CHAPTER XXXVIII

THE BASALT-PLATEAU OF THE PARISH OF SMALL ISLES--RIVERS OF

THE VOLCANIC PERIOD

iii. PARISH OF SMALL ISLES PLATEAU

The parish of Small Isles includes the islands of Eigg, Rum, Canna,

Sanday and Muck (Map VI.). The fragmentary basalt-plateau which it

contains, although the smallest of the whole series, is surpassed by

none in the variety and interest of its geology. It contains by far the

most complete records of the rivers which, during the volcanic period,

flowed across the lava plains. And it alone has preserved a relic of

the latest lava which, after the basalt-plateau had been built up and

had been greatly eroded, flowed over the denuded surface in streams of

volcanic-glass that found their way into a river-channel and sealed it

up.

That the fragments of the basaltic plateau preserved in each member

of the group of the Small Isles were once connected as a continuous

volcanic plain can hardly be doubted. Indeed, as already stated, they

were not improbably united with the plateau of Skye on the north, and

with that of Mull, Morven and Ardnamurchan oh the south. Taking the

whole space of land and sea within which the basalt of Small Isles

is now confined, we may compute it at not much less than 200 square

miles. In Eigg, Muck, Canna and Sanday the basalts retain their almost

horizontal position, and from underneath them the Jurassic strata

emerge in the first of these islands. The central part of the plateau

in the island of Rum has suffered greatly from denudation. It now

consists of four small outliers of basalt, which lie at levels of

1200 feet and upwards, on the western slope. The basalt is underlain

by a thick mass of red Torridon Sandstone, which, with some gneisses

and schists, forms the general underlying platform of this island.

These rocks are doubtless a continuation of the red sandstone and

schists of Sleat, in Skye, and like them have been subjected to those

post-Cambrian convolutions and metamorphism whereby the Lewisian

Gneiss and Torridon Sandstone have been brought above younger rocks,

and have been crushed and rolled out so as to assume a new schistose

arrangement. Before the time when volcanic action began, a mass of high

ground, consisting of these ancient rocks, stood where the island of

Rum is now situated. The streams of basalt spread around it, not only

covering the surrounding low tracts of Jurassic rocks, but gradually

accumulating against the hills, and thus reducing them both in area

and in height above the plain.[246] Viewed from Canna the western coast

of Rum presents a striking picture of the general relations of the

volcanic masses of the Inner Hebrides and of the enormous denudation

which they have undergone (Fig. 267). The Torridon Sandstones are

there seen to mount into ranges of hills, capped with outliers of

the basalt-plateau, while behind rise the great eruptive bosses of

gabbro and granophyre. The edges of the sheets that form the outliers

would, if prolonged, cover the northern or lower half of the island,

where pre-Cambrian rocks form the surface. In the southern half,

the continuity of the basalt has been partly obscured and partly

destroyed by the protrusion of the great masses of gabbro that form the

singularly picturesque mountain group to which this island owes its

prominence as a landmark far and wide along the West Coast of Scotland.

[Footnote 246: That the lava-fields did not completely bury this nucleus

of older rocks has been supposed to be shown by the fragments of red

sandstone found in the ancient river-bed of Eigg, which was scooped

out of the basalt-plateau and sealed up under pitchstone. But I am

disposed to think that these fragments, together with those of Jurassic

sandstone, came, not from Rum, but from some district more to the north

and east, as will be explained in a later page. At Canna, a few miles

to the west, fragments of red sandstone not improbably derived from Rum

are abundant in the conglomerates between the basalts.]

[Illustration: Fig. 267.--View of Rum from the harbour of Canna.

The ground indicated by single birds is the area of Torridon Sandstone;

two birds, the plateau basalts; three birds, the gabbro just seen at

one point above the granophyre hills; four birds, the granophyre.]

The most varied and interesting of the fragments of the basaltic

plateau in the area of the Small Isles is that which forms the island

of Canna, with its appendage Sanday. Canna measures five miles in

length by from half a mile to a mile in breadth, and consists entirely

of the rocks of the plateau and their accompaniments. The basalts are

exposed along the north coast in a range of mural precipices rising

to a height of about 600 feet above the sea. From the top of that

escarpment the ground falls by successive rocky terraces and grassy

slopes to the southern shore-line. Sanday, connected with the large

island by a shoal and foot-bridge, is two miles long and 220 to about

1200 yards broad. Its highest cliffs range along its southern shore

to a height of 193 feet, whence they slope gently northward into the

hollow between the two islands. This peculiar topography accounts

for the manner in which the geological sections of most interest are

distributed.

The first, and still the best, account of the geology of these islands

is that of Macculloch. He showed that the rocks all belong to the

series of the plateau-basalts, and he described the presence among them

of a "trap-conglomerate." He noticed the occurrence also of trap-tuff

and the occasional appearance of carbonized wood in these deposits.

Reasoning upon these observations in his characteristically vague and

verbose manner, "bewildered in the regions of conjecture," he concludes

that the basalts instead of belonging to "one general formation" have

been successively deposited on the same spot, "since lapse of time is

evidently implied in the formation of a conglomerate." He inclines to

believe that they have been discharged by ancient volcanoes from which

in the course of time all traces of their original outline have been

more or less completely removed, the existing basalts being merely

fragments of once more extensive masses.[247]

[Footnote 247: _Western Isles_, vol. i. pp. 448-459, and pl. xix. Figs.

2, 3 and 4. See also Jameson's _Mineralogy of the Scottish Isles_.]

Macculloch regarded the intercalated-conglomerates as having been

arranged under water and as marking pauses in the deposition of the

sheets of "trap." He gave two diagrams in illustration of the relations

of these detrital deposits, but he expressed no definite opinion as to

their origin, though from one passage it would seem that he inclined

towards the belief that they were formed in the sea.[248] Since his time,

so far as I am aware, no fresh light has been thrown upon the subject.

[Footnote 248: _Op. cit._ pp. 449, 457, pl. xix. Figs. 2 and 3.]

During a yachting cruise in the summer of 1894 I visited Canna for

the first time and found so much that was new to me in regard to the

history of Tertiary volcanic action, and which demanded a careful

survey, that I returned to the locality the following summer and

remained in the island until I had mapped it and its dependencies upon

the Ordnance Survey sheets on the scale of six inches to a mile. The

following narrative is the result of the observations then made.

As far back as the year 1865 I published an account of an ancient

river-channel which, during the volcanic period, had been eroded on the

surface of the basalt-plateau, and of which a small portion had been

preserved under a stream of pitchstone-lava that had flowed into and

buried it.[249] This water-course, now marked by the picturesque ridge of

the Scuir of Eigg, was shown to have been excavated by a stream which

came from the north-east or east, and to be younger, not only than the

plateau-basalts of the district, but than even the dykes which cut

these basalts. Yet that it belonged to the volcanic period was proved

by the manner in which it had been sealed up and preserved under the

black glassy lava of the Scuir. Its history and the data from which

this history is compiled will be narrated in a later part of this

chapter.

[Footnote 249: _Scenery of Scotland_ (1865); _Quart. Journ. Geo. Soc._

vol. xxvii. (1871), p. 303.]

My examination of the islands of Canna and Sanday, however, brought

to light other and more abundant evidence of river-action in the same

region of the Inner Hebrides, but belonging to an earlier part of the

volcanic period. This evidence reveals that a powerful river, flowing

westwards from the Highland mountains, swept over the volcanic plain,

while the sheets of basalt were still being poured forth, and while

volcanic eruptions were taking place from cones of slag.

The basalt-plateau of Canna resembles in all essential particulars

those of the other Western Isles. Its base is everywhere concealed

under the sea, but from the fragments of Torridon Sandstone in its

agglomerates we may infer that it probably rests on that formation,

like the volcanic outliers in Rum. It is formed of successive sheets of

different basalts including the usual banded, amygdaloidal and columnar

forms. Some of them towards the west are specially marked by the great

abundance and large size of their porphyritic felspars. The magnetic

properties of the basalts at the east end of the island have long been

known, and have given rise to various modern myths regarding their

influence on the compasses of passing vessels.

[Illustration: Fig. 268.--Section of the cliffs below Compass Hill,

Isle of Canna.]

But it is in its conglomerates, tuffs and agglomerates and the light

they cast on some aspects of the volcanic period, elsewhere hardly

recorded, that the geology of Canna possesses a special importance. To

these, therefore, we may at once turn.

The conglomerates are best developed at the eastern end of the island,

where the cliffs present the structure represented in Fig. 268. At the

base, and passing under the level of the sea, lies the agglomerate

(_a_) of a vent which will be described in Chapter xli., together with

other eruptive orifices of the various plateaux (p. 288). This rock

has a somewhat uneven upper surface which rises in places about 150

feet above high tide-mark. Here and there it shades off upward into

the conglomerate that overlies it; water-worn pebbles appear among its

contents, and rude traces of bedding begin to show themselves, until,

within the course of a few feet, we pass upward into an undoubted

conglomerate. Elsewhere, however, and particularly along the precipices

west of Compass Hill, the two deposits are more distinctly marked

off from each other. The agglomerate has there a hummocky, irregular

upper surface, as if it had been thrown down in heaps. The hollows

between these protuberances have been filled up with conglomerate and

sandstone, forming the base of the thick overlying deposit.

It is thus clear that the loose materials of the vent were directly

exposed at the surface when the conglomerate was accumulated, and,

indeed, that these materials served to supply some of the detritus of

which the conglomerate consists. The absence of any trace of a cone and

crater at the vent may perhaps be explicable on the supposition that

their incoherent material was washed down by the currents that swept

along and deposited the conglomerate.

The mass of sedimentary material (_b_) which overlies the agglomerate

of the vent forms a conspicuous feature along the lower half of the

precipices at the eastern end of Canna. It rises to a height of 250

to 300 feet above sea-level, and must reach a maximum thickness of

probably not less than 100 to 150 feet. It gradually descends in a

westward direction, both along the northern cliffs and in the lower

ground round Canna Harbour, insomuch that in about a mile, owing to the

gentle westerly dip of the whole volcanic series, combined with the

effect of a number of small faults, it passes under the level of the

sea.

Great variation in the character of the detritus composing this thick

group of strata may be observed as it is followed westward. On the

cliffs below Compass Hill, as represented in Fig. 268, the coarse

conglomerate with water-worn stones, hardly to be distinguished from

the volcanic agglomerate of the vent, shows more or less distinct

bedding, or at least a succession of coarser and finer bands. Towards

its base it encloses numerous pieces of Torridon Sandstone, sometimes

subangular, but often so well and smoothly rounded as to show that they

must have been long subjected to the action of moving water. It is

further observable that, while in the agglomerate the volcanic stones

have rough surfaces, those in the conglomerate begin to show increasing

evidence of attrition, until, as the deposit is traced upwards, they

become almost as well rounded and water-worn as the non-volcanic stones.

Yet amidst and overlying these proofs of transport from some little

distance lie abundant huge slags and blocks of amygdaloidal lava,

sometimes closely aggregated, sometimes scattered through a volcanic

tuff or ashy sandstone. The composition and structure of these stones,

and the manner of their dispersion through the deposit, leave little

doubt that they were ejected from the vent. We are thus confronted

with the interesting fact that, while the materials of the volcanic

cone were being washed down by running water, eruptions were still

taking place. But by degrees these indications of contemporaneous

volcanic activity diminish. The detrital materials become coarser and

more distinctly water-rolled until they pass into greenish sandstones

and fine conglomerates. Yet the matrix even of these higher sediments

is largely composed of fine volcanic detritus, and probably points to

occasional discharges of dust and ashes.

Various sills or intrusive sheets have been injected into this

sedimentary group along the precipices at the east end of Canna, and

form there lenticular bands. One of these (_c_) is shown in Fig. 268.

Immediately above the massive greenish pebbly sandstone (_d_) which

caps the stratified series lies a group of basalts (_e_), composed of

several distinct beds, having a united thickness of from 80 to 100

feet. The lowest of these has a regular columnar structure, while those

overlying it exhibit the confused starch-like grouping of curved and

rather indistinctly-formed prisms.

The next band in upward succession is one of conglomerate (_f_), which

runs as a continuous and conspicuous feature along the upper part of

the cliff. This rock presents in many respects a strong contrast to the

conglomerates underneath. It is dull-green to yellow in colour, and

is well stratified, being marked by the interstratification of finer

layers, and passing down into a band of pebbly sandstone, which rests

immediately on the basalt (_e_). Its component stones are thoroughly

water-worn, ranging up to six inches or even more in length. But its

most distinctive character lies in the nature of its pebbles. Instead

of consisting mainly of volcanic materials, these stones have almost

all been transported for some distance. They include abundant fragments

of Torridon Sandstone, gneiss, schists, grits, and other rocks like

those in Rum and Western Inverness-shire. No such rocks exist _in situ_

in Canna. The nearest tract of Torridon Sandstone is in Rum, about four

miles to the eastward. But the pieces of schist and epidotic grit like

the rocks of the Western Highlands, have probably travelled at least 30

miles.

It is important to observe that all these transported stones indicate

a derivation from some source lying to the eastward of Canna. The

evidence in this respect agrees with that furnished by the ancient

river-gravel under the pitchstone of the Scuir of Eigg. It is clear

that the waters which found their way across the lava-fields of this

part of the Inner Hebrides took their rise somewhere to the eastward,

probably among the mountains of Inverness-shire.

The conglomerate now described is from 40 to 50 feet thick. It can be

followed along the face of the cliffs for more than a mile on the north

side of Canna. Less persistent on the south side, its outcrop strikes

from the edge of the precipice inland, keeping to the south of the top

of Compass Hill. It is well seen in the ravine above the Coroghon, but

cannot be followed further westward among the basalt-terraces. Yet,

though this stratified intercalation is not traceable far as a band of

conglomerate, the same stratigraphical horizon is probably indicated

elsewhere by other kinds of sedimentary deposits, to which further

reference will be made in the sequel.

The section now described establishes the existence of at least two

successive platforms of conglomerate in the volcanic series. Following

these platforms along their outcrop, we obtain additional light on

their origin, and on the topographical conditions under which they

were deposited, and we learn further that other prolonged intervals,

which were likewise marked by intercalations of sedimentary material,

occurred in the outpouring of the basalts.

Taking first the lower conglomerate of Compass Hill and tracing it

westward, we find it to form the depression in which the sheltered

inlet of Canna Harbour lies. It is exposed along the shores and also

in the islands enclosed within the same bay. But it is not traceable

further west, possibly because it seems to sink beneath the level of

the sea. To the south-east, though it is there likewise for the most

part concealed under the waves, it rises above them in one or two parts

of the coast-line of Sanday, particularly at the Uamh Ruadh or Red

Cave, and likewise on a surf-beaten skerry off Ceann an Eilein, the

highest part of the Sanday cliffs--a distance of about a mile and a

half from Compass Hill. Throughout this space it retains its remarkably

coarse character and is mainly made up of volcanic material.

The numerous sections exposed in Canna Harbour enable us to study the

composition and local variations of this curious deposit. On the north

side of the basin, while the lower part of the sedimentary series

continues to be an exceedingly coarse volcanic conglomerate, it passes

upward into finer conglomerates, tuffs, and shales. In front of Canna

House the imbedded blocks are of large size, occasionally as much as

three or four feet in diameter. They are still more gigantic on the

island of Eilean a' Bhaird, where I found one to contain 150 cubic feet

in the exposed part, the rest being still concealed in the matrix. As

they are generally somewhat rounded, here and there markedly so, most

of these stones have probably undergone a certain amount of attrition

in water. The great majority of them, and certainly all those of

larger size, are pieces of basalt, dolerite, andesite, etc. Among them

huge blocks of amygdaloid and coarsely vesicular lava are specially

abundant. Some of these look like pieces of slag torn from the upper

surface of lava-streams. Others, displaying a highly vesicular centre

and a close-grained outer crust, are suggestive of bombs. It is

interesting to note here again that the amygdaloidal blocks present

their zeolitic infiltrations so precisely like those of the amygdaloids

of the plateau that it seems reasonable to suppose the carbonate of

lime, zeolites, etc. to have been introduced before the blocks were

imbedded in the conglomerate.

The whole aspect of this deposit is eminently volcanic. It looks like

a vast sheet of lava-fragments swept away from one or more cones of

slags and cinders, or from the scoriaceous surface of a lava-stream.

Where the vesicles were still empty, the large boulders could be more

easily swept along by moving water. But a powerful current must have

been needed to transport and wear down into more or less rounded forms

blocks of basic lava, many of which must weigh several tons. The large

block on Eilean a' Bhaird probably exceeds 12 tons in weight.

Besides the obviously volcanic contents of the conglomerate there occur

here also, as in the Compass Hill cliffs, abundant pieces of Torridon

Sandstone. These stones are notably smaller in size and more perfectly

water-worn and even polished than the blocks of lava. Obviously they

have travelled further and have undergone more prolonged attrition.

The matrix of the rock consists essentially of the fine detritus of

basic lavas, probably mingled with true volcanic dust. The coarser

parts display only the feeblest indication of stratification; indeed,

in a limited exposure the rock might be regarded as a tumultuous

agglomerate. But the manner in which the deposit is intercalated

with, and sometimes overlies, green tuffs and shales, together with

the water-worn condition of its stones, shows that it has not been

accumulated in a volcanic chimney, but has been thrown down by some

powerful body of water, with probably the co-operation of volcanic

discharges.

While the composition of the conglomerate suffices to indicate that

this deposit was formed at a time when some volcano was active in the

immediate neighbourhood, singularly convincing proofs of the work of

this vent are to be seen in the form of intercalated sheets of lava.

Thus on Eilean a' Bhaird the boulders of the conglomerate are overlain

and wrapped round by a sheet of rudely prismatic basalt, with lines of

vesicles arranged in the direction of the bedding. A similar relation

can be traced along the beach between Canna House and the wooden pier,

where successive sheets of basalt have flowed over the conglomerate

(Fig. 269).

But, besides coarse volcanic detritus, the sedimentary platform

represented by the lower conglomerate of Compass Hill includes other

deposits of which good sections may be examined all round Canna

Harbour. Beds of fine well-stratified dull-green tuff pass by an

admixture of pebbles into fine ashy conglomerate or pebbly sandstone,

and by an increase in the proportion of their fine detritus into

volcanic mudstone and fine shales. The shales vary from a pale grey

or white tone into blackish grey, brown, and black. They are well

stratified and are frequently interleaved with layers of fine tuff.

The darker bands are carbonaceous, and are not infrequently full

of ill-preserved vegetation. Indeed, leaves and stems in a rather

macerated condition are of common occurrence in all the shaly layers.

Here and there, especially in some ashy shales in front of Canna House,

I observed a recognisable _Sequoia_. The mudstones are dull green,

close-grained shattery rocks composed of fine volcanic detritus,

and pass both laterally and vertically into shales, tuffs, and

conglomerates. They suggest showers of fine dust or streams of volcanic

mud. They, too, contain fragmentary plants.

It is a noteworthy fact that the sedimentary intercalations among the

Canna basalts generally end upward in carbonaceous shales or coaly

layers. The strong currents and overflows of water, which rolled

and spread out the coarse materials of the conglomerates, gave way

to quieter conditions that allowed silt and mud to gather over the

water-bottom, while leaves and other fragments of vegetation, blown

or washed into these quiet reaches, were the last of the suspended

materials to sink to the bottom. Good illustrations of this sequence

in the case of the lower conglomerate zone of Canna may be studied

along the shores of Sanday, from the Catholic Chapel eastwards. The

fine pebbly sandstones, tuffs, and shales, which there overlie the

coarse conglomerate, are surmounted by dark brown or black carbonaceous

shale, with lenticles of matted vegetation that pass into impure coal.

Immediately overlying this coaly layer comes a sheet of prismatic

vesicular basalt, followed by another with an exceedingly slaggy

texture.

Lenticles of shale and mudstone likewise occur in the heart of the

finer parts of the conglomerate, especially towards the top, as may be

seen in the section exposed beneath the basalt behind the first cottage

west from Canna House. One of the most interesting layers in this

section is a seam of tuff, varying up to about two inches in thickness,

which lies at the top of the lenticular band of tuffs and shales, and

immediately beneath the band of basalt-conglomerate, on which a basalt,

carrying a vesicular band near its bottom, rests. Traced laterally, the

dark brown tuff of this seam gradually passes into a series of rounded

bodies and flattened shells composed of a colourless mineral which has

evidently been developed _in situ_ after the deposition of the tuff.

Mr. Harker's notes on thin slices made from this band are as follows:--

"This is a rusty-brown, dull-looking rock, rather soft and seemingly

light, but too absorbent to permit of its specific gravity being

tested. The dark brown mass is in great part studded with little

spheroidal bodies, 1/50 to 1/10 inch in diameter, of paler colour,

but the larger ones having a dark nucleus. In other parts larger flat

bodies have been formed, as if by the coalescence of the spheroids,

extending as inconstant bands in the direction of lamination for

perhaps 1/2 inch, with a thickness of 1/10 inch or less. The appearance

is that of a spherulitic rather than an oolitic structure.

"A slice [6658 A] shows the general mass of the rock to be of an

extremely finely divided but coherent substance of brown colour, which

can scarcely be other than a fine volcanic dust, composed of minute

particles of basic glass or 'palagonite' compacted together. Scattered

through this are fragments of crystals recognizable as triclinic and

perhaps monoclinic felspars, green hornblende, augite, olivine (?), and

magnetite, usually quite fresh.

"The curious spheroidal and elongated growths already mentioned are

better seen in another slide [6658 B], where they occupy the larger

part of the field, leaving only an interstitial framework of the brown

matrix. The substance of the little spheroids is clear, colourless,

and apparently structureless. The centre is often occupied by an

irregularly stellate patch of brown colour, and sometimes cracks tend

to run in radiating fashion, but these are the only indications of

radial structure. The outer boundary is sharply defined, and where the

slice is shattered the spheroids have separated from the matrix. The

matrix is darker than in the normal rock, being obscured by iron-oxide

which we may conceive as having been expelled from the spaces occupied

by the spheroids. The little crystal-fragments are enclosed in the

spheroids as well as in the matrix, but there is no appearance of

their having served as starting-points for radiate growths. The flat

elongated bodies are like the spheroids, with merely the modifications

implied in their different shape.

"The identity of the clear colourless substance seems to be rather

doubtful. It is sensibly isotropic and of refractive power distinctly

lower than that of felspar. These characters would agree with analcime,

which is not unknown as a contact-mineral; but it is difficult to

understand how analcime, even a lime-bearing variety like that of Plas

Newydd,[250] could be formed in abundance from palagonitic material.

An alternative supposition, perhaps more probable, is that the clear

substance is a glass, modified from its former nature, especially by

the expulsion of the iron-oxide into the remaining matrix. A comparison

is at once suggested with certain types of 'Knotenschiefer,' but

respecting the thermal metamorphism of fine volcanic tuffs there seems

to be little or no direct information."

[Footnote 250: Henslow, _Trans. Camb. Phil. Soc._ (1821), vol. i. p.

408; Mr. Harker, _Geol. Mag._ (1887), p. 414. Mr. W. W. Watts suggests

a comparison with the hexagonal bodies figured by Mr. Monckton in an

altered limestone from Stirlingshire: _Quart. Journ. Geol. Soc._, vol.

li. p. 487.]

Lenticular interstratifications of shale and mudstone make their

appearance even in the coarser parts of the conglomerate, as may be

observed on the beach below Canna House where, as shown in Fig. 269,

some shales and tuffs (_a_) full of ill-defined leaves are surmounted

by a conglomerate (_b_). The deposition of this overlying bed of

boulders has given rise to some scooping-out of the finer strata

underneath. Subsequently both the conglomerate and shales have been

overspread by a stream of dolerite (_c_), the slaggy bottom of which

has ploughed its way through them.

[Illustration: Fig. 269.--Lava cutting out conglomerate and shale.

Shore below Canna House.]

Before discussing the probable conditions under which the group of

sedimentary deposits now described was formed, we may conveniently

follow the upper conglomerate band of the Compass Hill, and note the

variations in structure and composition which its outcrop presents.

This yellowish conglomerate can be traced along the cliffs for more

than a mile, when it descends below the sea-level at the solitary stack

of Bod an Stòl. A few hundred yards further west, what is probably

the same band appears again at the base of the precipice overlain

by prismatic basalts. But the conglomerate, here only 12 feet thick,

is made of much finer detritus which, largely composed of volcanic

material, includes small well-rounded and polished pebbles of Torridon

Sandstone. Beneath it lies a bed of dark shale, with remains of plants,

resting immediately on a zeolitic amygdaloid which plunges into the

sea. The chief interest of this locality is to be found in the shale

which, instead of being at the top of the sedimentary group, lies at

the bottom. I was informed by Mr. A. Thom that leaves had been obtained

from this shale; but I was not successful in my search for them. The

locality is only accessible by boat, and, as the coast is fully exposed

to the Atlantic swell, landing at the place is usually difficult and

often impossible.

About a mile and a half still further west, where a foreshore fronts

the precipice of Earnagream at the Camas Tharbernish, a band of

intercalated sedimentary material underlies the great escarpment of

basalts and rests upon the slaggy sheet with the singular surface

already referred to (p. 187). This band not improbably occupies the

same platform as the upper conglomerate of Compass Hill. It is only

about seven feet thick, the lower four feet consisting of a dull green

pebbly tuff or ashy sandstone, with small rounded pieces of Torridon

Sandstone, while the upper three feet are formed of dark shale with

crowded but indistinct remains of plants. Here the more usual order

in the sequence of deposition is restored. The shale is indurated

and shattery, so that no slabs can be extracted without the use of

quarrying tools.

[Illustration:

Fig. 270.--Section of shales and tuffs, with a coniferous stump

lying between two basalt-sheets, Cùl nam Marbh, Canna.

]

Rather less than half a mile towards the south, on the roadside

at the gully of Cùl nam Marbh, the basalts enclose a sedimentary

interstratification which not improbably lies on the same horizon as

those just described along the northern shore. The relations of the

rocks at this locality are shown in Fig. 270. A remarkable slaggy

basalt (_a_) rises into a hummock, against which have been deposited

some fine granular tuffs (_b_) whereof only a few inches are visible,

that pass up into a thin band of dark shale (_c_), including a layer

of pebbly ferruginous tuff, with small rounded pea-like pieces of

basalt, basic pumice, bole, limonite, etc. At the top of this shale an

irregular parting of coaly material (_d_) lies immediately under the

slaggy base of the succeeding basalt (_e_). It will be observed that

this upper lava cuts out the shale and thus comes to rest directly

upon the lower sheet. At the point where it begins to descend it has

caught up and enclosed a small tree-stump (_d´_) which stands upright

on the coaly parting and shale. This stump, at the time of my visit,

measured five inches in height by three inches in breadth; it had

been thoroughly charred and was crumbling away on exposure, but among

the pieces which I took from it sufficient trace of structure can be

detected with the microscope to show the tree to have been a conifer.

We have here another instance of the deposition of volcanic dust and

fine mud in a pool that filled a hollow in the lava-field. Again we see

that the closing act of sedimentation was the subsidence of vegetable

matter in the pool, which was finally buried under another outflow of

basalt.

[Illustration: Fig. 271.--Dùn Mòr, Sanday. (From a photograph by Miss

Thom.)]

It is on the southern coast of the isle of Sanday that the higher

intercalations of sedimentary material among the basalts are most

instructively displayed. At the eastern end of this island, as already

stated, the lowest and coarsest conglomerate is visible on a skerry

immediately to the south of the headland Ceann an Eilein. It doubtless

underlies the Sanday cliffs, but is not there visible, for the

basalts descend below sea-level. These volcanic sheets have a slight

inclination westward; hence in that direction we gradually pass into

higher parts of the series. In the Creag nam Faoileann (Seamews' Crag)

and the gully that cuts its eastern end, likewise in the two singularly

picturesque stacks of Dùn Mòr and Dùn Beag (Big and Little Gull Rocks),

which here rise from the foreshore, two distinct platforms of detrital

material may be noticed among the basalts. Both of these can be well

seen on Dùn Mòr, about 100 feet high, which is represented in Fig.

271. The lower band, four or live feet thick, is here a rather coarse

conglomerate which lies upon a sheet of scoriaceous basalt that extends

up to the base of the Creag nam Faoileann. It is directly overlain by

another basalt, about 30 feet thick, which dips seawards and forms a

broad shelving platform, whereon the tides rise and fall. On this stack

a second coarse conglomerate, about 10 feet thick, forms a conspicuous

band about a third of the height from the bottom; it is composed mainly

of well-rounded blocks of various lavas up to 18 inches or more in

diameter, but it contains also pieces of Torridon Sandstone. It is

covered by about 60 feet of basalt, which towards the base is somewhat

regularly columnar, but passes upward into the wavy, starch-like,

prismatic structure.

If now we trace these two intercalated zones of conglomerate along

the shore, we find them both rapidly to change their characters and

to disappear. The lower, though formed of coarse detritus under the

Dùn Mòr, passes on the opposite cliff in a space of not more than 60

yards, into fine tuff and shale, about six feet thick, which become

carbonaceous at the top where they are overlain by the next basalt.

A hundred yards to the east, the band likewise consists of tuffs and

ashy shales, which underlie the basalts on the Dùn Beag, and again

show the usual coaly layers at the top. On the east side of the gully

in the coast, about 160 yards to the north-east of Dùn Mòr, the same

band is reduced to not more than three feet in thickness, consisting

chiefly of fine conglomerate, wherein well water-worn pebbles of

Torridon Sandstone and epidotic grit appear among the predominant

volcanic detritus. This conglomerate is surmounted by a few inches of

dark carbonaceous mudstone or shale. Rough slaggy basalts lie above and

below the band.

The upper conglomerate dies out, both towards the east and the west,

in the cliff opposite to Dùn Mòr, dwindling down at last to merely a

few pebbles between the basalts. It lies in a kind of channel or hollow

among these lavas. This depression, in an east and west direction,

cannot be more than about 65 yards broad.

Probably still higher in the series of basalts is another intercalation

of sedimentary layers which may be seen in the little bay to the east

of Tallabric, rather more than a mile to the west of the Creag nam

Faoileann. It rests upon a coarsely slaggy amygdaloid, and is from six

to ten feet in thickness. The lower and larger part of the deposit

consists of greenish pebbly sandstone and fine conglomerate, largely

composed of basaltic detritus, but including abundant well-smoothed and

polished pebbles of Torridon Sandstone, green grit, quartzite, etc. The

stones vary from mere pea-like pebbles up to pieces two or three inches

long, the largest being generally fragments of slag and amygdaloid

which are less water-worn than the sandstones and other foreign

ingredients. The uppermost two or three feet of the intercalation

consist of dark carbonaceous mudstone or shale, made up in large

measure of volcanic detritus, which may have been derived partly from

eruptions of fine dust, partly from subærial disintegration of the

basalt-sheets. Some layers of these finer strata are full of remains of

much macerated plants.

Other thin coaly intercalations have been observed among the basalts

of Canna, some of which may possibly mark still higher horizons than

those now described. But, confining our attention to the regular

sequence of intercalations exposed along the Sanday coast, we find

at least four distinct platforms of interstratified sediment among

the plateau-basalts of this district. Each of these marks a longer or

shorter interval in the outflow of lava, and points to the action of

moving-water over the surface of the lava-fields.

We may now consider the probable conditions under which this

intervention of aqueous action took place. The idea that the sea had

anything to do with these conglomerates, sandstones, and shales may

be summarily dismissed from consideration. The evidence that the

basalt-eruptions took place on a terrestrial surface is entirely

convincing, and geologists are now agreed upon this question.

Excluding marine action, we have to choose among forms of fresh

water--between lakes on the one hand and rivers on the other. That the

agency concerned in the transport and deposition of these strata was

that of a river may be confidently concluded on the following grounds:--

1. The large size and rolled shape of the boulders in the

conglomerates. To move blocks several tons in weight, and not only to

move them but to wear them into more or less rounded forms, must have

required the operation of strong currents of water. The coarse detritus

intercalated among the basalts is quite comparable to the shingle of a

modern river, which descends with rapidity and in ample volume from a

range of hills.

2. The evidence that the materials of the conglomerates are not

entirely local, but include a marked proportion of foreign stones.

The proofs of transport are admirably exhibited by pieces of Torridon

Sandstone, epidotic grit, quartzite, and other hard rocks none of which

occur _in situ_ except at some distance from Canna. These stones are

often not merely rounded, but so well smoothed and polished as to show

that they must have been rolled along for some considerable time in

water.

3. The lenticular character and rapid lithological variations of the

strata, both laterally and vertically. The coarse conglomerates die

out as they are followed along their outcrop and pass into finer

sediment. They seem to occur in irregular banks, which may not be more

than 200 feet broad, like the shingle-banks of a river. The coarser

sediment generally lies in the lower part of the sedimentary group.

But cases may be observed, such as that shown in Fig. 269, where fine

sediment, laid down upon the bottom conglomerate, has subsequently been

overspread by another inroad of coarse shingle. Such alternations are

not difficult to understand if they are looked upon as indicating the

successive floods and quieter intervals of a river.

For these reasons I regard the platforms of sedimentary materials

intercalated among the basalts of Canna and Sanday as the successive

flood-plains of a river which, like the rivers that traverse the

lava-deserts of Iceland, flowed perhaps in many separate channels

across the basalt-fields of the Inner Hebrides, and was liable to have

its course shifted from time to time by fresh volcanic eruptions.

That this river came from the east or north-east and had its source

among the Western Highlands of Inverness-shire, may be inferred from

the nature of the stones which it has carried for 30 miles or more

along its bed. And that it crossed in its course the tract of Torridon

Sandstone, of which a portion still remains in Rum, is manifest from

the abundance of the fragments of that formation in the conglomerates.

With the remarkable exception of the section on Dùn Beag, to be

immediately referred to, no trace of any eroded channel of this river

through the lavas of the great volcanic plain has been preserved.

Possibly frequent invasions of its bed by streams of basalt from

different vents hindered it from remaining long enough in one course

to erode anything like a gorge or canon. But, in any case, the main

channel of the river probably lay rather to the east of the present

islands of Canna and Sanday, on ground which is now covered by the

sea. The banks or sheets of boulder-conglomerate undoubtedly show

where its current swept with great force over the lava-plain, but the

manner in which these coarser materials are so often covered with fine

silt suggests that the sedimentary materials now visible were rather

deposited on the low grounds over which the steam rushed in times of

flood. Pools of water would often be left after such inundations, and

in these depressions silt would gradually accumulate, partly carried

in suspension by the river, partly washed in by rain, while drift-wood

that found its way into these eddies, and leaves blown into them from

the trees and shrubs of the surrounding country, would remain for

some time afloat and would be the last of the detritus to sink to the

bottom. Hence, no doubt, the carbonaceous character of the hardened

silt in the upper part of each intercalation of sediment.

If we were to look upon the volcanic materials in the conglomerates

as derived from the subærial disintegration of the fields of basalt,

we should be compelled to admit a very large amount of erosion of the

surface of the volcanic plain during the period when the river flowed

over that tract. It would be necessary to suppose not only that there

was a considerable rainfall, but that the differences of temperature,

either from day to night, or from summer to winter, were so great as

to split up the lavas at the surface, in order to provide the river

with the blocks which it has rolled into rounded boulders. I do not

think, however, that such a deduction would be sound. If we compare

the materials that have filled up the large eruptive vent at the east

end of Canna (to be afterwards described) with the great majority of

the blocks in the coarse conglomerates, we cannot fail to note their

strong resemblance. The abundance of lumps of slaggy lava in the

river-shingle corresponds with their predominance in the agglomerate

of the vent. The boulders of basalt, dolerite, and andesite which

crowd the conglomerates need not have been derived from the action

of atmospheric waste on the lava-fields, but might quite well have

been mainly supplied by the demolition of volcanic cones of fragmental

materials.

[Illustration: Fig. 272.--View of the Dùn Beag, Sanday, seen from the

south.

(From a Photograph by Miss Thom.)]

That such has really been the chief source of the blocks in the

conglomerates I cannot doubt. At the east end of Canna we actually

detect a volcanic cone, partly washed down and overlain by a pile

of river-shingle. There were probably many such mounds of slag and

stones along lines of fissure all over the lava-fields. The river in

its winding course might come upon one cone after another, and during

times of flood, or when its waters burst through any temporary barrier

created by volcanic operations it would attack the slopes of loose

material and sweep their detritus onward. At the same time, the current

would carry forward its own natural burden of far-transported sediment,

and hence on its flood-plains, buried and preserved under sheets of

basalt, we find abundant pebbles of the old Highland rocks which it had

borne across the whole breadth of the basaltic lowland.

But the destruction of volcanic cones was probably not the only source

of the basaltic detritus in the conglomerates of Canna and Sanday. I

have shown that these conglomerates pass laterally into tuffs, and

are sometimes underlain, sometimes overlain, with similar material.

It is quite obvious that their deposition was contemporaneous with

volcanic action in the immediate neighbourhood, and that at least part

of their finer sediment was obtained directly from volcanic explosions.

In wandering over the coast-sections of these coarse deposits, I

have been impressed with the enormous size of many of the stones,

their resemblance to the ejected blocks of the agglomerate, and the

distinction that may sometimes be made with more or less clearness

between their rather angular forms and the more rounded and somewhat

water-worn aspect of the other boulders. It seems to me not improbable

that some of the remarkably coarse masses of unstratified conglomerate

in Canna Harbour consist largely of ejected blocks from the adjacent

vent.

[Illustration: Fig. 273.--View of Dùn Beag, Sanday, from the north. The

island of Rum in the distance.

(From a Photograph by Miss Thom.)]

The only instance which I have observed of erosion of the basalt

contemporaneous with the operations of the river that spread out this

conglomerate is to be found in the striking stack of Dùn Beag already

alluded to.[251] This extraordinary monument of geological history

forms an outlying obelisk which rises from the platform of the shore

to a height of about 70 feet. Seen from the south-west, it appears

to consist entirely of bedded basalt resting on some stratified tuff

and shale which intervene between these lavas and that of the broad

platform of basalt on which the obelisk stands. On that side it

presents no essential difference from the structure of the Dùn Mòr to

the west, save that the lower conglomerate of that outlier is here

represented by fine sediment, and the upper conglomerate is wanting.

The general aspect of this south-western front of the stack is shown in

Fig. 272. If, however, we approach the rock from the coast-gully to the

north, we form a very different impression of its structure. It then

appears to consist chiefly of conglomerate with a capping of basalt on

the top (Fig. 273). Not until a close scrutiny is made of the eastern

and western faces of the column do the true structure and history of

this singular piece of topography become apparent.

[Footnote 251: This pinnacle of rock is referred to by Macculloch in

his account of Canna, and is figured in Plate xix. Fig. 3 of his work

already cited. But neither his description nor his drawing conveys any

idea of the real structure of the rock.]

[Illustration: Fig. 274.--Section of eastern front of Dùn Beag.

_a_, Very shaggy amygdaloidal basalt; _b_, shales and tuff; _c_, slaggy

and jointed basalts; _d_, conglomerate; _e_, prismatic basalt.

The dotted lines indicate the supposed form of the ravine.]

On the eastern front, the section represented in Fig. 274 is exposed.

At the bottom, forming the pediment of the column, lies a sheet of

slaggy and vesicular or amygdaloidal basalt (_a_), which shelves gently

in a south-westerly direction into the sea. The lowest band (_b_)

in the structure of the stack is a thin group of lilac, brown, and

green shale and volcanic mudstone or tuff, which encloses pieces of

coniferous wood, and becomes markedly carbonaceous in its uppermost

layers. Above these strata on the south front comes the pile of bedded

basalts (_c_) with their slaggy lower and upper surfaces. But as we

follow them round the east side, we find them to be abruptly cut off by

a mass of conglomerate (_d_). That the vertical junction-line is not a

fault is speedily ascertained. The lower platform of slaggy basalt runs

on unbroken under both shales and conglomerate. Moreover, the line of

meeting of this conglomerate with the basalts that overlie the shales

is not a clean-cut straight wall, but displays projections and recesses

of the igneous rocks, round and into which the materials of the

conglomerate have been deposited. The pebbles may be seen filling up

little crevices, passing under overhanging ledges of the basalts, and

sharply truncating lines of scoriaceous structure in these rocks. The

same relations may be observed on the west front of the stack. There

the ashy shales and tuffs are sharply cut out by the conglomerate,

which wraps round and underlies a projecting cornice of the slaggy

bottom of the basalt that rests on the stratified band (Fig. 275).

[Illustration:

Fig. 275.--Enlarged Section on the western side of Dùn Beag.

_a_, amygdaloid; _b_, tuff; _c_, ashy shales; _d_, layer of coaly

shale; _e_, amygdaloidal basalts conglomerate.

]

The conglomerate is rudely stratified horizontally, its bedding being

best shown by occasional partings of greenish sandstone. It consists

of well-rounded, polished, and water-worn stones, chiefly of members

of the volcanic series--basalts, and dolerites, both compact and

amygdaloidal or slaggy--but with a conspicuous admixture of Torridon

Sandstone, gneiss, grey granite, grit and different schists. The

coarsest part of the deposit lies toward the bottom where the volcanic

blocks, some of them being six and eight feet in diameter, may have

originally fallen from the basalts against which the conglomerate now

reposes. The far-transported stones are also of considerable size,

pieces of granite and gneiss frequently exceeding a foot in length. The

well-rounded pebbles of foreign materials have been washed into the

interstices between the large volcanic blocks.

It is, I think, tolerably clear that the wall of basalt against which

this conglomerate has been laid down is one of erosion. The beds of

basalt have here been trenched by some agent which has likewise scooped

out the soft underlying shales, and even cut them away from under their

protecting cover of basalt. There can be little hesitation in regarding

this agent as a water-course, which for some considerable interval of

time continued to dig its channel through the hard basalts. There is

not room enough between the basalt-wall of Dùn Beag and the opposite

cliffs of the shore (where no trace of this conglomerate is to be seen)

for any large stream to have found its way. I do not therefore seek to

identify this relic of an ancient waterway with the channel of the main

river which deposited the conglomerate bands of Canna and Sanday. More

probably it was either a mere torrential chasm, or a tributary stream

draining a certain part of the volcanic plateau and allowed to retain

its channel long enough to be able to erode it to a depth of nearly

50 feet. Erosion had reached down through the underlying tuffs to the

slaggy basalt below, but before it had made any progress in that sheet

its operations were brought to an end at this locality by the floods

that swept in the coarse shingle, and by the subsequent stream of

basalt of which a mere outlying fragment now forms the upper third of

the stack (_e_, Fig. 274).

That the ravine or gully of Dùn Beag probably lay within the reach

of the floods of the main river, may be inferred from the number and

size of the far-transported rocks in its conglomerate. It was filled

up gradually, but the conditions of deposition remained little changed

during the process, except that the largest blocks of rock were swept

into the chasm in the earlier part of its history, while much smaller

and more water-worn shingle were introduced towards the close.

Denudation, which has performed such marvels in the topography of the

West of Scotland since older Tertiary time, has here obliterated every

trace of this ancient gully, save the little fragment of one of the

walls which survives in the stack of Dùn Beag. When in the course of

centuries this picturesque obelisk shall have yielded to the action

of the elements, the last leaflet of one of the most interesting

chapters in the geological history of the Inner Hebrides will have been

destroyed.

The question naturally arises--What was the subsequent history of the

river which has left so many records of its floods entombed among the

basalts of Canna and Sanday? In particular, can any connection be

traced or plausibly conjectured between it and the river-bed preserved

under the Scuir of Eigg? To this question I shall return after the

evidence for the existence and date of the latter stream has been laid

before the reader.

In the chain of the Inner Hebrides, broken as it is in outline and

varied in its types of scenery, there is no object more striking than

the island of Eigg. Though only about five miles long and from a mile

and a half to three miles and a half broad, and nowhere reaching

a height of so much as 1300 feet, this little island, from the

singularity of one feature of its surface, forms a conspicuous and

familiar landmark. Viewed in the simplest way, Eigg may be regarded as

consisting of an isolated part of the basaltic plateau which, instead

of forming a rolling tableland or a chain of hills with terraced sides,

as in Antrim, Mull and Skye, has been so tilted that, while it caps

a lofty cliff about 1000 feet above the waves at the north end, it

slopes gently along the length of the island to the south end. In its

southern half, however, the ground rises, owing to the preservation of

an upper mass of lavas, which denudation has removed from the northern

half. On this thicker part of the plateau stands the distinguishing

feature of the island, the strange fantastic ridge of the Scuir, which,

seen from the north or south, looks like a long steep hill-crest,

ending in a sharp precipice on the east. Viewed from the east, this

precipice is seen to be the end of a huge mountain-wall, which rises

vertically above the basalt-plateau to a height of more than 350 feet.

The accompanying map (Fig. 276) shows that the ridge of the Scuir

corresponds with the area occupied by a mass of pitchstone, and that

while the basaltic rocks cover the whole of the rest of the southern

half of the island, they gradually rise towards the north, successive

members of the Jurassic series making their appearance until, at the

cliffs of Dunan Thalasgair, the latter cover the greater part of the

surface, and leave the volcanic rocks as a mere stripe capping the

cliffs. In the section (Fig. 277) the general structure of the island

is represented.

[Illustration: Fig. 276.--Geological Map of the Island of Eigg.

P, Pitchstone-lava of the Scuir; R, old river gravel under

pitchstone; _p p_, small veins of Pitchstone; _b b_, dykes, veins

and sheets of intrusive basalt; the short black lines running

north-west and south-east are basalt dykes; _f f_, granophyre

sills; D, bedded basalts with occasional tuffs; F, andesite; 1, 2,

3, 4, clays, shales, sandstones, limestones, etc. (Jurassic); xx,

Loch Beinn Tighe; x, Loch a Bhealaich. --> General dip of the rocks.

]

[Illustration: Fig. 277.--Section of the geological structure of the

Island of Eigg.

P, Pitchstone-lava of Scuir; _c_, ancient river-gravel; _p p_,

pitchstone veins; _f f_, intrusive granophyre, etc.; _b b_, dolerite

and basalt dykes and veins; B, intrusive dolerite and basalt-sheets; D,

bedded dolerites and basalts; F, andesite bed; 1-4, Jurassic rocks.]

In Eigg the fragment of the basalt-plateau which has been preserved,

rests unconformably on successive platforms of the Jurassic formations.

Its component sheets of lava rise in cliffs around the greater part

of the island. As they dip gently southwards their lower members are

seen along the northern and eastern shores, while on the south-west

side their higher portions are exposed in the lofty precipices which

there plunge vertically into the sea. The total thickness of the

volcanic series may here be about 1100 feet. The rocks consist of the

usual types--black, fine-grained, columnar and amorphous basalts,

more coarsely crystalline dolerites, dull earthy amygdaloids with red

partings, and occasional thin bands of basalt-conglomerate or tuff.

The individual beds range in thickness from 20 to 50 or 60 feet.

Though they seem quite continuous when looked at from the sea, yet, on

closer examination, they are found not unfrequently to die out, the

place of one bed being taken by another, or even by more than one,

in continuation of the same horizon. The only marked petrographical

variety which occurs among them is a light-coloured band which stands

out conspicuously among the darker ordinary sheets of the escarpment on

the east side of the island. The microscopic characters of this rock

show it to belong to the same series of highly felspathic, andesitic,

or trachitic lavas as the "pale group" of Ben More, in Mull. It is

strongly vesicular, and the cells are in some parts so flattened and

elongated as to impart a kind of fissile texture to the rock. There

can be no doubt that this band is a true lava, and that it was poured

out during the accumulation of the basalt-plateau. It supplies an

interesting example of the intercalation of a lighter and less basic

lava among the ordinary heavy basic basalts and dolerites.

That feature of the island of Eigg which renders it so remarkable

and conspicuous an object on the west coast is the long ridge of the

Scuir. Rising gently from the valley which crosses the island from

Laig Bay to the Harbour, the basaltic plateau ascends south-westwards

in a succession of terraces, until along its upper part it forms a

long crest, from 900 to 1000 feet above the sea, to which it descends

on the other or south-west side, first by a sharp slope, and then by

a range of precipices. Along the watershed of this crest runs, in a

graceful double curve, the abrupt ridge of the Scuir, terminating

on the north-west at the edge of the great sea-cliff (975 feet), and

ending off on the south-east in that strange well-known mountain-wall

(1272 feet high) which rises in a sheer cliff nearly 300 feet above

the basalt-plateau on the one side and more than 400 feet on the other

(Fig. 278). The total length of the Scuir ridge is two miles and a

quarter, its greatest breadth 1520, its least breadth 350 feet. Its

surface is very irregular, rising into minor hills and sinking into

rock-basins, of which nine are small tarns, besides still smaller

pools, while six others, also filled with water, lie partly on the

ridge and partly on the basaltic plateau. No one, indeed, who looks

on the Scuir from below, and notes how evenly it rests upon the

basalt-plateau, would be prepared for so rugged a landscape as that

which meets his eye everywhere along the top of the ridge. Two minor

arms project from the east side of the ridge; one of these forms the

rounded hill called Beinn Tighe (968 feet), the other the hill of A

chor Bheinn.

[Illustration: Fig. 278.--View of the Scuir of Eigg from the east.]

Singular as the Scuir of Eigg is, regarded merely as one of the

landmarks of the Hebrides, its geological history is not less peculiar.

The natural impression which arises in the mind when this mountain

comes into view for the first time is, that the huge wall is part of a

great dyke or intrusive mass which has been thrust through the older

rocks.[252] It was not until after some time that the influence of this

first impression passed off my own mind, and the true structure of the

mass became apparent.

[Footnote 252: Hay Cunningham remarks:--"In regard to the relations of

the pitchstone-porphyry of the Scuir and the trap-rocks with which it

is connected, it can, after a most careful examination around the whole

mass, be confidently asserted that it exists as a great vein which

has been erupted through the other Plutonic rocks--thus agreeing in

age with all the other pitchstones of the island." Macculloch leaves

us to infer that he regarded the rock of the Scuir to be regularly

interstratified with the highest beds of the dolerite series (_Western

Isles_, i. p. 522). Hugh Miller speaks of the Scuir of Eigg as "resting

on the remains of a prostrate forest."--_Cruise of the Betsy_, p. 32.]

The ridge of the Scuir, presenting as it does so strong a

topographical contrast to the green terraced slopes of the

plateau-basalts on which it rests, consists of some very distinct

bands of black and grey lava, long known as "pitchstone-porphyry." To

the nature and history of these rocks I shall return after we have

considered a remarkable bed of conglomerate which lies below them. On

the lower or southern side of the ridge the bottom of the pitchstone,

dipping into the hill, is exposed on the roof of a small cave where the

ends of its columns form a polygonal reticulation. It is there seen

to repose upon a bed of breccia or conglomerate, having a pale-yellow

or grey felspathic matrix like the more decomposing parts of the grey

devitrified parts of the pitchstone. Through this deposit are dispersed

great numbers of angular and subangular pieces of pitchstone, some

of which have a striped texture. Fragments of basalt, red (Torridon)

sandstone, and other rocks are rare; and the bed suggests the idea that

it is a kind of brecciated base or floor of the main pitchstone mass. A

similar rock is found along the bottom of the pitchstone on both sides

of the ridge (_c_, in Fig. 279). Here and there where this breccia is

only a yard or two in thickness, it consists of subangular fragments

of the various dolerites and basalts of the neighbourhood, together

with pieces of red sandstone, quartzite, clay-slate, etc. The matrix

is in some places a mass of hard basalt debris; in others it becomes

more calcareous, passing into a sandstone or grit in which chips and

angular or irregular-shaped pieces of coniferous wood are abundant.[253]

A little further east, beyond the base of the Scuir, a patch of similar

breccia is seen, but with the stones much more rounded and smoothed.

This outlier rests against the denuded ends of the basalt-beds forming

the side of the hill. Its interest arises from the evidence it affords

of the prolongation of the deposit eastward, and consequently of the

former extension of the precipice of the Scuir considerably beyond its

present front.

[Footnote 253: The microscopic structure of this wood was briefly

described by Witham (_Fossil Vegetables_, p. 37), and two magnified

representations were given to show its coniferous character. Lindley

and Hutton further described it in their "Fossil Flora," naming it

_Pinites eiggensis_, and regarding it as belonging to the Oolitic

series of the Hebrides--an inference founded perhaps on the erroneous

statement of Witham to that effect. William Nicol corrected that

statement by showing that the wood-fragments occurred, not among the

"lias rocks," but "among the debris of the pitchstone" (_Edin. New

Phil. Journal_, xviii. p. 154). Hay Cunningham, in the paper already

cited, states that the fossil wood really lies in the pitchstone

itself! The actual position of the wood, however, in the breccia and

conglomerates underlying the pitchstone is beyond all dispute. I have

myself dug it out of the bed. The geological horizon assigned to this

conifer, on account of its supposed occurrence among Oolitic rocks,

being founded on error, no greater weight can be attached to the

identification of the plant with an Oolitic species. Our knowledge

of the specific varieties of the microscopic structure of ancient

vegetation is hardly precise enough to warrant us in definitely fixing

the horizon of a plant merely from the examination of the minute

texture of a fragment of its wood. From the internal organization of

the Eigg pine, there is no evidence that the fossil is of Jurassic age.

From the position of the wood above the dolerites and underneath the

pitchstone of the Scuir it is absolutely certain that the plant is not

of Jurassic but of Tertiary date.]

It is at the extreme north-western extremity of the pitchstone ridge,

however, that the most remarkable exposure of this intercalated

detrital band is now to be seen. Sweeping along the crest of the

plateau the ridge reaches the edge of the great precipice of Bideann

Boidheach, by which its end is truncated, so as to lay open a section

of the gravelly deposit along which the pitchstone flowed.

The accompanying diagram (Fig. 279) represents the natural section

there exposed. Rising over each other in successive beds, with a hardly

perceptible southerly dip of 2°, the sheets of basalt form a mural

cliff about 700 feet high. The bedded character of these rocks and

their alternations of compact, columnar, amorphous and amygdaloidal

beds are here strikingly seen. They are traversed by veins and dykes

of an exceedingly close-grained, sometimes almost flinty, basalt. But

the conspicuous feature of the cliff is the hollow which has been

worn out of these rocks, and which, after being partially filled with

coarse conglomerate, has been buried under the huge pitchstone mass of

the Scuir. The conglomerate consists of water-worn fragments, chiefly

of dolerite and basalt, but with some also of the white Jurassic

sandstones, imbedded in a compacted sand derived from the waste of the

older volcanic rocks. The grey devitrified bands in the pitchstone, so

conspicuous at the east end of the Scuir, here disappear and leave the

conglomerate covered by one huge overlying mass of glassy pitchstone.

[Illustration: Fig. 279.--Natural Section at the Cliff of Bideann

Boidheach, north-west end of the Scuir of Eigg.

_a a_, Bedded dolerites and basalts; _b_, basalt dykes and veins; _c_,

ancient river-bed filled with conglomerate; _p_, pitchstone of the

Scuir.]

If any doubt could arise as to the origin of the mass of detritus

exposed under the pitchstone at the east end of the Scuir it would be

dispelled by the section at the west end, which shows with unmistakable

clearness that the conglomerate is a fluviatile deposit and lies in the

actual channel of the ancient river which was eroded out of the basalt

plateau, and was subsequently sealed up by streams of pitchstone-lava.

An examination of the fragments of rock found in the conglomerate

affords here, as in Canna and Sanday, some indication of the direction

in which the river flowed. The occurrence of pieces of red sandstone,

which no one who knows West-Highland geology can fail to recognize

as of Torridonian derivation, at once makes it clear that the higher

grounds from which they were borne probably lay to the north or

north-east. The fragments of white sandstone may also have been

derived from the same quarter, for the thick Jurassic series of Eigg

once extended further in that direction. The pieces of quartzite and

clay-slate bear similar testimony to an eastern or north-eastern

source. In short, there seems every probability that this old Tertiary

river flowed through a forest-clad region, of which the red Torridon

mountains of Ross-shire, the white sandstone cliffs of Raasay and Skye,

and the quartzite and schist uplands of Western Inverness-shire are but

fragments, that it passed over a wide and long tract of the volcanic

plateau, and continued to flow long enough to be able to carve out for

itself a channel on the surface of the basalt. Its course across what

is now the island of Eigg took a somewhat north-westerly direction,

probably guided by inequalities on the surface of the lava-plain. It is

there marked by the winding ridge of the Scuir, the pitchstone of which

flowed into the river-bed and sealed it up. Several minor spurs, which

project from the eastern side of the main ridge, show the positions

of small tributary rivulets that entered the principal channel from

the slopes of the basaltic tableland. One of these, on the south-east

side of the hill called Corven, must have been a gully in the basalt

with a rapid or waterfall. The pitchstone has flowed into it, and some

of the rounded pebbles that lay in the channel of this vanished brook

may still be gathered where the degradation of the pitchstone has once

more exposed them to the light. That the Eigg river here flowed in a

westerly direction may be inferred from the angle at which the beds of

the small tributaries meet the main stream, and also from the fact that

the old river-bed at the east end of the Scuir is considerably higher

than at the west end.

Several features in the geological structure of this locality serve to

impress on the mind the great lapse of time represented by the erosion

of the river-channel of Eigg. Thus at the narrowest point of the

pitchstone ridge, near the little Loch a' Bhealaich, the bottom of the

glassy lava is about 200 feet above its base on the south side, so that

the valley cut out of the plateau-basalts must have been more than 200

feet deep. Even the little tributaries had cut ravines or cañons in the

basalts before the ground was buried under the floods of pitchstone. In

the most northerly spur of the ridge, for example, the hill of Beinn

Tighe, which represents one of these tributaries, shows a considerable

difference between the level of the bottom of the pitchstone on the

east and west sides.

Again, all along the ridge of the Scuir, the basalt-dykes are abruptly

cut off at the denuded surface on which the pitchstone rests. This

feature is conspicuously displayed on the great sea-wall at the

west end (Fig. 279). The truncation of the dykes demonstrates that

a considerable mass of material must have been eroded before these

lava-filled fissures could be laid bare at the surface. And the removal

of this material shows that the denudation must have been continued for

a long period of time.

The river-channel of Eigg, since it was eroded long after the cessation

of the outflows of basalt in the plateau of Small Isles, must be much

later in origin than those of Canna and Sanday which, as we have seen,

were contemporaneous with the basalt-eruptions. But the river that

excavated the channels and deposited the gravels may have been the same

in both areas.

In dealing with this subject, though the evidence is admittedly scanty,

we are not left wholly to conjecture. A consideration of the general

topographical features of the wide region of the Inner Hebrides, from

the beginning of the volcanic period onward, will convince us that, in

spite of the effects of prolonged basalt-eruptions, the persistent flow

of the drainage of the Western Highlands must have taken a westerly

direction. It was towards the west that the low grounds lay. Though

the long and broad valley which stretched northwards from Antrim,

between the line of the Outer Hebrides and the West of Scotland, was

gradually buried under a depth of two or three thousand feet of lava,

the volcanic plain that overspread it probably remained even to the

end lower than the mountainous Western Highlands. Hence the rivers,

no matter how constantly they may have had their beds filled up and

may have been driven into new channels, would nevertheless always seek

their way westwards into the Atlantic.

On Canna and Sanday the traces of a river are preserved which poured

its flood-waters across the lava-fields in that part of the volcanic

region, while streams of basalt were still from time to time issuing

from vents and fissures. Not more than fourteen miles to south-east

stands the Scuir of Eigg, with its buried river-channel and its

striking evidence that there, also, a river flowed westwards, but at a

far later time, when the basalt-eruptions had ceased and the volcanic

plain had been already deeply trenched by erosion, yet before the

subterranean fires were finally quenched, as the pitchstone of the

Scuir abundantly proves.

When one reflects upon the enormous denudation of this region, to which

more special reference will be made in the sequel, one is not surprised

that many connecting links should have been effaced. The astonishment

rather arises that so continuous a story can still be deciphered.

Even, however, had the original record been left complete, it would

have been exceedingly difficult to trace the successive mutations of a

river-channel during long ages of volcanic eruptions. Such a channel

would have been concealed from view by each lava-stream that poured

into it, and would not have been again exposed save by the very process

of erosion that destroys while it reveals.

While, therefore, there is not and can never be any positive proof that

in the fluviatile records of Canna, Sanday and Eigg successive phases

are registered in the history of one single stream, I believe that this

identity is highly probable. It was a river which seems to have risen

among the mountains of Western Inverness-shire, and it had doubtless

already taken its course to the sea before any volcanic eruptions

began. It continued to flow westwards across the lava-floor that

gradually spread over the plains. Its channel was constantly being

filled up by fresh streams of basalt or deflected by the uprise of new

cinder-cones. But, fed by the Atlantic rains, it maintained its seaward

flow until the general subsidence which carried so much of the volcanic

plain below the sea. Yet the higher part of this ancient water-course

is no doubt unsubmerged, still traversing the schists of the Western

Highlands as it has done since older Tertiary time. It may, perhaps, be

recognized in one of the glens which carry seaward the drainage of the

districts of Morar, Arisaig, or Moidart.

[Illustration: Fig. 280.--View of the Scuir of Eigg from the South.]

Let us now turn to the remarkable lava which has sealed up the

river-channel of Eigg, and of which the remaining fragment stands up

as the great ridge of the Scuir. This rock presents characters that

strongly distinguish it from the surrounding basalts. It is not one

single uniform mass, but consists of a number of distinct varieties,

some of which are a volcanic glass, while others are a grey "porphyry,"

or devitrified pitchstone. These are arranged in somewhat irregular,

but well-marked, and, in a general sense, horizontal sheets. On the

great eastern terminal gable of the Scuir this bedded structure is not

clearly displayed, for the cliff seems there to be built up of one

homogeneous mass, save a markedly columnar band that runs obliquely up

the base of the precipice (Fig. 278). If, however, the ridge is looked

at from the south, the truly bedded character of its materials becomes

a conspicuous feature. Along the cliffs on that side the two varieties

of rock are strongly distinguished by their contrasting colour and

mode of weathering, the sombre-hued pitchstone standing up in a huge

precipice striped with columns, and barred horizontally with bands of

the pale-grey "porphyry," which, from its greater proneness to decay,

seems sunk into the face of the cliff. At the south-east end of the

ridge the bedding is especially distinct. West of the precipices, to

the south of the Loch a' Bhealaich, the dark pitchstone which forms the

main mass is divided by two long parallel intercalations of grey rock,

and two other short lenticular seams of the same material (see Figs.

280, 281). It is clear from these features, which are not seen by most

travellers who pass Eigg in the tourist-steamer that the Scuir is in

no sense of the word a dyke.

But although the Scuir is thus a bedded mass, the bedding is far

different from the regularity and parallelism of that which obtains

among the bedded basalt-rocks below. Even where no intervening

"porphyry" occurs, the pitchstone can be recognized as made up of many

beds, each marked by the different angle at which its columns lie. And

when the "porphyry" does occur and forms so striking a division in the

pitchstone, its beds die out rapidly, appearing now on one horizon, now

on another, along the face of the cliffs, and thickening and thinning

abruptly in short distances along the line of the same bed. Perhaps the

best place for examining these features is at the Bhealaich, the only

gully practicable for ascent or descent, at the south-eastern face of

the ridge.

[Illustration: Fig. 281.--View of the Scuir of Eigg from the South-west

of the Loch a' Bhealaich, showing the bedded character of the mass.]

By much the larger part of the mass of the Scuir consists of vitreous

material. As a rule this rock is columnar, the columns being much

slimmer and shorter than those of the basalt-rocks. They rise sometimes

vertically, and often obliquely, or project even horizontally from the

face of the cliff. They are seldom quite straight, but have a wavy

outline; and when grouped in knolls here and there along the top of

the ridge they remind one of gigantic bunches of some of the Palæozoic

corals, such as _Lithostrotion_. In other cases they slope out from

a common centre, and show an arrangement not very unlike that of a

Highland peat-stack.

The pitchstone of the Scuir differs considerably in petrographical

character from other pitchstones of the island which occur in dykes

and veins. Its base is of a velvet-black colour, and is so much less

vitreous in aspect than ordinary pitchstone as to have been described

by Jameson and later writers as intermediate between pitchstone and

basalt.[254] A chemical analysis of the rock by Mr. Barker North,[255] gave

the following composition:--

Silica 65·81

Alumina 14·01

Ferric oxide 4·43

Lime 2·01

Magnesia 0·89

Soda 4·15

Potash 6·08

Loss in ignition 2·70

------

100·08

[Footnote 254: _Mineralogy of the Scottish Isles_, vol. ii. p. 47. See

also Macculloch, _Western Isles_, vol. i. p. 521, and Hay Cunningham,

_Mem. Wern. Soc._ vol. viii. p. 155.]

[Footnote 255: _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 379.]

The grey devitrified bands, which occur as a subordinate part of

the mass of the Scuir ridge, are usually somewhat decomposed. Where

a fresh fracture is obtained, the material shows a fine-grained,

sometimes almost flinty, grey felsitic base, containing clear granules

of quartz, and facets of glassy felspar. In some places the rock is

strongly porphyritic. Examined under the microscope it presents a more

thoroughly devitrified groundmass, with the minutest depolarizing

microlites, large porphyritic crystals of plagioclase and sanidine,

grains of augite, and sometimes exceedingly abundant particles of

magnetite.[256]

[Footnote 256: The microscopic structure of the identical pitchstone of

Hysgeir is given on p. 247.]

[Illustration: Fig. 282.--Section at the base of the Scuir of Eigg

(east end).]

Although the line of separation between the grey dull felsitic sheets

and the more ordinary glassy pitchstone is usually well defined,

the two rocks may be observed to shade into each other in such a

manner as to show that the lithoid material is only a devitrified and

somewhat decomposed condition of the glassy rock. This connection is

particularly to be observed under the precipice at the east end of the

Scuir. At that locality the pitchstone is underlain by a very hard

flinty band, varying in colour from white through various shades of

flesh-colour and brown into black, containing a little free quartz and

crystals of glassy felspar. Where it becomes black it passes into a

rock like that of the main mass of the Scuir. Such vitreous parts of

the bed lie as kernels in the midst of the more lithoid and decomposed

rock. The lower six feet of the "porphyry" are white and still more

decomposed. The relations of this mass are represented in Fig. 282,

where the basalt-rocks of the plateau (_a_) are shown to be cut through

by basalt dykes (_b b_), and overlain by the "porphyry" (_c_) and the

pitchstone (_d_). In the porphyry are shown several pitchstone kernels

(_p_, _p_). It is deserving of remark also that in different parts

of the Scuir, particularly along the north side, the bottom of the

pitchstone beds passes into a dull grey earthy lithoid substance, like

that now under description.

The bedded character of the rock of the Scuir and the well-marked

lithological distinction between its several component sheets show the

lava to have been the product of a number of separate outflows that

found their way one after another into the river-valley, which was the

lowest ground in the vicinity of the active vent. There can be little

doubt, I think, that the lava flowed down the valley. Its successive

streams are still inclined from east to west. The vent of eruption,

therefore, ought to be looked for towards the east. Nowhere within the

Tertiary volcanic region is there any boss of pitchstone or any mass

the shape or size of which is suggestive of this vent. In the island of

Eigg no boss of any kind exists, save those of granophyric porphyry to

be afterwards referred to. But none of these affords any satisfactory

links of connection with the rock of the Scuir. More probably the

vent lay somewhere to the east on ground now overflowed by the sea.

The pitchstone veins of Eigg may represent some of the subterranean

extrusions from the same volcanic pipe, and if so, its site could not

be far off.

The rock of the Scuir of Eigg has a special importance in the history

of the volcanic plateaux. It is, so far as we know, the latest of all

the superficial lavas of Britain.[257] From the basalts on which it rests

it was separated by an enormous interval of time, during which these

older lavas were traversed by dykes and were worn down into valleys.

Its presence shows that long after the basalts of Small Isles had

ceased to be erupted, a new outbreak of volcanic activity took place in

this district, when lavas of a more acid composition flowed out at the

surface. Whether this outburst was synchronous with the appearance of

the great granophyric protrusions of the Inner Hebrides, or with the

still later extravasation of pitchstone dykes, can only be surmised.

[Footnote 257: The rocks of Beinn Hiant in Ardnamurchan have been

claimed by Professor Judd as superficial lavas. For reasons to be

afterwards given (p. 318) I regard them as intrusive sheets. Professor

Cole believes the rhyolites and pitchstones of Tardree to be probably

evidence of a volcano later than the basalts of Antrim. As I have not

been able to detect any actual proofs of superficial outflow there, I

relegate the description of the rocks to a future chapter, in which the

acid protrusions will be discussed (p. 426).]

When one scans the great precipice on the west side of Eigg, with

its transverse section of the pitchstone-lava, buried river-bed and

basalt-plateau underneath, there seems no chance of any further

westward trace of the pitchstone being ever found. The truncated end

of the Scuir looks from the top of the cliff out to sea, and the

progress of denudation might have been supposed to have effectually

destroyed all evidence of the continuation of the rock in a westerly

direction. Some years ago, however, my friend Prof. Heddle, while

cruising among the Inner Hebrides, landed upon the little uninhabited

islet of Hysgeir, which, some eighteen miles to the westward of Eigg,

rises out of the open sea. He at once recognized the identity of the

rock composing this islet with that of the Scuir, and in the year 1892

published a brief account of this interesting discovery.[258]

[Footnote 258: Appendix C to _A Vertebrate Fauna of Argyle and the Inner

Hebrides_, by Messrs. J. A. Harvie-Brown and Thomas E. Buckley, p. 248.]

I have myself been able to land on Hysgeir in two successive summers,

and can entirely confirm Prof. Heddle's identification. The islet

stands on the eastern edge of the submarine ridge which, running in a

north-easterly direction, culminates in the island of Canna. Hysgeir

is a mere reef or skerry, of which the top rises only 38 feet above

the Ordnance datum-level. Its surface is one of bare rock, save where

a short but luxuriant growth of grasses has found root on the higher

parts of two or three of its ridges, and on the old storm-beach of

shingle which remains on the summit. The rock undulates in long low

swells, that run in a general direction 20° to 45° west of north, and

are separated by narrow channels or hollows. The place is a favourite

haunt of gulls, terns, eider-ducks and grey seals, and is used by the

proprietor of Canna for the occasional pasturage of sheep or cattle.

So numerous are the sea-fowl during the breeding-season that the

geologist, intent upon his own pursuits, may often tread on their nests

unawares, while he is the centre of a restless circle of white wings

and anxious cries.

The pitchstone of Hysgeir, like that of Eigg, is columnar, the columns

being irregularly polygonal and varying from three to ten inches in

diameter. They are packed so close together that the domes of rock on

which their ends appear look like rounded masses of honeycomb. They may

here and there be observed to be arranged radially with their ends at

right angles to the curved exterior of the ridges, as if this external

surface represented the original form of the cooled pitchstone, and

were not due to mere denudation. There can be no doubt, however, that

the island has been well ice-worn.

At the north-west promontory a beautiful example of fan-shaped grouping

of columns may be observed on a face of rock which descends vertically

into the sea. Here, too, is almost the only section on which the sides

of the columns may be examined, for, as a rule, it is merely their ends

on the rounded domes which are to be observed, and which everywhere

slip under the waves. The columns in a cliff from 15 to 20 feet high

show the slightly wavy, starch-like arrangement so often to be met with

among the plateau-basalts.

The rock presents a tolerably uniform texture throughout, though

in some parts it is blacker, more resinous, and less charged with

porphyritic enclosures than in the general body of the rock. Large

fresh felspars are generally scattered through it. To the naked eye it

reproduces every feature of the pitchstone of the Scuir of Eigg.

A microscopic examination completes our recognition of the identity of

these two rocks. Mr. Harker has examined a thin slice prepared from the

Hysgeir pitchstone, and remarks regarding it that "the large felspars

are not the only porphyritic element. The microscope shows the presence

also of smaller imperfect crystals of augite, very faint green in the

slice, and small grains of magnetite. The felspars have been deeply

corroded by the enveloping magma, and irregular included patches of the

groundmass occupy nearly half the bulk of some of the crystals. This

latter feature is seen especially in some of the larger crystals, which

seem to be sanidine. They are, for the most part, apparently simple

crystals, but in places there is a scarcely defined lamellar twinning,

or, again, small patches not extinguishing with the rest; so that

we are probably dealing with some perthitic intergrowth on a minute

scale.[259]

[Footnote 259: Comp. Prof. Judd's remarks on the Scuir of Eigg rock,

_Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 380.]

"Rather smaller felspar-crystals are rounded by corrosion, but

lack the inclusions of groundmass; these have albite-and sometimes

pericline-lamellation, and may be referred to oligoclase-andesine.

The groundmass of the rock is a brown glass with perlitic cracks,

enclosing very numerous microlites of felspar about ·001 inch in length

[6619]. The rock is probably to be regarded as a dacite rather than a

rhyolite, and thus agrees with Mr. Barker North's analysis of the Eigg

pitchstone."[260]

[Footnote 260: _Op. cit._ p. 379.]

There is no trace of any conglomerate _in situ_ like that under the

Scuir of Eigg, nor of any other rock, aqueous or igneous. As the

pitchstone everywhere slips under the sea, its geological relations are

entirely concealed.

The great variety of materials met with in the form of boulders on

the island is a testimony to the transport of erratics from the

neighbouring islands and the mainland during the Glacial Period. The

most abundant rock in these boulders is Torridon Sandstone, derived

no doubt from the hills of Rum, but there occur also various kinds of

schist, gneisses, quartzites, granites, porphyries, probably from the

west of Inverness-shire, as well as pieces of white sandstone, probably

Jurassic, which may have come from Eigg.

That the pitchstone of Hysgeir is a continuation of that of the Scuir

may be regarded as highly probable. If not a continuation, it must be

another stream of the same kind, and doubtless of the same date. If

it be regarded as probably a westward prolongation of the Eigg rock,

and if it be about as thick as that mass at the west end of the Scuir,

then its bottom lies 200 or 300 feet under the waves. The river-channel

occupied by the Eigg pitchstone undoubtedly sloped from east to west.

The position of Hysgeir, 18 miles further west, may indicate a further

fall in the same direction at the rate of perhaps as much as 35 feet in

the mile.[261] Unfortunately, however, as no trace of the river-bed can

now be seen on this island, any statement in regard to it must rest on

mere conjecture.

[Footnote 261: _Rep. Brit. Assoc._ 1894, p. 653.]

Although the question of the denudation of the basalt-plateaux since

the close of the volcanic period will be the subject of a special

chapter in a later part of this volume, I cannot here refrain from

calling attention to the pitchstone of Eigg and Hysgeir as one of the

most impressive monuments of denudation to be found within the British

Isles. Though now so prominent an object in the West Highlands, this

rock once occupied the bottom of a valley worn out of the basaltic

tableland. Prolonged and stupendous denudation has destroyed the

connection with its source, has cut down its ends into beetling

precipices, has reduced the former surrounding hills into gentle slopes

and undulating lowland, and has turned the bottom of the ancient valley

into a long, narrow and high crest. Moreover, we see that the erosion

has not been uniform. The great wall of the Scuir does not stand fairly

on the crest of the basalt-plateau but on the south side of it, so that

the southern half of the old valley, with all its surrounding hills,

has been entirely cut away. That subsidence has also come into play in

the destruction of even the youngest parts of the volcanic plateaux

will be more fully discussed in a later chapter. I need only remark

here that the submergence of Hysgeir probably points to extensive

depression of the land-surface on which the lavas were poured out.