CHAPTER XVII_The Ancient Volcanoes of Great Britain, Volume 1 (of 2)

DISTRIBUTION OF THE VOLCANIC CENTRES IN THE LOWER OLD RED

SANDSTONE--CHARACTERS OF THE MATERIALS ERUPTED BY THE VOLCANOES

i. DISTRIBUTION OF VOLCANIC CENTRES

The area within which volcanic rocks belonging to the Lower Old Red

Sandstone appear is one of the most extensive regions over which the

volcanic eruptions of any geological period can be traced in the

British Isles (Map I.). Its northern limit reaches as far as the

islet of Uya in Shetland, and its southern appears in England in the

Cheviot Hills--a distance of about 250 miles. But volcanic rocks of

probably corresponding age occur even as far to the south as the hills

near Killarney. The most easterly margin of this area is defined by

the North Sea on the coast of Berwickshire, and its extreme western

boundary extends to near Lough Erne in the north of Ireland--a distance

of some 230 miles. If we include the post-Silurian bosses and dykes,

like those of Shap, and likewise the Devonian volcanic rocks of Devon

and Cornwall, as contemporaneous with those of the Old Red Sandstone,

the area of eruption will be greatly enlarged. But leaving these out

of account for the present, and confining our attention to the Lower

Old Red Sandstone series, we find that, within the wide limits over

which the volcanic rocks are distributed, a number of distinct and

often widely separated centres of eruption may be traced. Taking

these as they lie from north to south, we may specially enumerate the

following:--

1. The Shetland and Orkney Islands, together with the basin of the

Moray Firth. This region includes several distinct volcanic groups,

of which the most northerly extends through the centre to the

north-western headlands of the mainland of Shetland, another lies in

the island of Shapinshay, one of the Orkneys, while at least two can be

recognized on the south side of the Moray Firth. To this wide region

of Old Red Sandstone I have given the general designation of "Lake

Orcadie."[322]

[Footnote 322: _Trans. Roy. Soc. Edin._ vol. xxviii. (1878), p. 354.]

2. The basin of Lorne, on the west of the mainland of Argyllshire,

extending from Loch Creran to Loch Melfort and the hills on the west

side of Loch Awe.

3. The great central basin of Scotland, which, for the sake of

distinctness, I have called "Lake Caledonia,"[323] stretching

between the Highlands and the Southern Uplands, from the east coast

south-westwards across Arran and the south end of Cantire into Ireland

as far as Lough Erne. Numerous distinct volcanic groups occur in this

great basin, and their volcanic history will be discussed in detail in

later chapters (see Map III.).

[Footnote 323: _Op. cit._]

4. The basin of the Cheviot Hills and Berwickshire, with these hills as

the chief area, but including also other tracts, probably independent,

which are cut off by the sea along the eastern coast of Berwickshire

between St. Abb's Head and Eyemouth.

5. The Killarney tract, including the hills lying around Lough Guitane

in the east of County Kerry.

At the outset we may take note of a feature in the volcanic history of

Britain, first prominently noticeable in the records of the Old Red

Sandstone, and becoming increasingly distinct during the rest of the

long sequence of Palæozoic eruptions, namely, the persistence with

which the vents have been opened in the valleys and have avoided the

high grounds. I formerly dwelt on this relation, with reference to

the Carboniferous volcanic phenomena,[324] but the observation may

be greatly extended. With regard to the Old Red Sandstone of Central

Scotland, though the lavas and tuffs that were discharged over the

floor of the sheet of water which occupied that region gradually rose

along the flanks of the northern and southern hills, yet it was on the

lake-bottom and not among the hills that the orifices of eruption broke

forth.

[Footnote 324: _Trans. Roy. Soc. Edin._ vol. xxix. (1879), p. 454.]

So far as I am aware, no undoubted vents of the age of the Lower

Old Red Sandstone have been detected among the high grounds of the

Highlands on the one hand, or among the Silurian uplands on the other,

although a fringe of the lavas may be traced here and there along the

base of the hills.[325] In some cases, doubtless, the position of the

valleys may have been determined by lines of fault that might well

serve as lines of relief along which volcanic vents would be opened.

But in many instances it can be proved that, though the vents have

risen in valleys and low grounds, they have not selected lines of fault

visible at the surface, even when these existed in their neighbourhood.

Any fissures up which the volcanic ejections made their way must have

lain at great depths beneath the formations that now form the surface

rocks.

[Footnote 325: Certain remarkable necks of breccia have been detected

by Mr. J. R. Dakyns rising through the schists at the upper end of

Loch Lomond; but there is not sufficient evidence to connect them

with the volcanic series of the Lower Old Red Sandstone. Some of the

younger granite bosses are not improbably to be referred to this

volcanic series. The latest granites of the eastern Grampians, as

already stated, have lately been found by Mr. Barrow cutting the

band of probably Lower Silurian strata along the southern border of

the Highlands. Those of Galloway are younger than the Upper Silurian

formations, which they invade, and older than the conglomerates of the

Upper Old Red Sandstone, which contain pebbles of them. These eruptive

bosses will be further discussed in the sequel.]

ii. CHARACTERS OF THE MATERIALS ERUPTED BY THE VOLCANOES

A general summary of the petrographical characters of the igneous rocks

of the Lower Old Red Sandstone may here find a place. Further details

will be given in the account of "Lake Caledonia," which is the typical

area for them; but, on the whole, the prevailing types in one region

are found to be repeated in the others.

1. _Bedded Lavas._--Beginning with the lavas which were poured out

at the surface, we have to notice a considerable range of chemical

composition among them, although, as a rule, they are characterized

by general similarity of external appearance. At the one end, come

diabases and other ancient forms of basalt or dolerite, wherein the

silica percentage is below or little above 50. By far the largest

proportion of the lavas, however, are porphyrites or altered andesites,

having about 60 per cent of silica. With these are associated lavas

containing more or less unstriped felspar and a somewhat higher

proportion of silica, which may be grouped as trachytes, though no very

sharp line can be drawn between them and the andesites. In the Pentland

Hills, and some other areas, orthophyres flowed out alternately with

the more basic lavas, and were associated with felsitic tuffs and

breccias.

It is noteworthy that the lava-sheets of the Lower Old Red Sandstone,

if we consider the character of the prevalent type, hold an

intermediate grade between the average chemical composition of those

of Silurian and of those of later Carboniferous time. On the one hand,

they rarely assume the character of thoroughly acid rocks, like the

nodular rhyolites of the Bala and Upper Silurian series;[326] on the

other hand, they seldom include such basic lavas as the basalts, so

common among the puy-eruptions of the Carboniferous system, and never,

so far as I know, contain varieties comparable to the "ultra-basic"

compounds which I shall have occasion to allude to as characteristic of

a particular volcanic zone in that system.

[Footnote 326: The only examples known to me are those of Benaun More

and other hills in County Kerry.]

(_a_) The Diabase-lavas are typically developed in the chain of the

Pentland Hills, where they form long bands intercalated between

felsitic tuffs--a remarkable association, to which I shall make more

detailed reference in a later chapter. They range in texture from a

compact dark greenish base to a dull earthy amygdaloid. One of their

most remarkable varieties is a fine-grained green porphyry, with

large flat tabular crystals of plagioclase arranged parallel to the

direction of flow (Carnethy Hill). Most of them, however, are more

or less amygdaloidal, and some of them (Warklaw Hill) strongly so.

The following analyses, made in the laboratory of the Royal School of

Mines under the direction of Prof. E. Frankland, show the chemical

composition of some of the diabases of the Pentland Hills:[327]--

[Footnote 327: For analyses of some Shetland diabases of Old Red

Sandstone age, see Mr. R. R. Tatlock, _Trans. Roy. Soc. Edin._ vol.

xxxii. (1887), p. 387.]

+------------+-----------+---------+-----------+---------+-----------+

+------------+-----------+---------+-----------+---------+-----------+

+------------+-----------+---------+-----------+---------+-----------+

| SiO_{2} | 51·16 | ... | 52·00 | ... | 47·77 |

| Al_{2}O_{3}| 22·27 | 1·30 | 17·46 | 5·23 | 13·08 |

| Fe_{2}O_{3}| 2·94 | 1·53 | 7·85 | 7·32 | 0·84 |

| FeO | 4·02 | 1·14 | ... | ... | ... |

| CaO | 5·61 | 2·43 | 6·80 | 7·88 | 4·07 |

| MgO | 3·46 | 0·98 | 1·06 | 3·65 | 0·30 |

| K_{2}O | 2·42 | ... | 1·66 | ... | 1·17 |

| Na_{2}O | 2·58 | ... | 4·17 | ... | 2·30 |

| H_{2}O | 3·42 | ... | 2·68 | ... | 2·48 |

| P_{2}O_{5} | 0·48 | 0·32 | ... | 0·12 | ... |

| CO_{2} | 1·28 | ... | ... | 5·01 | ... |

+------------+-----------+---------+-----------+---------+-----------+

[Footnote 328: There was a trace of manganous oxide in this specimen.]

(_b_) The Andesites, or, as they were formerly called, Porphyrites,

which constitute by far the largest proportion of the lavas, have

a characteristic but limited range of lithological varieties. The

prevailing type presents a close-grained, rather dull texture, and a

colour varying from pinkish grey, through many shades of green and

brown, to purplish red, which last is, on the whole, the predominant

hue. Minute lath-shaped felspars may frequently be detected with the

naked eye on fresh surfaces, while scattered crystals, which are

generally hæmatitic pseudomorphs after some pyroxene, occasionally

after hornblende or mica, may often be observed. The usual porphyritic

constituents are plagioclase felspars, occasionally in abundant

tabular crystals measuring half an inch or more across, also one or

more pyroxenes (augite, enstatite), and sometimes brown or black mica.

Where large felspar-crystals occur in a compact green matrix, the rock

assumes a resemblance to the _verde antique_ of the ancients.[329]

One of the Cheviot andesites lying at the bottom of the series is

distinguished by its large and abundant plates of black mica.[330]

[Footnote 329: An instance of this rock occurs in Kincardineshire, from

which the large flat twins of labradorite have been analyzed by Dr.

Heddle (_Trans. Roy. Soc. Edin._ vol. xxviii. (1879), p. 257).]

[Footnote 330: C. T. Clough, "The Cheviot Hills," _Mem. Geol. Survey_

(1888), p. 12.]

The texture of the andesites occasionally becomes faintly resinous,

where a considerable proportion of glass still remains undevitrified,

as in the well-known varieties from the Cheviot Hills, and in another

pitchstone-like rock from above Airthrey Castle in the Ochil Hills,

near Bridge of Allan. It sometimes presents a nodular or coarsely

perlitic character, weathering out in nut-like balls, like the rock

of Buckham's Wall Burn in the Cheviot Hills.[331] Much more frequent

is a well-developed amygdaloidal structure, which indeed may be said

to be the most obvious characteristic of these rocks as a whole. The

steam-vesicles, now filled with agate, quartz, calcite or zeolite, vary

in size from mere granules up to large irregular cavities a foot or

more in diameter. Where the kernels are coated with pale-green earth

and lie in a dark brown matrix, they give rise to some of the most

beautiful varieties of rock in any volcanic series in this country,

as may be seen on the Ayrshire coast at Culzean and Turnberry. Some

rocks contain the vesicles only as rare individuals, others have them

so crowded together as to form the greater part of the cubic contents

of the mass. When the infiltration-products have weathered out, some of

the amygdaloids present a striking resemblance to recent slaggy brown

lavas; lumps of them must have been originally light enough to float in

water.

[Footnote 331: _Ibid._ p. 11.]

My colleague in the Geological Survey, Mr. J. S. Grant Wilson, some

years ago made for me a large series of determinations of the specific

gravity of the volcanic rocks of the Lower Old Red Sandstone of

Scotland. He found that the andesites collected from various districts

to illustrate the more typical varieties of rock averaged about 2·66.

He also made a series of chemical analyses of a number of the same

rocks from the Cheviot Hills, where they are well preserved. The

results are shown in the following table:--

+-----------+-----+-------+-------+--------+-------+--------+-----------+-------+

|-----------+-----+-------+-------+--------+-------+--------+-----------+-------+

|SiO_{2} |59·29| 62·81 | 63·38 | 59·44 | 60·70 | 60·58 | 62·29 | 59·82 |

|Al_{2}O_{3}|16·30| 16·40 | 15·77 | 16·15 | 17·98 | 12·25 | 17·03 | 16·96 |

|Fe_{2}O_{3}| 1·77| ·55 | ·73 | 1·05 | ·66 | 1·01 | ·93 | ·20 |

|FeO | 3·70| 3·27 | 2·65 | 2·83 | 2·58 | 4·13 | 2·44 | 6·57 |

|MnO | ·41| ·81 | ·08 | ·37 | ·20 | ·15 | ·21 | ·15 |

|CaO | 4·81| 4·46 | 4·44 | 6·70 | 7·07 | 4·40 | 3·92 | 4·73 |

|MgO | 3·15| 1·64 | 1·88 | 2·46 | 2·20 | 2·86 | 2·71 | 2·84 |

|K_{2}O | 4·19| 3·60 | 1·88 | 3·18 | 3·57 | 2·19 | 1·14 | 2·63 |

|Na_{2}O | 3·44| 3·02 | 4·54 | 3·70 | 2·95 | 3·61 | 3·20 | 3·04 |

|H_{2}O | 3·84| 4·04 | 4·69 | 3·35 | 3·45 | ... | ·29[332]| ... |

|H_{2}SO_{4}| ...| ... | ... | ... | ... | ·55 | ·37 | trace |

|Loss. | ...| ... | ... | ... | ... | 2·15 | 4·81 | 1·98 |

+-----------+-----+-------+-------+--------+-------+--------+-----------+-------+

[Footnote 332: This is CO_{2}.]

The microscopic structure of the andesites of the Lower Old Red

Sandstone has been partially investigated, especially those of the

Cheviot Hills, by Mr. Teall[333] and by Dr. Petersen,[334] who both

give chemical analyses of the rocks. Much, however, still remains to

be done before our knowledge of this branch of British petrography

can be regarded as adequate. The groundmass in some of the rocks

consists mainly of a brown glass with a streaky structure (as in

the well-known variety of Kirk Yetholm, and in the rock, still more

like pitchstone, from near Airthrey Castle in the Ochil chain);

more usually it has been devitrified more or less completely by the

appearance of felspathic microlites, until it presents a confused

felspar aggregate. The porphyritic felspars are often large, generally

striped, but sometimes including crystals that show no striping.

They are frequently found to be full of inclusions of the base, and

these sometimes consist of glass. The ferro-magnesian constituents

are usually rather decomposed, being now represented by chloritic

pseudomorphs; but augite, and perhaps still more frequently enstatite,

may be recognized, or its presence may be inferred among them. The

beautiful resinous or pitchstone-like rock from near Airthrey Castle

has been found by Mr. Watts to be a glassy hypersthene-augite-andesite,

since among its phenocrysts of plagioclase, augite and hypersthene both

occur. Magnetite is commonly traceable, and apatite may be occasionally

detected. As the result of decomposition, calcite, chlorite and

limonite are very generally diffused through the rocks.[335]

[Footnote 333: _Geol. Mag. for 1883_, pp. 100, 145, 252.]

[Footnote 334: _Mikroskopische und chemische Untersuchungen am

Enstatit-porphyrit aus den Cheviot Hills_, Inaug. Dissert. Kiel, 1884.

Descriptions have also been published of detached rocks from other

districts, such as those by Prof. Judd and Mr. Durham of specimens from

the Eastern Ochils, _Quart. Journ. Geol. Soc._ vol. xlii. (1886), p.

418.]

[Footnote 335: Dr. F. H. Hatch supplied notes on microscopic structure

which are incorporated in the text, together with particulars

afterwards furnished by Mr. Watts.]

(_c_) The lavas which may be separated as Trachytes offer no

distinctive features externally by which they may be distinguished from

the andesites. Indeed, both groups of rocks appear to be connected by

intermediate varieties. In the Cheviot Hills some of the lavas are

found, on microscopic examination, to contain a large admixture of

unstriped porphyritic felspars, which can occasionally be recognized

as sanidine in Carlsbad twins. The groundmass is sometimes a brown

glass, but is usually more or less completely devitrified, portions of

it being inclosed in the large felspars. Chlorite, pseudomorphic after

augite or enstatite, may be detected, and sometimes a brown mica. A

specimen of one of these rocks, from a locality to the north-west of

Whitton, near Jedburgh, was found by Mr. J. S. Grant Wilson to have the

following composition:--

+-----------------------+

| N.W. of Whitton Hill, |

| Jedburgh (No. 1938) |

| Sp. gr. 2·55. |

+-------------+---------+

| SiO_{2} | 62·44 |

| Al_{2}O_{3} | 18·99 |

| Fe_{2}O_{3} | 3·35 |

| FeO | 1·8 |

| MnO | ·25 |

| CaO | 1·84 |

| MgO | 1·37 |

| K_{2}O | 5·02 |

| Na_{2}O | 2·65 |

| H_{2}O | 2·48 |

+-------------+---------+

| Total. | 100·19 |

+-------------+---------+

(_d_) Acid rocks such as Felsites and Rhyolites are rare among the

lavas poured out at the surface during the time of the Lower Old Red

Sandstone. They occur in the Pentland Hills, also near Dolphinton

in the Biggar district, and in the Ochil Hills near Auchterarder,

associated with extensive accumulations of felsitic tuffs and breccias.

They are usually so much decomposed that it is hardly possible to

procure fresh specimens of them. Some of them display beautiful

flow-structure. They appear to be generally orthoclase-felsites or

orthophyres. Dull, fine-grained to flinty in texture, they hardly

ever display free quartz, so that they can seldom be placed among the

typical rhyolites, though in their banded flow-structure they often

strongly resemble some lithoid varieties of these rocks, especially

such varieties as that represented in Fig. 9.

Mr. Watts, to whom I submitted, for microscopic examination, a number

of specimens from the Pentland and Ochil Hills, has found them to

"consist of a brown felsitic groundmass in which are embedded a

generation of small stumpy prisms of orthoclase and a set of larger

phenocrysts, generally consisting of orthoclase and plagioclase in

equal proportions. Brown mica is usually present and zircons are not

uncommon." The rocks, when they undergo weathering, pass into the

varieties formerly comprised under the name claystone.

The only nodular felsite of this age which I have met with is that

of Lough Guitane among the "Dingle Beds," near Killarney, to which

reference will be made in later pages.

2. _Intrusive Bosses, Sills and Dykes._--While the interbedded

lava-sheets are mainly andesites, the intrusive rocks are generally

more acid, and most of them may be grouped under the convenient head

of felsites. Some intrusive andesites, and even more basic rocks, do

indeed occur in dykes and sills, as well as also filling vents. But

the rule remains of general application over the whole country that

the materials which have consolidated in the volcanic orifices of

the Old Red Sandstone, or have been thrust among the rocks in dykes,

bosses or sills, are decidedly acid. In this series of rocks a greater

range of types may be traced than among the extrusive lavas. At the

one end we find true granites or granitites, as in the intrusive

bosses of Spango Water and of Galloway, which, for reasons which I

will afterwards adduce, may with some probability be assigned to the

volcanic history of the Lower Old Red Sandstone period. Among the

bosses, many of which probably mark the positions of eruptive vents,

orthophyres are especially prominent. These rocks frequently contain

no mica, but, on the other hand, they sometimes show abundant quartz

in their groundmass. The augite-granitite of the Cheviot Hills, so

well described by Mr. Teall, has invaded the bedded andesites of that

region.[336] A similar rock has been noticed by my brother, Prof. James

Geikie, associated with the Lower Old Red Sandstone volcanic rocks of

the east of Ayrshire. A remarkable petrographical variety has been

mapped by Mr. B. N. Peach, rising as a small boss through the lower

part of the great lava-sheets of the Ochil Hills, above Tillicoultry.

It is a granophyric quartz-diorite, which, under the microscope, is

seen to be composed of short, thick-set prisms of plagioclase, with

abundant granophyric quartz, a pleochroic hypersthene, and needles

of apatite. Sometimes the pyroxene is replaced by green chloritic

pseudomorphs.[337]

[Footnote 336: _Geol. Mag._ for 1883, pp. 100, 145, 252; and _British

Petrography_, pp. 272, 278.]

[Footnote 337: Notes by Dr. Hatch.]

At the other end of the series come the felsites, quartz-porphyries,

mica-porphyrites, minettes, vogesites, "hornstones" and "claystones"

(or decayed felsites), which have a close-grained texture, often with

porphyritic felspars, quartz or black mica, generally a whitish, pale

buff, orange, pink or purplish-grey colour, and a specific gravity of

about 2·55.[338]

[Footnote 338: The intrusive "porphyry" of Lintrathen in Forfarshire

(which may be younger than the Old Red Sandstone) is a bright red rock

with porphyritic felspar, quartz, white mica and a very singular black

mica (Mr. Teall's _British Petrography_, p. 286).]

Though I class these rocks as intrusive, I am not prepared to assert

that in none of the instances where they occur as sheets may they

possibly have been erupted at the surface as lavas. In one or two cases

the evidence either way is doubtful, but as the great majority of the

acid rocks can be shown to be intrusive in their behaviour, I have

preferred to keep them all in the same category. I am prepared to find,

however, that, as so vast an amount of felsitic debris was ejected to

form the tuffs, more of this material may have flowed out in streams of

lava than is at present recognized.

The following table shows the chemical composition of some acid sills

and dykes from the Lower Old Red Sandstone, as determined in the

laboratory of Prof. E. Frankland:[339]--

[Footnote 339: Two analyses of rhyolites from Shetland by Mr. Tatlock

will be found in _Trans. Roy. Soc. Edin._ vol. xxxii. (1887), p.

387. Their silica percentage is 72·32 and 73·70. An analysis of a

quartz-felsite from the Cheviot Hills by Mr. T. Waller is given in the

Geological Survey Memoir on the Cheviot Hills, p. 25. The proportion of

silica in this rock is 67·9.]

+-----------+------------+------------+-------------+-----------+

+-----------+------------+------------+-------------+-----------+

|SiO_{2} | 73·91 | 64·73 | ·04 | 70·28 |

|Al_{2}O_{3}| 14·41 | 17·01 | 1·01 | 12·54 |

|Fe_{2}O_{3}| ·76 | 2·35 | 1·24 | ·43 |

|MnO | ·07 | ·24 | ... | ... |

|CaO | 1·21 | 4·19 | ·92 | ·91 |

|MgO | 4·90 | ·66 | ·52 | ... |

|K_{2}O | 3·36 | 3·27 | ... | 3·92 |

|Na_{2}O | 1·57 | 3·75 | ... | 5·84 |

|P_{2}O_{5} | ... | ·26 | ·16 | ... |

|H_{2}O | ·90 | 2·78 | ... | 1·99 |

+-----------+------------+------------+-------------+-----------+

[Footnote 340: This specimen also yielded 0·13 of ferrous oxide, and 2·42

of carbon dioxide.]

The rock of Tinto, which may be considered typical of the prevailing

acid intrusive rocks of the series, presents several slightly different

varieties. Dr. Hatch, as the result of his examination of a number of

microscopic slides prepared from specimens taken by me from various

parts of the hill, found some to be minettes, showing small isolated

crystals of orthoclase and rare flakes of biotite, sometimes granules

of quartz, imbedded in a brown, finely microlitic groundmass of felspar

powdered over with calcite; while other specimens had a granular

instead of a microlitic groundmass, and contained a considerable amount

of quartz in addition to the constituents just mentioned. A conspicuous

knob on the south side of Tinto, called the Pap Craig, is a mass of

augite-diorite, which has risen through the other rocks[341] (see Fig.

93). The sills in the same region show still further differences. Some

are true "felspar-porphyries," and "quartz-porphyries" varying in the

relative abundance and size of their porphyritic orthoclase and quartz,

while others, by the introduction of hornblende or pseudomorphs after

that mineral, pass into vogesites.

[Footnote 341: This rock differs considerably from the other intrusive

masses in its neighbourhood. Dr. Hatch found it to be composed chiefly

of lath-shaped striped felspar, with some granular augite, magnetite

and interstitial quartz.]

Basic sills and bosses are chiefly developed among the Ochil and

Sidlaw Hills. They may generally be classed as diabases. But sometimes

their pyroxenic constituent is partly hypersthene, as in a coarsely

crystalline boss about a mile south of Dunning, which has been

determined by Mr. Watts to "consist of augite and hypersthene imbedded

in and occurring amongst large plagioclase prisms. Some iron-ore is

also present; the rock is a hyperite."

3. _Tuffs and Agglomerates._--The fragmental materials, ejected from

or filling up the vents, vary from the finest compacted dust up to

some of the coarsest agglomerates in this country. In general they

consist mainly of detritus of andesite, and have been derived from the

blowing up of already consolidated masses of that rock. The fragments

are usually angular, and range from minute grains up to blocks as large

as a cottage. The tuffs are often more or less mixed with ordinary

non-volcanic sediment, and as they are traced away from the centres of

eruption they pass insensibly into sandstones and conglomerates.

But while, as might be expected, the tuffs are most commonly made

up of debris of the same kind of lavas as those that usually form

the sheets which were poured out at the surface, they include also

bands of material derived from the destruction of much more acid

rocks. Throughout the chain of the Ochil Hills, for example, in the

midst of the bedded andesite-lavas, many of the thin courses of fine

tuff consist largely of felsitic fragments, with scattered felspar

crystals. The most remarkable examples of this nature, however, are to

be met with at the great vent of the Braid Hills, in the chain of the

Pentland Hills which runs south-westward from it, and in the Biggar

volcanic district still further south. These acid tuffs are generally

pale flesh-coloured or lilac in tint, and compact in texture, but,

like the felsitic lavas from which they were derived, they are apt

to weather into yellow or buff "claystones." The finer varieties are

so compact as to present to the naked eye no distinguishable grains;

they might be mistaken for felsites, and indeed, except where they

contain recognizable fragments of rock or broken crystals of felspar,

can hardly be discriminated from them. They consist of an exceedingly

fine compacted felsitic dust. Here and there, however, the scattered

crystals of felspar and small angular fragments of felsite, which may

be detected in them, increase in number until they form the whole of

the rock, which is then a brecciated tuff or fine volcanic breccia,

made up of different felsites, among which, even with the naked eye,

delicate flow-structures may be detected. In these pale acid tuffs,

fragments of different andesites may often be observed, which increase

in number as the rocks are traced away from the main vents of eruption.

At my request my colleague, Mr. George Barrow, determined the silica

percentages in a few specimens which I selected as showing some of

the more characteristic varieties of these tuffs from the Braid and

Pentland Hills. His results are exhibited in the following table:--

Silica

percentage.

1. Quarry above Woodhouselee 63·3

2. South-west side of Castlelaw Hill 73·15

3. Quarry on road, ½ mile N.E. of Swanston (Braid Hill vent) 74·1

4. South-west side of Castlelaw Hill 75·0

5. Castlelaw Hill 76·00

6. South side of White Hill Plantation 90·00

From these analyses it may be inferred that the average amount of

silica in the more typical varieties is between 70 and 75 per cent.

The last specimen in the table, with its abnormally high percentage

of acid, must be regarded as an exceptional variety, where there has

either been an excessive removal of some of the bases, or where silica

has been added by infiltration.

The microscopic examination of these rocks has not added much to

the information derivable from a study of them in the field. In

their most close-grained varieties, as above remarked, they are

hardly to be distinguished from felsites. But they generally show

traces of the minute detrital particles of felsite of which they are

essentially composed. The brecciated varieties exhibit finely-streaked

flow-structure in some of the fragments. Pieces of andesite, grains of

quartz, and other extraneous ingredients appear in these rocks towards

the southern limits of the volcanic area of the Pentland Hills, where

the acid tuffs are associated with and pass laterally and vertically

into ordinary non-volcanic sedimentary strata. Further details as to

the part which these tuffs play in the volcanic history of the regions

wherein they occur will be given in later pages.