not spread out over the older rocks, and they have evidently
been erupted from orifices situated along the line of the valley.
It is another example of the relation between the trend of hollows
and the outbreak of volcanic vents, which I have referred to as so
strikingly displayed in the distribution of the Permian volcanic rocks
of south-western Scotland.
The volcanic materials of the Devonshire Permian district consist
mainly of lavas, but include also red sandy and gravelly tuffs. The
whole volcanic group is remarkably thin, never attaining even the
limited development of the Ayrshire series. No adequate petrographical
investigation of these rocks has yet been made. Externally, as seen
in the quarries and lanes, the lavas present the closest resemblance
to those of the Permian basins of Ayrshire and Nithsdale. They show
considerable differences of texture even within the same mass, some
portions being dull, fine-grained purplish-red rocks, with scattered
pseudomorphs of hæmatite and a few porphyritic felspars, other parts
passing into an exceedingly coarse amygdaloid or slaggy pumice. Dr.
Hatch, after a microscopical examination of a small collection of
specimens, found that while most are olivine-basalts, containing
ferruginous pseudomorphs after olivine (Raddon Court, Pocombe, and near
Budlake), others are true andesites (Ide, Kellerton Park) and even
mica-trachytes (Copplestone, near Knowle Hill).[109] As already remarked,
some of the older writers mention the existence of quartz-porphyries.[110]
[Footnote 109: _Geol. Mag._ 1892, p. 250. The rocks have been more
recently described by Mr. B. Hobson, _Quart. Jour. Geol. Soc._ vol.
xlviii. (1892), p. 502. The rock of Kellerton Park is called by Mr.
Hobson "mica-augite-andesite," and he gives a chemical analysis of it
by Mr. E. Haworth, _op. cit._ p. 507. Mr. Watts has lately found one of
the orthoclase rocks to be rich in olivine.]
[Footnote 110: See De la Beche, _Report_, pp. 203, 204. My colleague, Mr.
Ussher, found close to the Thurlestone outlier of conglomerate near
Kingsbridge, Devonshire, a small boss of quartz-porphyry which rises
through and alters the Devonian rocks. The actual junction of this
mass with the conglomerate is not seen, nor have any fragments of the
porphyry been noticed among the pebbles.
Mr. Ussher informs me that in the quarry the visible exposure of
the acid rock is surrounded an covered by mica-porphyrite, probably
andesite.]
The geographical conditions in which the red rocks of Devonshire
accumulated were those so characteristic of the Permian and Trias
formations throughout Britain. The red sandstones and sandy marls
gathered in inland basins, where the water seems to have become too
saline and bitter to support animal life. The strata are consequently
singularly devoid of organic remains. The climate was probably arid,
and the absence or scarcity of traces of terrestrial vegetation
indicates that the land around the water-basins stretched in wide
sandy and rocky wastes. In the dry atmosphere and under the influence
of rapid radiation the cliffs and crags of Culm-measures would
disintegrate into angular rubbish, and this material, slipping into the
lakes or washed down by occasional rain-storms, forms now the breccias
that constitute so typical a feature in the Permian system.
[Illustration: Fig. 228.--Section at Belvedere, S.W. of Exeter.
_a_, Culm-measures; _b_, breccia and marls; _c_, lavas; _d_, red pebbly
sandstones.]
[Illustration: Fig. 229.--Diagram to show the unconformability and
overlap of the Permian rocks in the Crediton Valley.
_a_, Culm-measures; _b_, breccias and sandstones; _c_, lava-group; _d_,
breccias with fragments of lava passing up into sandstones and marls
(_e_).]
It was while this geographical type continued in the South-west of
England that the volcanic eruptions took place which we are now
considering. De la Beche correctly referred these eruptions to the
early part of the red sandstone series. A brief examination of the
ground suffices to show that although, as he pointed out, the volcanic
rocks lie towards the base of that series, as shown in Fig. 228, they
do not all occupy the same platform. That in some cases the lavas lie
directly on the Culm-measures, while in others they are separated
from these strata by 100 feet or more of red sandstones and breccias
(Fig. 229), would not in itself be proof of any difference of age or
stratigraphical position in the igneous rocks, for the floor on which
the Permian formations were here laid down can be shown to have been
singularly uneven. Prominent hills of Culm grit, several hundred feet
high, rose above the basins in which the earliest Permian sediments
were deposited, and these eminences were gradually submerged and buried
under the detritus.
But that the volcanic zone includes in some places more than one
outflow of lava with layers of sandstone, breccia and tuff between the
successive sheets may be proved in different parts of the district.
Thus the two conspicuous hills at Kellerton are composed of several
sheets of highly slaggy lava, separated by breccia, and a third much
thinner sheet lies above these, intercalated in a mass of breccia,
sandstone and sandy tuff (Fig. 230). Again, at Budlake the sandstones
and fine breccias include a thin band of vesicular lava, while farther
to the east they are interrupted by a higher and thicker zone of
similar material.
[Illustration: _North Hill_ _South Hill_
Fig. 230.--Section of the volcanic series at Kellerton, Devonshire.
_a_, Breccias and sandstones; _b_, lavas.]
These igneous sheets can be shown by many interesting sections to have
been poured out contemporaneously with the deposit of the sedimentary
material among which they occur. At Crabtree, for instance, near
Kellerton, the uppermost lava is a thin sheet of highly slaggy texture,
which rests immediately on the gravelly red sandstone and catches
up parts of it, while the pebbles include fragments of some of the
andesites below. The dark lavas are occasionally traversed by veins of
fine hard sandstone, which descending from above, like those in the Old
Red Sandstone and Permian lavas of Scotland, have been produced by the
silting or drifting of fine sand into cracks in the lava, before the
igneous material was entirely buried. These features are well exposed
in the high ridge of the Belvedere near Exeter (Fig. 228), where, over
a thin and inconstant band of red breccia and marl which rests on the
upturned ends of the Culm-measures, a band of dull-red andesite may
be seen. This rock, partly compact and partly highly amygdaloidal, is
in some portions full of irregular fissures and cavities filled with
sandstone.
Nowhere among the Palæozoic volcanic rocks of Britain are more
remarkable examples of the slaggy structure to be found than in these
Devonshire lavas of probably Permian age. I would especially cite the
rock of Knowle Farm, a few miles to the west of Crediton, as in part a
mere spongy pumice, blocks of which would originally have floated in
water.
One of the best sections in the district for the exemplification of
the internal structures of these lavas is that in the large quarry at
the top of Posbury Hill. On the west side of this quarry the rock is
tolerably compact, but contains vesicles and irregular steam-holes. On
the east side it passes upward and laterally into a coarse agglomerate
of its own fragments, and in its mass it encloses similar agglomerate.
No sharp passage can be traced between the two rocks. So far as I could
judge, it seemed to me that the lava had broken up as it moved along,
possibly shattered by coming in contact with water. The agglomerate is
overlain by some reddish ashy sandstone, which fills up the interstices
between the slags, and is immediately covered by a bed of lilac
andesite, marking another distinct outflow.
[Illustration:
Fig. 231.--Section of agglomerate overlain with sandstone and
andesite, Posbury, Crediton.
]
As in Ayrshire, the lavas of Devonshire are not accompanied by any
thick accumulation of tuff. The fragmentary discharges consisted
in both areas of fine dust and gravelly detritus of small lapilli,
which were not ejected in such quantities as entirely to conceal the
ordinary non-volcanic sediment of the water-basin. The dust and cinders
mingled with the red sand and angular scree-material, so that we now
see a group of red, somewhat ashy sandstones and breccias. Among the
component fragments of the breccias, a considerable variety of igneous
material may be observed. While the most of the non-volcanic stones
may have been derived by ordinary processes of weathering from rocks
exposed at the surface, it is by no means improbable that some of them,
including even pieces of Culm grit, killas and baked slate, may have
been ejected from volcanic vents.[111]
[Footnote 111: On the composition of the Devonshire breccias see Mr. R.
N. Worth, _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 69. This
author has adopted the view that the granite of Dartmoor represents the
neck of a great volcano from which these later volcanic materials were
ejected. But all the evidence seems to me in favour of numerous small
vents situated not far from the outcrops of the lavas, as stated in
the text. See Mr. B. Hobson, _Quart. Journ. Geol. Soc._ vol. xlviii.
(1892), p. 498. The Dartmoor granite is later than the surrounding
Carboniferous rocks, but no good evidence has been obtained to connect
it with the Permian volcanic phenomena of Devonshire.]
Taking the volcanic rocks of this district as a whole, I regard them
as the mere edges of sheets that have flowed from vents which not
improbably lie concealed somewhere along the centres of these old
Permian valleys. No visible necks have been described from any part
of the area, and though I have not examined the whole of it, nothing
of that nature was detected by me either in the Crediton Valley or
between Silverton and the Exeter neighbourhood. The Tiverton district,
which has not yet been searched, appears to be the only tract where any
chance remains of finding some of the vents.
No indication of any sills has been met with among the Devonshire
Permian rocks. None of the lavas which I have seen have the internal
characters of true sills, while in the field their association with the
sandstones and breccias in no observed case points to intrusion.
Though much remains to be done in this region before an adequate
account can be given of the interesting series of eruptions which
concludes the long volcanic history of the South-west of England,
enough is known to indicate the general character of the phenomena.
The eruptions were on even a feebler scale than those of the Permian
period in Scotland, but they seem to have resembled them in their
general character. Small puy-like vents were opened, from which
dark scoriaceous lavas and showers of gravelly tuff and stones were
discharged over the floor of the inland sea or lake-basin in which
the red sandstones and breccias were accumulated. These outflows and
explosions took place too, as in Scotland, towards the beginning of
the deposition of the red strata, and entirely ceased long before that
deposition came to an end. In each area the eruptions mark the close
of Palæozoic volcanic activity in Britain. The varied and recurrent
volcanic episodes which distinguished each successive geological period
from the Archæan onwards now definitely terminate, not to be resumed
until after the passing of the whole of the vast cycle of Mesozoic ages.
2. ERUPTIVE ROCKS IN THE MIDLAND COAL-FIELDS
Between the thick and thoroughly marine development of the
Carboniferous Limestone in Derbyshire and in South Wales, there lies
the region, already referred to, wherein both the Carboniferous
Limestone and Millstone Grit die out against what must have been a
ridge of land or group of islands that stretched in a general east
and west direction from the high grounds of Wales through Shropshire,
Staffordshire and Leicestershire. On the slopes of this ridge the
limestone is gradually overlapped by the Millstone Grit, and both are
in turn overlapped by the Coal-measures, which are then found lying
immediately on the more ancient rocks of the region--Cambrian or
pre-Cambrian, Silurian and Old Red Sandstone. The gradual subsidence
that led to the deposit of several thousand feet of Carboniferous
strata over the regions to north and south, before the beginning of
the Coal-measure period, does not seem to have sensibly affected the
persistence of this old terrestrial surface, which probably lay on an
axis of upward movement, so that, amidst the surrounding depression,
its position above water was on the whole maintained. But there are
indications that the inequality of movement in this part of the earth's
crust was of much older date than the Carboniferous period. The Old Red
Sandstone is conformably continuous below the base of the Carboniferous
system, and in Wales is estimated to be some 10,000 feet thick. No
break has yet been detected in this vast accumulation of sedimentary
material, though it is highly probable that some such unconformability
must exist in it as that between the Scottish Lower Old Red Sandstone,
which passes down into the Upper Silurian shales, and Upper Old Red
Sandstone, which graduates upward into the base of the Carboniferous
formations. But even if such a break should be discovered, it will
not account for the position of the Coal-measures on Cambrian or even
perhaps older rocks. It is hardly conceivable that, had these rocks
been covered with a full development of Old Red Sandstone, they could
have been stripped of it by denudation before the deposition of the
Coal-measures. It seems much more probable that the discrepancy in the
terrestrial movements had commenced in Old Red Sandstone time, and that
these ridges of ancient Palæozoic rocks never sank below the waters in
which the vast thickness of red sandstones, marls and conglomerates was
laid down.[112]
[Footnote 112: See a discussion of this subject in Jukes' Preface to his
_South Staffordshire Coal-field_.]
But apart from the question of its antiquity, this tract of persistent
land has a special interest in the history of volcanic action in
Britain, for it was the scene of some remarkable protrusions of
eruptive material which took place after a part, and possibly after
the whole, of the Coal-measures were accumulated. The date of these
protrusions cannot be fixed with greater precision; but there can be
no doubt that they belong to one of the later volcanic periods in the
geological history of Britain, and the account of them is therefore
included in the present Chapter of this work.
In the English Midlands south of Stafford, over a tract of country
about 700 square miles in extent, stretching from Birmingham on the
east, across the vale of the Severn, to the uplands of Shropshire
on the west, the Coal-measures, partly isolated into outliers by
denudation and partly separated by overlying younger formations, are
pierced by masses of intrusive igneous rocks. Many of these masses
have long been familiar to geologists. Those, for example, of the
Clee Hills of Shropshire, and the Rowley, Barrow and Pouk Hills of
Staffordshire and Worcestershire, have been frequently described, their
relations to the surrounding strata have been minutely sought out,
their composition has been chemically determined, and their microscopic
structure has been investigated. But they have been studied rather as
individual masses of local importance. No attempt has yet been made to
ascertain how far they are capable of being grouped together as one
connected series, linked with each other in chemical and mineralogical
characters, and containing a definite record in the volcanic history of
the country. This is a task which, it is to be hoped, some competent
inquirer will before long undertake.
In the meantime it is only possible to review here the already
published information, and to gather from it what may at present be
surmised to have been the history of these later eruptions of the
Midlands.
The areas where the igneous rocks now to be described are exhibited may
be conveniently placed in the following five groups:--1st, Titterstone
Clee Hill; 2nd, Brown Clee Hill; 3rd, The Forest of Wyre Coal-field;
4th, The Coalbrookdale Coal-field; and 5th, The South Staffordshire
Coal-field.
1. _The Titterstone Clee Hill_ forms a ridge about seven miles long
and a mile and a quarter broad, running in a north-easterly direction
over the Old Red Sandstone uplands of the south of Shropshire. The
ground rises gradually towards the south-west, until it reaches there a
height of 1754 feet (Fig. 232). On the north-western side of the ridge,
the last vanishing representative of the Carboniferous Limestone can
be seen to be overlapped the Millstone Grit, which, as it is traced
towards the south-west, is in turn overlapped by the Coal-measures,
and these, about 400 feet thick, then rest immediately on the Old
Red Sandstone. Two sheets of columnar olivine-dolerite, possibly
originally connected, lie as cakes on the summit and eastern slope
of the ridge, and cover in all a space of about a square mile and a
half. The larger sheet, which varies from 60 to 180 feet in thickness,
overlies the Coal-measures, and the coals of the Cornbrook coal-field
have been worked underneath it. The smaller mass, which may be 300
feet in thickness, forms the summit of the ridge. On its eastern side
it reposes on Coal-measures, which are there much disturbed; but on
the west side, where it forms a bold capping to the escarpment, it is
underlain at once by the Old Red Sandstone. There cannot be any doubt
that these masses of eruptive material are sills, which have been
injected into the Carboniferous strata, and partly between these strata
and the Old Red Sandstone. One or more dykes of eruptive rock have
been met with in mining, and the coal on approaching them undergoes
alteration.[113]
[Footnote 113: See J. R. Wright, _Trans. Geol. Soc._ (2nd ser.) iii.
(1832), p. 487. Titterstone Clee Hill is shown on Sheet 55 N.E. and
N.W. of the Geological Survey, and in Horizontal Sections, Sheets 33
and 36, from which Fig. 232 is reduced. The microscopic structure of
the dolerite has been described by Mr. Allport, _Geol. Mag._ 1870, p.
159; _Quart. Journ. Geol. Soc._ xxx. (1874), p. 550.]
[Illustration: Fig. 232.--Diagrammatic section across Titterstone Clee
Hill.
1. Old Red Sandstone; 2. Carboniferous Limestone; 3. Millstone Grit; 4.
Coal-measures; 5 5. Columnar olivine-dolerite.]
2. _Brown Clee Hill_ consists of two outliers of Coal-measures, each
about a mile long, placed on the summit of a broad ridge of Old Red
Sandstone, and rising to a height of 1800 feet above the sea. Both of
the outliers is capped with a cake of dolerite, and a third smaller
patch of the same material lies on the southern outlier between the
cappings. Neither at this locality nor around Titterstone Clee have any
eruptive rocks been observed rising through the older strata. It is
evident that in both cases the orifices or fissures up which the molten
material rose lie concealed under the surviving cakes of dolerite.[114]
[Footnote 114: Brown Clee Hill is mapped in Sheet 61 S.W. of the
Geological Survey, and its structure is shown in Sheet 36 of the
Horizontal Sections.]
3. _Forest of Wyre Coal-field._--On both sides of this extensive tract
of Coal-measures, the strata near the base of the series are traversed
by sills or dykes of olivine-dolerite like that of the Clee Hills. The
sandstones in contact with the eruptive rock have been indurated. In
this district, also, the evidence shows that the sheets are intrusive,
and later than the portion of the Coal-measures there visible.[115]
[Footnote 115: This district is represented in Sheets 55 N.E. and 61 S.E.
of the Geological Survey. The microscopic structure of the larger mass
on the west side of the coal-field, and the variations in the minute
structure of the intrusion which forms a long ridge on the east side,
are described by Mr. Allport, _Quart. Journ. Geol. Soc._ xxx. pp. 550,
551.]
4. _Coalbrookdale Coal-field._--In this interesting district a sill of
rather finely crystalline olivine-dolerite, which is estimated to be
nearly 200 feet thick, is traceable from near Little Wenlock for three
miles to the north, intercalated between the Carboniferous Limestone
and the Silurian rocks underneath. It appears to underlie the western
part of the Coal-field, for it is exposed by denudation in several
valleys between Little Wenlock and Great Dawley. Owing to the thinning
out of the Carboniferous Limestone in an easterly direction, the sill
gradually comes to have the Millstone Grit on its upper surface, and at
one point is represented on the Geological Survey map as even intruded
into the Coal-measures. Here again we have an intrusive sheet of later
date than at least the earlier part of the Coal-measures, and no
evidence of any superficial outflow of volcanic material.[116]
[Footnote 116: The Coalbrookdale coal-field has been described by Sir
Joseph Prestwich, _Trans. Geol. Soc._ (2) v. p. 428; and Prof. E. Hull,
_Quart. Jour. Geol. Soc._ xxxiii. (1877), p. 629. The minute structure
of the sill at Little Wenlock is referred to by Mr. Allport, _op. cit._
p. 550. The ground is mapped on Sheet 61 N.E. of the Geological Survey,
and its structure is shown on Sheet 54 of the Horizontal Sections.]
5. _South Staffordshire Coal-field._--This district, in respect to its
igneous intercalations, has been much more fully examined and described
than any of the others. It forms the subject of an exceedingly able
memoir by Jukes, who carefully studied its geology and delineated it
on the maps and sections of the Geological Survey. Since his time
the rocks have been studied microscopically, but no material facts
regarding the stratigraphy have been obtained in addition to those
which he patiently collected and generalized upon.[117]
[Footnote 117: Jukes, "South Staffordshire Coal-field," _Mem. Geol.
Surv._ 2nd edit. (1859). The area is embraced in Sheet 62 N.W. and S.W.
of the Geological Survey, and is illustrated in Sheets 23, 24 and 25 of
the Horizontal Sections.]
This coal-field is above 20 miles long and 5 miles broad. Its strata
rest unconformably on Upper Silurian strata, which, as part of the
ancient ridge or island already referred to, project here and there
from amidst the Coal-measures. The boundaries of the field on the east
and west sides are chiefly made by faults which bring down Permian and
Triassic formations against the Carboniferous strata.
Throughout this coal-field sheets of igneous rock are abundant. In
the detailed account of them given by Jukes in his admirable essay
on the South Staffordshire Coal-field,[118] he distinguished two kinds
of igneous material--"basalt," which comes out at the surface, and
sometimes overlies the Coal-measures in large cakes like that of
the Rowley Hills, which extends for two miles in one direction and
more than a mile in another; and "greenstone," which burrows among
the coal-bearing strata, and gives off dykes and veins of "white
rock-trap." There does not appear, however, to be any essential
difference in composition, age or origin between these contrasted kinds
of igneous material. They not improbably all belong to one series of
extrusions, their distinctions being due rather to the conditions
under which they were erupted, and in particular to their comparative
thickness, and the influence of adjacent coals and carbonaceous shales
upon them.
[Footnote 118: _Op. cit._ p. 117.]
The igneous rocks seen at the surface in this district form a series
of well-marked eminences. Of these the largest extends as a ridge from
Dudley to beyond Rowley Regis, a distance of more than two miles. To
the west of this tract, a number of small patches of the same material
crop out at the surface, the most important forming Barrow Hill. Six
miles farther north another group of similar patches may be seen. Of
these the largest occurs at Wednesfield, but the most noted forms the
Pouk Hill, which has long been noted for the beauty of its columnar
structure.
The sheets of "greenstone" met with in the coal-field are more
numerous and extensive than the detached areas of more compact rock
visible above ground, a single sheet being sometimes traceable in the
coal-workings for two miles in one direction.
The eruptive rocks of this district, when examined in their freshest
form, consist of well-preserved olivine-dolerite. An examination of
the "greenstone" and the "white rock-trap," which runs in fingers and
threads through the coal, shows that these are really the same dolerite
which has undergone alteration, the ferruginous silicates having
especially been decomposed.[119]
[Footnote 119: Allport, _Quart. Journ. Geol. Soc._ xxx. (1874), p. 547.
Chemical analysis also shows the identity of the rocks and the nature
of the alteration of the "white rock." See Jukes, "South Staffordshire
Coal-field," pp. 117, 118.]
The sills of greenish decomposed material that have been injected
amongst and alter the coals, vary from 15 feet to 80 or 90 feet in
thickness. The largest of the dolerite cakes on the surface, that of
the Rowley Hills, is somewhat irregular in its thickness, but may reach
as much as 100 feet.
That nearly the whole of the igneous material is intrusive is admitted
by all observers who have studied the ground. The manner in which the
"basalts" and "greenstones" send out veins into the Coal-measures shows
conclusively that they have been injected into the strata. The only
rock about which some doubt has been expressed is that of the Rowley
Hills, which Jukes was disposed, though not without some hesitation,
to consider as part of an actual lava-stream. He based this inference
chiefly on the occurrence, immediately under the dolerite, of what
he looked upon as a "trappean breccia or brecciated ash, containing
rounded and angular fragments of igneous rock lying in a brown
rather ferruginous paste, that looks like the debris of a basaltic
rock."[120] This breccia he regarded as belonging to and passing into the
Coal-measures, and he was thus inclined to regard the dolerite as a
lava of Coal-measure age.
[Footnote 120: _Op. cit._ p. 119.]
It is possible, however, that the "trappean breccia" may be of the
same nature as the "uncompressed balls of basalt bedded in a mass of
decomposed basalt or basaltic 'wacke' or clay"[121]--that is, a decayed
contact layer of the eruptive rock. But if it be regarded as the
fragmental accompaniment of a lava-stream, it can hardly belong to
the Coal-measures. If the dolerite had been a lava of that age, it
ought to be found lying conformably on the Coal-measures. But this it
does not appear to do. Making every allowance for the way in which an
advancing current of lava might plough up soft sediment on the bottom
of the sea or of a lake, we can hardly thus account for the very uneven
surface of Coal-measures on which the sheet of igneous rock rests.
If the Rowley rock be looked upon as a lava which flowed out at the
surface, it must, I think, be assigned to a time subsequent to that of
the Coal-measures, when these strata had been upraised and had suffered
some amount of denudation. I confess, however, that the petrographical
characters of the rock, the alteration of the coals which have been
worked underneath it, and the abundant veins of "white rock" which
there traverse the seams, induce me to regard this rock as forming
no exception to the general rule in the Midlands, but as having been
intruded as a sill, now laid bare by denudation. Its fresher condition
may arise from its thickness, or from some other circumstance which has
not been ascertained.
[Footnote 121: _Op. cit._ p. 126.]
We have now to consider the probable geological date of the various
intrusions of basic igneous material which can be traced over so wide
an area in the centre of England. In discussing the subject, Jukes
pointed out that in the surrounding district "no igneous rocks of any
kind are found in any formation newer than the Coal-measures."[122]
This statement is, with the exception of one locality, undoubtedly
true.[123] But on any view there must have been a long interval of time
between the formation of the highest strata of the South Staffordshire
coal-field and that of the lowest Permian deposits of the district. It
is quite conceivable, though at present incapable of proof, that the
extravasation of eruptive material took place after the close of the
Carboniferous period and during the earlier part of the Permian period.
[Footnote 122: _Op. cit._ p. 131.]
[Footnote 123: See note on next page.]
Jukes further shows that "at whatever period these igneous rocks were
produced, they were all existent before the production of the faults
and dislocations that have traversed the Coal-measures, and before any
great denudation had been effected on the country." This argument may
be readily granted. But, so far as we know, many, if not most, of the
faults traverse also the surrounding Permian and Triassic rocks, so
that igneous masses protruded during those periods would be affected by
the same dislocations.
When we consider the history of Palæozoic time in this country, and
especially the proof, obtainable everywhere else in Britain, that
volcanic energy became quiescent during the accumulation of the
Coal-measures, we may well demand better evidence than has hitherto
been forthcoming that any portion of the dolerites of the Midlands
is of Carboniferous age. It is important to notice that though the
dolerite sills and veins are so abundant in the South Staffordshire
coal-field, coming even in many places up to the present surface of
the ground, no single case has been observed where they rise into
the Permian rocks that overlie the Coal-measures unconformably. It
is difficult to believe that, had these intrusions taken place after
the deposition of the younger formation, they should not be found
penetrating it.[124] It seems almost certain that they must be of an age
intermediate between the Coal-measures of South Staffordshire and the
surrounding breccias and sandstones of the Permian series. And as there
is clear evidence of contemporaneous volcanic action in the lowest part
of the Permian system to the north in Scotland and to the south in
Devonshire, the inference seems not unreasonable that these intrusive
basalts of the Midlands are most probably of Permian age.
[Footnote 124: Only one instance is known where in Staffordshire
any igneous rock has been intruded into rocks younger than the
Coal-measures (Allport, _Quart. Journ. Geol. Soc._ vol. xxx. p. 551;
Sheet 72 S. W. of the Geological Survey, and Horizontal Sections,
Sheet 57). It forms a dyke which has been traced near Norton Bridge,
Swinnerton and Butterton, running for 8 miles in a N.N.W. direction,
and rising through Permian, Bunter and Keuper strata. It is a highly
basic olivine-basalt, and is unquestionably a dyke. Mr. J. Kirkby,
who has recently mapped and described it (_Trans. North Staffordshire
Naturalists' Field-Club_, xxviii. (1894), p. 129), suggests that it
may be connected with the igneous rocks of the South Staffordshire
coal-field. But of this idea there is no evidence. The last point to
which the dyke has been traced is some five-and-twenty miles from the
nearest known portion of the dolerites of the coal-field. I have little
doubt that this dyke is really an outlying member of the great system
of Tertiary dykes described in Book VIII. of the present work.]
No trace of vents has been met with in the Coal-measures of the
Midland district or among the surrounding older rocks, nor any proof
that the abundant sills and veins were connected with the eruption
of volcanic materials at the surface. Nevertheless, from the analogy
of the structure of these intrusive sheets to that of the sills in
such volcanic districts as the southern half of Scotland, we may well
believe that they were connected here and there with eruptive vents,
and thus that besides the northern and southern districts of Permian
volcanoes, there rose a central group among the lagoons of the heart
of England. Though no vestige of any such group has been detected, we
must remember that a large portion of the Midlands is overspread with
Permian and Triassic deposits, and that much more igneous rock may be
concealed than appears at the surface. Possibly there may be buried
under these younger sheets of red sandstone and marl, lavas and tuffs
with their connected vents, such as may be seen where the Permian
volcanic series has been laid bare by denudation in Ayrshire and
Devonshire. In this respect it would be interesting to make a thorough
examination of the Permian breccias of the district, with the view of
discovering whether, though the volcanic rocks _in situ_ may still lie
covered up, fragments of them may not be found in these deposits.
[Illustration: TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES
OF BRITAIN"
Map V. MAP OF THE PERMIAN VOLCANIC DISTRICTS OF SCOTLAND
_The Edinburgh Geographical Institute_ Copyright J. G. Bartholomew.]
BOOK VIII
THE VOLCANOES OF TERTIARY TIME