VOLCANOES OF THE LOWER OLD RED SANDSTONE OF THE CHEVIOT HILLS, LORNE,
"LAKE ORCADIE" AND KILLARNEY
THE CHEVIOT AND BERWICKSHIRE DISTRICT
In the south-east of Scotland, and extending thence into the north of
England, the remains of several distinct volcanic centres of the Lower
Old Red Sandstone may still be recognized. Of these the largest and
most interesting forms the mass of the Cheviot Hills; a second has
been partially dissected by the sea along the coast south from St.
Abb's Head; while possibly relics of others may survive in detached
bosses of eruptive rock which rise through the Silurian formations
of Berwickshire. The water-basin in which these volcanic groups were
active was named by me "Lake Cheviot,"[371] to distinguish it from the
other basins of the same geological period (Map I.).
[Footnote 371: _Trans. Roy. Soc._ Edin. xxviii. (1878), p. 354.]
The volcanic rocks of the Cheviot Hills, though their limits have
been reduced by faults, unconformable overlap of younger formations
and severe denudation, still cover about 230 square miles of ground,
and rise to a height of 2676 feet above the sea. As they have been
mapped in detail by the Geological Survey, both on the English and the
Scottish sides of the Border, their structure is now known.[372] No
good horizontal section, however, has yet been constructed to show this
structure--a deficiency which, it is hoped, may before long be supplied.
[Footnote 372: The Geology of the Cheviot Hills is comprised in Sheets
108 N.E., 109 N.W., and 110 S.W. of the Geological Survey of England
and Wales, and in Sheets 17, 18 and 26 of the Geological Survey of
Scotland. For descriptive accounts the Memoirs to some of these Sheets
may be consulted, particularly "Geology of the Cheviot Hills" (English
side), by C. T. Clough (_Mem. Geol. Surv._ 1888); "Geology of Otterburn
and Elsdon," by H. Miller and C. T. Clough (_Mem. Geol. Surv._ 1887);
"Geology of Part of Northumberland between Wooler and Coldstream," by
W. Gunn and C. T. Clough, with Petrographical Notes by W. W. Watts
(_Mem. Geol. Surv._ 1895). Other descriptions have been published by
Professor James Geikie, _Good Words_, vol. xvii. (1876), reprinted in
_Fragments of Earth-lore_ (1893), and by Prof. Lebour, _Outlines of
the Geology of Northumberland_, 2nd edit. 1886. For the petrography
of the rocks consult Mr. J. J. H. Teall, _Geol. Mag._ 1883, pp. 100,
145, 252, 344; 1884, p. 226; 1885, p. 106; _Proc. Geol. Assoc._ ix.
(1886) p. 575; and his _British Petrography_, 1888; Dr. J. Petersen,
_Mikroskopische und chemische Untersuchungen am Enstatit-porphyrit aus
den Cheviot Hills_, Inaugural Dissertation, Kiel, 1884.]
This volcanic pile, consisting mainly of bedded andesites which rest
unconformably on the upturned edges of Wenlock shales and grits,
presents a most typical display of the lavas of the Lower Old Red
Sandstone. These rocks range from vitreous or resinous pitchstone-like
varieties to coarsely porphyritic forms, on the one hand, and to highly
vesicular and amygdaloidal kinds, on the other. Analyses of some of
these rocks, and an account of their petrography, have already been
given.
The lavas are often separated by thin partings of tuff, and their
upper surfaces show the fissured character with sandstone infillings,
so characteristic among the lavas of "Lake Caledonia."[373] Tuffs
form a very subordinate part of the whole volcanic series. One of the
most important bands is a thick mass at the base of the series, lying
immediately on the highly inclined Silurian shales. The fragments are
generally of a fine-grained purple mica-andesite, often two or three
feet and sometimes at least five feet long. For a few feet near the
bottom of this mass of tuff, pieces of Silurian shale an inch in length
may be noticed. Mr. Clough remarks that distinct bedding is not usual
among the tuffs. Though no doubt most of the fragmental materials
really lie intercalated between successive lava-streams, yet some of
the isolated patches of coarse volcanic breccia may mark the sites of
eruptive vents. One such probable neck has been mapped on the Scottish
side between Cocklawfoot, at the head of the Bowmont Water, and King's
Seat, while others may perhaps occur among the detached patches that
have been observed on the Northumbrian side. No thick conglomerates
or sandstones have been noticed in the Cheviot District. The volcanic
eruptions appear to have usually succeeded each other without the
spread of any notable amount of ordinary detritus over the floor of
the water-basin. It is difficult to estimate the total thickness of
volcanic material here piled up, but it probably amounts to several
thousand feet. The top of the series is not visible, having been
partly removed by denudation and partly buried under the Carboniferous
formations.
[Footnote 373: Clough, _Geology of the Cheviot Hills_, p. 15.]
It will thus be seen that the Cheviot area stands apart from the
other volcanic districts of the Lower Old Red Sandstone in the great
relative thickness of its accumulated lavas, the comparative thinness
of its tuffs, and the absence of the thick intercalations of coarse
conglomerate so abundantly developed among the volcanic series all over
Central Scotland. But there is yet another characteristic in which this
area is pre-eminently conspicuous. In the heart of the andesites lies a
core of augite-granitite, around which these rocks are traversed with
dykes.
This interesting granitic boss rises into the highest summit of the
whole Cheviot range, and covers an area of rather more than 20 square
miles. While its petrographical characters have been described by Mr.
Teall, its boundary has been mapped by Mr. Clough, who found the line
difficult to trace, owing partly to the prevalent covering of peat, and
partly to the jagged and irregular junction caused by the protrusion of
dykes from and into the boss. He obtained evidence that the granite has
broken through the bedded andesites, and that it is in turn traversed
by dykes composed of a material indistinguishable from that of some
of the flows. He therefore considered that it is essentially of the
same age as the rest of the volcanic series, and "not improbably the
deep-seated source of it."[374] Mr. Teall also, from a chemical and
microscopical examination of the rocks, drew a similar conclusion.[375]
[Footnote 374: _Op. cit._ p. 24.]
[Footnote 375: _Geol. Mag._ 1885, p. 106.]
The andesites around the granite have undergone contact-metamorphism,
but the nature and extent of the change have not yet been
studied. There occur around the granite many dykes of felsite and
quartz-felsite, to the petrographical character of which reference has
already been made. But the most abundant and remarkable dykes of the
district are those of a reddish mica-porphyrite, of which Mr. Clough
has mapped no fewer than forty, besides those in the granitic area. He
has called attention to the significant manner in which all the dykes
of the district tend to point in a general way to the great core of
granite, as if that were the nucleus from which they had radiated.[376]
[Footnote 376: _Op. cit._ pp. 26-28.]
The central granite of the Cheviot Hills, with its peripheral dykes,
has no accompanying agglomerates nor any decided proof that it
ever communicated with the surface. When, however, we consider its
petrographical and chemical constitution, its position as a core
among the bedded lavas, and the intimate way in which it is linked
with these rocks by the network of dykes, we are, I think, justified
in accepting the inference that it belongs to the volcanic series.
It possesses some curious and interesting features in common with
the great granophyre bosses of Tertiary age in the Inner Hebrides.
Like these it has no visible accompaniment of superficial discharges.
Yet it may have ascended by means of some central vent or group of
vents which, offering to it a weak part of the crust, allowed it to
communicate with the surface and give rise to the outflow of lavas and
fragmental ejections. In any case, it affords us a most interesting and
instructive insight into one of the deeper-seated ducts of a volcanic
region, and the relation of a volcanic focus to the ascent of the
granitic magma.
* * * * *
About twenty miles to the north of the Cheviot Hills, and separated
from them by the Carboniferous and Upper Old Red Sandstones which
spread across the broad plain of the Merse, a group of volcanic rocks
has been laid open in a singularly instructive manner along the coast
of Berwickshire, between the village of Eyemouth and the promontory
of St. Abb's Head. Not only the actual vents, but the lavas and tuffs
connected with them, have there been admirably dissected by the forces
of denudation.
That this volcanic area was quite distinct from that of the Cheviot
Hills may be inferred from its coarse agglomerates, and from the fact
that when the rocks are followed inland in a south-westerly direction,
that is, towards the Cheviot area, they are found to diminish in
thickness and to disappear among the ordinary sediments. For the same
reason we may regard the area as independent of any vents which may
have risen further west about Cockburn Law and the Dirrington Laws.
Unfortunately, however, only a small part of the area comes into view,
the rest of it lying beneath the waters of the North Sea.[377]
[Footnote 377: This area lies in Sheet 34 Geological Survey of
Scotland, and was described by myself in the Memoir to accompany that
Sheet ("Geology of Eastern Berwickshire," 1864, p. 20). More recently
the shore between St. Abb's Head and Coldingham has been re-mapped by
Professor James Geikie who has also studied the microscopic character
of the rocks, _Proc. Roy. Soc. Edin._ xiv. (1887).]
Of the several vents dissected along this coast-line, one may be seen
at Eyemouth, filled with a very coarse tumultuous agglomerate of
andesite fragments embedded in a compact felspathic matrix, through
which are scattered broken crystals of felspar, and imperfect tabular
crystals of black mica. Another of similar character is exposed for
more than a mile and a half along the shore at Coldingham. It contains
blocks, sometimes more than a yard in diameter, of different varieties
of andesite, and, as at Eyemouth, is much invaded by veins and bosses
of intrusive andesite.
[Illustration: Fig. 98.--Section across the volcanic area of St. Abb's
Head (after Prof. J. Geikie).
11. Silurian formations; 2. Lower Old Red Conglomerate and
Sandstone; 3 3. Sheets of andesitic lava; 4. Volcanic tuffs,
largely composed of scoriæ in the higher parts; 5. Volcanic
agglomerate of neck on shore; 6. Intrusive andesites. _f_, Fault.
]
To the north of Coldingham, a series of bedded volcanic rocks which
form the picturesque headland of St. Abb's Head, are, according to the
estimate of Professor James Geikie, about 1000 to 1200 feet thick,
but neither their bottom nor their top is seen. The same observer
found them to consist of three groups of andesite sheets separated and
overlain by bedded tuffs. The lowest lavas have their base concealed
under the sea, and are covered by a thick band of coarse agglomeratic
tuff, above which lies the second group of andesites, about 250 feet
thick. An intercalation of various tuffs from 40 to 50 feet thick then
succeeds, followed by the third lava-group, 250 or 300 feet in depth.
The highest member of the series is a mass of bedded tuffs some 400
feet thick.
The andesites lie in beds varying from about 15 to about 50 feet
or more in thickness. They are fine-grained, purplish-blue, or
greyish-blue, often reddish rocks, of the usual type. Generally rather
close-grained, they are not as a rule very porphyritic, but often
highly scoriaceous and amygdaloidal, especially towards the top and
bottom of each bed. The more slaggy portions are sometimes so filled in
with fine tuff that the rock might be mistaken for one of fragmental
origin.
The bedded tuffs are usually well stratified deposits. The most
important band of them is that which forms the highest member of the
volcanic series. It consists of successive beds that vary from fine
red mudstones up to volcanic breccias with blocks one foot or more
in diameter. The materials have been derived from the explosion of
andesitic lavas. Most of the lapilli are vesicular or amygdaloidal,
and many of them have evidently come from vitreous scoriaceous
lavas. Professor Geikie remarks that "from their highly vesicular
character, they might well have floated in water at the time of their
ejection--they are in short mere cinders." He could detect no trace of
ordinary sediment in the matrix, the whole material being thoroughly
volcanic in origin.
The lavas, tuffs and agglomerates have been abundantly invaded by
intrusive rocks, chiefly andesites.[378]
[Footnote 378: See Prof. J. Geikie, _op. cit._]
The agglomerates of this Berwickshire coast extend for a short way
inland from the Coldingham and Eyemouth vents, but the fragmental
material soon becomes finer and more water-rolled, and assumes a
distinctly stratified structure, as it is gradually and increasingly
interleaved with layers of ordinary sediment. Hence in passing towards
the south-west, away from the coast-line, we are obviously receding
from the vents of eruption and entering into the usual non-volcanic
deposits of the time. That these deposits belong to the Lower Old Red
Sandstone was first ascertained during the progress of the Geological
Survey in this district by the discovery of abundant plant-remains in
the form of linear grass-like strips, and also pieces of _Pterygotus_
in some of the green shales interstratified among fine tuffs and ashy
sandstones.[379] Before the volcanic detritus disappears from the
strata as they are followed in a south-westerly direction, the whole
series is unconformably overlain by the Upper Old Red Sandstone. The
lower division of the formation is not again seen until it rises from
under the southern margin of the plain of the Merse into the Cheviot
Hills.
[Footnote 379: "Geology of Eastern Berwickshire," _Mem. Geol. Surv.
Scotland_ (1864), pp. 26, 27, 57.]
About ten miles to the south-west of the large Coldingham neck the
great boss of Cockburn Law and Stoneshiel Hill rises out of the
Silurian rocks.[380] Five miles still further in the same direction
the group of the beautiful cones of Dirrington (Fig. 70) overlooks
the wide Merse of Berwickshire,[381] and six miles to the north of
these hills, in the very heart of Lammermuir, lies the solitary
boss of the Priestlaw granite.[382] To these protrusions of igneous
material reference has already been made as possible volcanic vents
connected with the eruptions of the Lower Old Red Sandstone. As regards
their age they must certainly be younger than the Llandovery rocks
which they disrupt, and older than the Upper Old Red Sandstone, of
which the conglomerates, largely made from their debris, lie on them
unconformably. It seems therefore probable that these great bosses may
form a part of the volcanic history of the Lower Old Red Sandstone
period. But no positive proof has yet been obtained that any one of
them was the site of an eruptive vent, and no trace has been detected
around them of any lavas or tuffs which might have proceeded from them.
[Footnote 380: See "The Geology of Eastern Berwickshire" (Sheet 34),
_Mem. Geol. Surv. Scotland_ (1864), p. 29.]
[Footnote 381: These hills are chiefly represented in Sheet 25. But see
"The Geology of East Lothian," _Mem. Geol. Surv. Scotland_ (1866), p.
26.]
[Footnote 382: "Geology of East Lothian," _Mem. Geol. Surv. Scotland_,
p. 15, and authorities there cited.]
"THE LAKE OF LORNE"
[Illustration: Fig. 99.--View of terraced andesite hills resting on
massive conglomerate, south of Oban.]
The basin of Lorne has not yet been carefully examined and described,
though various writers have referred to different parts of it (Map I.).
My own observations have been too few to enable me to give an adequate
account of it. The volcanic sheets of this area form a notable feature
in the scenery of the West Highlands, for they are the materials out
of which the remarkable terraced hills have been carved, which stretch
from Loch Melfort to Loch Creran (Fig. 99), and which reappear in
picturesque outliers among the mountains traversed by Glen Coe. Between
the ancient schists that form the foundation-rocks of this district and
the base of the volcanic series, lies a group of sedimentary strata
which in the western part of the district must be 500 feet thick.
This group consists of exceedingly coarse breccias at the bottom,
above which come massive conglomerates, volcanic grits or tuffs,
fine sandstones and courses of shale. While the basement-breccias
are composed mainly of detritus of the underlying schists, including
blocks six feet long, they pass up into coarse conglomerates made up
almost entirely of fragments of different lavas (andesites, diabases,
etc.), and more than 100 feet thick, which often show little or no
trace of stratification, but break up into large quadrangular blocks
by means of joints which cut across the imbedded boulders. These
volcanic conglomerates form some of the more conspicuous features of
the coast to the south and north of Oban, and are well exposed in the
Isle of Kerrera. They offer many points of resemblance to those of
Lake Caledonia, but no certain proof has been noted that they belong
to the Lower Old Red Sandstone. They have obviously been derived from
lava-sheets that were exposed to strong breaker-action, which at
the same time transported and rounded blocks of granite, schist and
other crystalline rocks derived from the adjacent hills. During the
intervals of quieter sedimentation indicated by the fine sandstones and
shales, volcanic explosions continued, as may be seen by the occurrence
of occasional large bombs which have fallen upon and pressed down the
fine ashy silt that was gathering on the bottom.
It would seem from the characters of some of the strata in this
sedimentary series that over the area of deposit portions of the
shallower waters were occasionally laid bare to the sun and air.
Among the conglomerates there lie certain bands of reddish sandy,
ripple-marked, sun-cracked and rain-pitted shales and fine sandstones.
Such accumulations, indicative of the ultimate exposure of fine
sediment that silted up hollows in the great banks of coarse shingle,
may be noticed at the south end of the Island of Kerrera, on at least
two horizons which are separated from each other by thick masses of
conglomerate and fine felspathic grit. We may infer, therefore, that
the fine littoral mud, which gathered during pauses in the heaping
up of the coarse gravel and shingle, was occasionally laid dry.
Similar strata may be observed behind Oban, where the alternation of
exceedingly fine sediment and granular ashy bands is well exhibited.
[Illustration: Fig. 100.--Section of lava-escarpment at Beinn Lora,
north side of mouth of Loch Etive, Argyllshire.
1. Phyllites; 2. Thick conglomerate; 3. Successive sheets of andesite.]
But the explosions that gave rise to the volcanic materials so largely
represented in these lower conglomerates, were merely preliminary to
those which led to the outflow of the great sheets of lava that now
constitute so large a part of the hills of Lorne. In the few traverses
which I have made across different parts of this district I have noted
the general resemblance of the lavas to those of the Lower Old Red
Sandstone of the Midland Valley of Scotland, their bedded character,
and the occurrence of occasional layers of stratified material between
them. The prominent features of these rocks, and their relation to the
volcanic conglomerates below them, and to the underlying slates and
schists are well displayed on Beinn Lora at the mouth of Loch Etive
(Fig. 100). There the black slates of the district are unconformably
covered by the coarse volcanic conglomerate, formed chiefly of blocks
of andesite, cemented in a hard matrix of similar composition. About
150 or 200 feet of this material underlie the great escarpment of the
lavas, which here rise in successive beds to the top of the hill, 1000
feet above its base.
On the south side of Loch Etive the base of the volcanic series, with
its underlying conglomerate, may be followed westward to Oban and
thence southward to Loch Feochan. The lavas cover most of the ground
from the western shore eastwards to near Loch Awe. But this area is
still very imperfectly known. The Geological Survey, however, has now
advanced to this part of the country, so that we shall before long
be in possession of more detailed information regarding the character
and sequence of its volcanic history and the geological age of the
eruptions.
Mr. H. Kynaston, who has begun the mapping of the eastern portion of
the district, finds that there, as further west, the bottom of the
volcanic series is generally a breccia or conglomerate. He has met with
two leading types among the lavas, the more abundant being strongly
vesicular, the other more compact. He has observed also numerous dykes
and sills of intrusive porphyrite, trending in a general N.N.E. and
S.S.W. direction, and pointing towards the great granite mass of Ben
Cruachan.[383]
[Footnote 383: _Ann. Report Geol. Surv._ (1895), p. 29 of reprint.]
Mr. R. G. Symes has traced the volcanic series to the north and south
of Oban. While visiting with him part of his ground, I was much struck
with the evidence of an intrusive mass at the base of the volcanic
series in the Sound of Kerrera. A prominent feature on the east side
of the channel, known as Dun Uabairtich and 270 feet high, consists of
andesite which appears to combine both a central boss and a sill. The
rock breaks through the black slates and the overlying conglomerates
and sandstones, and has wedged itself into the unconformable junction
between the two formations. It is beautifully columnar on its
sea-covered face, some of the columns being 120 feet or more in length,
and gently curved.
"LAKE ORCADIE"
We now cross the whole breadth of the Scottish Highlands in order to
peruse the records of another of the great detached water-basins of
the Old Red Sandstone, which for the sake of brevity of reference I
have named and described as "Lake Orcadie" (Map I.). This area has
its southern limits along the base of the hills that enclose the wide
Moray Firth. It spreads northward over the Orkney and much of the
Shetland Islands, but its boundaries in that direction are lost under
the sea. In the extensive sheet of water which spread over all that
northern region the peculiar Caithness Flags, with their associated
sandstones and conglomerates, were deposited to a total depth of 16,000
feet. A sigillaroid and lycopodiaceous vegetation flourished on the
surrounding land, together with ferns, _Psilophyton_ and conifers. The
waters teemed with fishes of which many genera and species have now
been described. The remains of these creatures lie crowded upon each
other in the flagstones in such a manner as to indicate that from time
to time vast quantities of fish were suddenly killed. Not impossibly,
these destructions may have been connected with the volcanic activity
which has now to be described.
In the year 1878 I called attention to the evidence for the existence
of contemporaneous volcanic rocks in the Old Red Sandstone north of the
range of the Grampians, and specially noted three localities where this
evidence could be seen--Strathbogie, Buckie and Shetland.[384] Since
that time Messrs. B. N. Peach and J. Horne have added a fourth centre
in the Orkney Islands. At present, therefore, we are acquainted with
the records of four separate groups of volcanic vents in Lake Orcadie.
[Footnote 384: _Trans. Roy. Soc. Edin._ xxviii. (1878).]
The southern margin of this water-basin appears to have indented the
land with long fjord-like inlets. One of these now forms the vale of
Strathbogie, which runs into the hills for a distance of fully 20 miles
beyond what seems to have been the general trend of the coast-line. In
this valley I found a bed of dark vesicular diabase intercalated among
the red sandstones and high above the base of the formation, as exposed
on the west side of the valley near Burn of Craig. On the east side a
similar band has since been mapped for the Geological Survey by Mr. L.
Hinxman who has traced it for some miles down the Strath.[385] This
latter band, as shown in Fig. 101, lies not far above the bottom of
the Old Red Sandstone of this district, and is thus probably distinct
from the Craig outcrop. There would thus appear to be evidence of two
separate outflows of basic lava in this fjord of the Old Red Sandstone
period.
[Footnote 385: See Sheet 76 of the Geological Survey of Scotland.]
[Illustration: Fig. 101.--Section across Strathbogie, below Rhynie,
showing the position of the volcanic band.
1. Knotted schists; 2. Diorite intrusive in schists; 3. Old Red
Conglomerate; 4. Volcanic band; 5. Shales with calcareous nodules; 6.
Sandstones of Rhynie; 7. Shales and sandstones. _f_, Fault.]
No vestige has here been found of any vent, nor is the lava accompanied
with tuff. The eruptions took place some time after the earlier
sediments of the basin were accumulated, but ceased before the thick
mass of upper sandstones and shales was deposited. A section across
the valley gives the structure represented in the accompanying diagram
(Fig. 101).
Twenty-five miles further north a still smaller andesite band has been
detected by Mr. J. Grant Wilson among the sandstones and conglomerates
near Buckie.[386] It is a truly contemporaneous flow, for pebbles of it
occur in the overlying strata. But again it forms only a solitary bed,
and no trace of any accompanying tuff has been met with, nor of the
vent from which it came. Both this vent and that of Strathbogie must
have been situated near the southern coast-line of the lake.
[Footnote 386: See Sheet 95 of the Geological Survey of Scotland and
_Trans. Roy. Soc. Edin._ vol. xxviii. (1878), p. 435.]
At a distance of some 90 miles northward from these Moray Firth
vents another volcanic district lies in the very heart of the Orkney
Islands.[387] The lavas which were there ejected occur at the
south-eastern corner of the island of Shapinshay, where they are
regularly bedded with the flagstones. They consist of dark green
olivine-diabases with highly amygdaloidal and vesicular upper
surfaces. Their thickness cannot be ascertained, as their base is not
seen, but they have been cut by the sea into trenches which show them
to exceed 30 feet in depth. The position of the vent from which they
came has not been ascertained. Neither here nor in the Moray Firth area
do any sills accompany the interbedded sheets, and in both cases the
volcanic action would seem to have been of a feeble and short-lived
character.
[Footnote 387: Messrs. B. N. Peach and J. Horne, _Proc. Roy. Phys. Soc.
Edin._ vol. v. (1879), p. 80.]
Much more important were the volcanoes that broke out nearly 100
miles still further north, where the Mainland of Shetland now lies.
I shall never forget the pleasure with which I first recognized the
traces of these eruptions, and found near the most northerly limits
of the British Isles proofs of volcanic activity in the Lower Old Red
Sandstone. Since my observations were published,[388] Mr. Peach, who
accompanied me in Shetland, has returned to the district, and, in
concert with his colleague Mr. Horne, has extended our knowledge of
the subject.[389] The chief vent or vents lay towards the west and
north-west of the Mainland and North Mavine; others of a less active
and persistent type were blown out some 25 miles to the east, where
the islands of Bressay and Noss now stand. In the western district
streams of slaggy andesite and diabase with showers of fine tuff and
coarse agglomerate were ejected, until the total accumulation reached
a thickness of not less than 500 feet. The volcanic eruptions took
place contemporaneously with the deposition of the red sandstones, for
the lavas and tuffs are intercalated in these strata. The lavas and
volcanic conglomerates are traceable from the southern coast of Papa
Stour across St. Magnus' Bay to the western headlands of Esha Ness,
a distance of more than 14 miles. They have been cut by the Atlantic
into a picturesque range of cliffs, which exhibit in some places, as
at the singular sea-stalk of Doreholm, rough banks of andesitic lava
with the conglomerate deposited against and over them, and in other
places, as along the cliffs of Esha Ness, sheets of lava overlying the
conglomerates.
[Footnote 388: _Trans. Roy. Soc. Edin._ vol. xxviii. (1878), p. 418.]
[Footnote 389: _Ibid._ vol. xxxii. (1884), p. 359.]
No trace of any vents has been found in the western and chief volcanic
district, but in Noss Sound a group of small necks occurs, filled with
a coarse agglomerate composed of pieces of sandstone, flagstone and
shale. Messrs. Peach and Horne infer that these little orifices never
discharged any streams of lava. More probably they were opened by
explosions which only gave forth vapours and fragmentary discharges,
such as a band of tuff which is intercalated among the flagstones in
their neighbourhood.
But one of the most striking features of the volcanic phenomena of
this remote region is the relative size and number of the sills and
dykes which here as elsewhere mark the latest phases of subterranean
activity. Messrs. Peach and Horne have shown us that three great sheets
of acid rocks (granites and spherulitic felsites, to which reference
has already been made, p. 292) have been injected among the sandstones
and basic lavas, that abundant veins of granite, quartz-felsite and
rhyolite radiate from these acid sills, and that the latest phase of
igneous action in this region was the intrusion of a series of bosses
and dykes of basic rocks (diabases) which traverse the sills.
The Killarney District
In the south of Ireland the Upper Silurian strata are followed upwards
conformably by the great series of red sandstones and conglomerates
known as the "Dingle Beds." Lithologically these rocks present the
closest resemblance to the Lower Old Red Sandstone of Central Scotland.
They occupy a similar stratigraphical position, and though they have
not yielded any palæontological data for comparison, there can, I
think, be no hesitation in classing them with the Scottish Lower Old
Red Sandstone, and regarding them as having been deposited under
similar geographical conditions. They offer one feature of special
interest for the purpose of the present inquiry, since they contain a
well-marked group of contemporaneous volcanic rocks, including nodular
felsites, like those so characteristic of the Silurian period.
The area where this remote and isolated volcanic group is best
developed forms a range of high rugged ground along the northern front
of the hills that stretch eastward from the Lakes of Killarney. Their
general distribution is shown on Sheets 184 and 185 of the Geological
Survey of Ireland;[390] though I may again remark that petrography has
made great strides during the thirty years and more that have passed
since these maps and their accompanying Memoirs were published, and
that, were the district to be surveyed now, probably a considerable
tract of ground coloured as ash would be marked as felsite. At the same
time the existence of both these rocks here cannot be gainsaid.
[Footnote 390: See the Memoir (by J. B. Jukes and G. V. Du Noyer) on
Sheet 184, p. 15. Other volcanic rocks have been mapped at Valentia
Harbour in the Dingle Beds, but these I have not had an opportunity of
personally examining.]
The felsite was long ago brought into notice by Dr. Haughton,
who published an analysis of it.[391] It is also referred to by
Mr. Teall for its spherulitic structure.[392] Seen on the ground
it appears as a pale greenish-grey close-grained rock, sometimes
exhibiting flow-structure in a remarkably clear manner, the laminæ of
devitrification following each other in wavy lines, sometimes twisted
and delicately puckered or frilled, as in some schists. Portions of the
rock are strongly nodular, the nodules varying in size from less than a
pea to that of a hen's egg.
[Footnote 391: _Trans. Roy. Irish Acad._ vol. xxiii. (1859), p. 615.]
[Footnote 392: _British Petrography_, p. 349.]
The close resemblance of this rock to many of the Lower Silurian
nodular felsites of Wales cannot but strike the geologist. It presents
analogies also to the Upper Silurian felsites of Dingle. But its
chief interest arises from the geological horizon on which it occurs.
Lying in the so-called "Dingle-Beds," which may be regarded as the
equivalents of the Lower Old Red Sandstone of England and Scotland, it
is, so far as my observations go, the only example of such a nodular
felsite of later date than the Silurian period. We recognize in it a
survival, as it were, of the peculiar Silurian type of acid lava, the
last preceding eruption of which took place not many miles to the
west, in the Dingle promontory. But elsewhere this type does not appear
to have survived the end of the Silurian period.
The detrital rocks accompanying the felsite, in the district east of
Killarney, vary from such closed-grained felsitic material as cannot
readily be distinguished from the felsite itself to unmistakable
felsitic breccias. Even in the finest parts of them, occasional rounded
quartz-pebbles may be detected, while here and there a reddish shaly
band, or a layer of fine pebbly conglomerate with quartz-pebbles an
inch in length, shows at once the bedding and the dip. Mr. W. W. Watts,
who, with Mr. A. M'Henry of the Irish Staff of the Geological Survey,
accompanied me over this ground, found that a microscopic examination
of the slides which were prepared from the specimens we collected
completely confirmed the conclusions reached from inspection of the
rocks in the field.[393] He detected among the angular grains slightly
damaged crystals of felspar, chiefly orthoclase. Many portions of these
felspathic grits much resemble the detrital Cambrian rocks which in
the Vale of Llanberis have been made out of the pale felsite of that
locality.
[Footnote 393: Mr. Watts also examined the microscopic structure of
the felsite of Benaun More. He found that the spherulites appear
to have a micropegmatitic structure, owing to the intergrowth of
quartz and felspar. In some parts of the rock the spherulites, from
·02 to ·01 inch in diameter, are surrounded by exceedingly minute
green needles, possibly of hornblende, while inside some of them are
small quartz-grains. Larger porphyritic felspars occur outside the
spherulites, some being of plagioclase, but most of orthoclase. The
spherulitic structure is not so well developed near the felspars. A few
of the large nodules are hollow and lined with crystals, while some of
them show a finely concentric lamination like the successive layers of
an agate.]