We have now to consider the structure and history of those volcanic masses which, during Tertiary time, were ejected to the surface within the area of the British Islands, and now remain as extensive plateaux. Short though the interval has been in a geological sense since these rocks were erupted, it has been long enough to allow of very considerable movements of the ground and of enormous denudation, as will be more fully discussed in Chapters xlviii. and xlix. Hence the superficial records of Tertiary volcanic action have been reduced to a series of broken and isolated fragments. I have already stated that no evidence now remains to show to what extent there were actual superficial outbursts of volcanic material over much of the dyke-region of Britain. The subsequent waste of the surface has been so enormous that various lava-fields may quite possibly have stretched across parts of England and Scotland, whence they have since been wholly stripped off, leaving behind them only that wonderful system of dykes from which their molten materials were supplied.

There can be little doubt, however, that whether or not other Phlegrean fields extended over portions of the country whence they have since been worn away, the chief volcanic tract lay in a broad and long hollow that stretched from the south of Antrim to the Minch. From the southern to the northern limit of the fragmentary lava-fields that remain in this depression is a distance of some 250 miles, and the average breadth of ground within which these lava-fields are preserved may be taken to range from 20 to 50 miles. If, therefore, the sheets of basalt and layers of tuff extended over the whole of this strip of country, they covered a space of some 7000 or 8000 square miles. But they were not confined to the area of the British Islands. Similar rocks rise into an extensive plateau in the Faroe Islands, and it may reasonably be conjectured that the remarkable submarine ridge which extends thence to the North-west of Scotland, and separates the basin of the Atlantic from that of the Arctic Ocean, is partly at least of volcanic origin. Still further north come the extensive Tertiary basaltic plateaux of Iceland, while others of like aspect and age cover a vast area in Southern Greenland. Without contending that one continuous belt of lava-streams stretched from Ireland to Iceland and Greenland, we can have no doubt that in older Tertiary time the north-west of Europe was the scene of more widely-extended volcanic activity than had shown itself at any previous period in the geological history of the whole continent. The present active vents of Iceland and Jan Mayen are not improbably the descendants in uninterrupted succession of those that supplied the materials of the Tertiary basaltic plateaux, the volcanic fires slowly dying out from south to north. But so continuous and stupendous has been the work of denudation in these northern regions, where winds and waves, rain and frost, floe-ice and glaciers reach their highest level of energy, that the present extensive sheets of igneous rock can be regarded only as magnificent relics, the grandeur of which furnishes some measure of the magnitude of the last episode in the extended volcanic history of Britain.

The long and wide western valley in which the basalt-plateaux of this country were accumulated seems, from a remote antiquity, to have been a theatre of considerable geological activity. There are traces of some such valley or depression even back in the period of the Torridon Sandstone of the north-west. This formation, as we have seen, was laid down between the great ridge of the Outer Hebrides and some other land to the east, of which a few of the higher mountains, once buried under the sandstone, are now being revealed by denudation between Loch Maree and Loch Broom, and also in Assynt. The conglomerates and volcanic rocks of Lorne may represent the site of one of the older water-basins of this ancient hollow. The Carboniferous rocks, which run through the North of Ireland, cross into Cantyre, and are found even as far north as the Sound of Mull, mark how, in later Palæozoic time, the same strip of country was a region of subsidence and sedimentation. During the Mesozoic ages, similar operations were continued; the hollow sank several thousand feet, and Jurassic strata to that depth filled it up. Before the Cretaceous period, underground movements had disrupted and irregularly upheaved the Jurassic deposits, and prolonged denudation had worn them away, so that when the Cretaceous formations came to be laid down on the once more subsiding depression, they were spread out with a strong unconformability on everything older than themselves, resting on many successive horizons of the Jurassic system, and passing from these over to the submerged hillsides of the crystalline schists. Yet again, after the accumulation of the Chalk, the sea-floor along the same line was ridged up into land, and the Chalk, exposed to denudation, was deeply trenched by valleys, and entirely removed from wide tracts which it once covered.

It was in this long broad hollow, with its memorials of repeated subsidences and upheavals, sedimentation and denudation, that the vigour of subterranean energy at last showed itself in volcanic outbreaks, and in the gradual piling up of the materials of the basalt-plateaux. So far as we know, these outbursts were subærial. At least no trace of any marine deposit has yet been found even at the base of the pile of volcanic rocks. Sheet after sheet of lava was poured out, until several thousand feet had accumulated, so as perhaps to fill up the whole depression, and once more to change entirely the aspect of the region. But the volcanic period, long and important as it was in the geological history of the country, came to an end. It, too, was merely an episode during which denudation still continued active, and since which subterranean disturbance and superficial erosion have again transformed the topography. In wandering over these ancient lava-fields, we see on every hand the most stupendous evidence of change. They have been dislocated by faults, sometimes with a displacement of hundreds of feet, and have been hollowed out into deep and wide valleys and arms of the sea. Their piles of solid rock, hundreds of feet thick, have been totally stripped off from wide tracts of ground which were once undoubtedly buried under them. Hence, late though the volcanic events are in the long history of the land, they are already separated from us by so vast an interval that there has been time for cutting down the wide plateaux of basalt into a series of mere scattered fragments. But the process of land-sculpture has been of the utmost service to geology, for, by laying bare the inner structure of these plateaux, it has provided materials of almost unequalled value and extent for the study of one type of volcanic action.

I. NATURE AND ARRANGEMENT OF THE ROCKS OF THE PLATEAUX

The superficial outbursts of volcanic action during Tertiary time in Britain are represented by a comparatively small variety of rocks. These consist almost wholly of basalts, but include a number of less basic rocks which may be classed as andesites. Many andesitic sheets, like the andesitic dykes, have been intruded into the basalts, and are really sills.

Besides the lavas of the basaltic-plateaux there are intercalated deposits of tuffs and breccias and large masses of agglomerate. A brief notice of the general petrography of the various constituents of the plateaux and their mode of occurrence will here be given. The intrusive bosses which have disrupted the superficial lavas will be discussed in subsequent chapters.

i. LAVAS

1. Petrographical Characters

(a) Basalts and Dolerites.—In external characters these rocks range from coarsely crystalline varieties, in which the constituent minerals may be more or less readily detected with the naked eye or a field-lens, to dense black compounds in which only a few porphyritic crystals may be megascopically visible. One of their characteristic features is the presence of the ophitic structure, sometimes only feebly developed, sometimes showing itself in great perfection. Many of the rocks are holocrystalline, but usually show more or less interstitial matter; in others the texture is finer, and the interstitial matter more developed; in no case, as far as I have observed, are there any glassy varieties, which are restricted to the dykes and sills, though in some of the basalts the proportion of glassy or incompletely devitrified substance is considerable. The felspars are generally of the characteristic lath-shaped forms, and are usually quite clear and fresh. The augite resembles that of the dykes, occurring sometimes in large plates that enclose the felspars, at other times in a finely granular form. Olivine is frequently not to be detected, even by green alteration products. Magnetite is sometimes present in such quantity as to affect the compass of the field-geologist. Porphyritic varieties occur with large felspar phenocrysts; but such varieties are, I think, less frequent among the plateau-rocks than among the dykes. They are well developed in the west part of the island of Canna, and have been described from the Faroe islands. Occasionally the plateau lavas are full of enclosed fragments of other rocks which have been carried up in the ascending magma.

(b) Andesites and Trachytes.—Probably the majority of these rocks where they occur intercalated between the basalts of the plateaux are, as already remarked, intrusive sheets rather than true lavas. But they have also been poured out intermittently among the basalts and dolerites. The most extensive development of lavas which are readily distinguishable from the group of plateau-basalts, and must be placed in the present series, occurs in the island of Mull. These rocks form part of a group of pale lavas which overlie the main mass of the plateau-basalts, and cap the mountain Ben More, together with several of its lofty neighbours. They are interstratified with true ophitic dolerites, and basalts showing characteristic granular augite. They are not so heavy as the ordinary plateau-lavas, their specific gravity ranging from 2·55 to 2·74. Externally they are light grey in colour and dull in texture, sometimes strongly amygdaloidal, sometimes with a remarkable platy structure, which, in the process of weathering, causes them to split up like stratified rocks. In some of their amygdaloidal varieties the cells are filled with epidote, which also appears in the fissures, and sometimes even as a constituent of the rock.

Specimens from this "pale group" of Ben More, when examined in thin slices under the microscope, were found by Dr. Hatch to consist almost wholly of felspar in minute laths or microlites, but in no instance sufficiently definite for satisfactory determination. In one of them he observed that each lath of felspar passed imperceptibly into those adjacent to it; the double refraction being very weak, and the twin-striation, if present, not being traceable.[220] More recently my colleague, Mr. W. W. Watts, has looked at some of the same slides. He is disposed to class the rocks rather with the trachytes than the andesites. He remarks that "in the apparent holocrystalline character, the size and shape of the felspars, the sort of damascened appearance in polarized light, the finely scattered iron-ores and the presence of a pale green hornblende, possibly augite, in small, often complex, grains, these rocks much resemble the Carboniferous trachytes of the Garlton Hills in Scotland."

One of the most interesting lavas of the Tertiary volcanic series is the "pitchstone-porphyry" of the Scuir of Eigg. This rock, the latest known outflow of lava in any of the volcanic areas of Britain, was formerly classed with the acid series. Microscopical and chemical analyses prove it, however, to be of intermediate composition, and to be referable to the andesites or dacites. It is more particularly described in Chapter xxxviii.

Professor Judd, collecting the andesitic rocks as a whole (both lavas and sills), has grouped them into amphibole and mica-andesites, and pyroxene-andesites.[221] The thick lumpy and non-persistent sheets of these rocks sometimes found near the centres of protrusion of the gabbros and granophyres are probably sills.

(c) Rhyolites.—In the Antrim plateau a group of rhyolite bosses occurs, some of which have been claimed as superficial lavas. In some cases it can be demonstrated that they are intrusive, and in no instance can they be decisively shown to have escaped in streams at the surface. It is probable, however, that some of these bosses did actually communicate with the outer air, for between the lower and upper group of basalts in this plateau, bands of rhyolitic conglomerate occur which may indicate the degradation of exposed masses of rhyolite. The description of these Antrim bosses will be given in Chapter xlvii., in connection with the acid eruptive rocks of the Tertiary volcanic series.

2. Structure in the Field

Passing now to the consideration of the lavas as they are built up into the plateaux, we have to note their distinctive characters as individual sheets of rock, and their influence on the topography of the regions in which they occur. Every tourist who has sailed along the cliffs of Antrim, Mull, Skye, or the Faroe Islands is familiar with the singular terraced structure of the great volcanic escarpments which stretch as mural precipices along these picturesque shores. Successive sheets of lava, either horizontal or only gently inclined, rise above each other from base to summit of the cliffs as parallel bars of brown rock with intervening strips of bright green grassy slope.

The geologist who for the first time visits these coast-lines is impressed by the persistence of the same lithological characters giving rise to the same topographical features. He soon realises that the plateaux, so imposingly truncated by the great escarpments that spring from the edge of the sea, are built up essentially of dark lavas—basalts and dolerites—and that fragmental volcanic accompaniments, though here and there well developed, play, on the whole, a quite insignificant part in the structure and composition of these thick piles of volcanic material. Closer examination in the field enables him to ascertain that, regarded as rock-masses, the lavas include four distinct types:—

1st. Thick, massive, prismatic or rudely-jointed sheets, rather more coarsely crystalline and obviously more durable than the other types, inasmuch as they project in tabular ledges and tend to retain perpendicular faces owing to the falling away of slices of the rock along lines of vertical joints. Many rocks of this type are undoubtedly intrusive sheets, and as such will be further referred to in a later chapter. But the type includes also true superficial lavas which show the characteristic slaggy or vesicular bands at their upper and lower surfaces. The mere presence of such bands may not be enough, indeed, absolutely to establish that the rock possessing them flowed at the surface as a lava, for they are occasionally, though it must be confessed rarely, exhibited by true sills. But the rough scoriaceous top of a lava-stream, and the presence of fragments of this surface in the overlying tuff, or wrapped round by the next succeeding lava, sufficiently attest the true superficial outflow of the mass.

2nd. Prismatic or columnar basalts, which, as at the Giant's Causeway and Staffa, have long attracted notice as one of the most striking topographical elements of the plateaux. Columnar structures are typical of the more compact heavy basalts. A considerable variety is observable in the degree of perfection of their development. Where they are least definite, the rock is traversed by vertical joints, somewhat more regular and close-set than those in the dolerites, by the intersection of which it is separated into rude quadrangular or polygonal columns. The true columnar structure is shown in two chief forms. (a) The rock is divided into close-fitting parallel, usually six-sided columns; the number of sides varying, however, from three up to nine. The columns run the whole thickness of the bed, and vary from 8 or 10 to 40 or even 80 feet in length. They are segmented by cross joints which sometimes, as at Giant's Causeway, take the ball-and-socket form. Occasionally they are curved, as at the well-known Clam-shell cave of Staffa. (b) The prisms are much smaller, and diverge in wavy groups crowded confusedly over each other, but with a general tendency upwards. This starch-like aggregation may be observed superposed directly upon the more regular columnar form as at the Giant's Causeway and also at Staffa. Excellent illustrations of both these types may be seen at many points along the sea-cliffs of the Inner Hebrides; the western coast of Skye, the south-west side of Mull, and the cliffs of the island of Canna may be specially cited.

Though generally rather compact, becoming indeed dense, almost vitreous rocks in some sheets, the columnar basalts are often more or less cellular throughout, and highly slaggy along their upper and under surfaces. In some cases, as in that of a prismatic sheet which overlies the rough scoriaceous lava of Camas Tharbernish, in the island of Canna, the rows of vesicles are disposed in lines parallel to the under surface of the sheet (Fig. 259.)

As already remarked with regard to the massive, rudely-jointed sheets, many of the most perfectly columnar rocks of the plateaux are not superficial lavas, but intrusive sills, bosses or dykes. Conspicuous examples of such sills are displayed on the coast of Trotternish in Skye, and of the bosses and dykes at the eastern end of Canna. To these further reference will be made in the sequel. It is not always possible to be certain that columnar sheets which appear to be regularly intercalated among the undoubted lavas of the volcanic series may not be really intrusive. In some instances, indeed, we can demonstrate that they are so, when after continuing perfectly parallel with the lavas above and below them, they eventually break across them. One of the most remarkable examples of this feature is supplied by the great sill of the south-west of Stromö, in the Faroe Islands, of which I shall give some account in Chapter xlii. (Figs. 312, 328, 329).

3rd. Slaggy or amygdaloidal lavas without any regular jointed structure, but often with roughly scoriform upper and under layers, and tending to decay into brown earthy debris. Some of the upper surfaces of such sheets among the Tertiary basalt-plateaux must have resembled the so-called "Aa" of the Sandwich Islands. A striking example of the structure may be noticed at Camas Tharbernish, on the north coast of the Island of Canna. There the hummocks on the upper surface of a slaggy basalt measure about 15 feet in breadth, and rise about three feet above the hollows between them, like a succession of waves (see Fig. 259). The steam-holes are disposed in a general direction parallel to the strike of the hummocks.

Great variety obtains in the size and shape of the vesicles. Huge cavities a foot or more in diameter may occasionally be found, and from such extremes every gradation may be traced down to minute pore-like vacuoles that can hardly be made out even with a strong lens. In regard to the deformation of the vesicles, it is a familiar general rule that they have been drawn out in the direction of the flow of the original lava. Occasionally they have become straight, narrow, sometimes bifurcating pipes, several inches long, and only an eighth of an inch or so in diameter.[222] A number of such pipes, parallel to each other, resembles a row of worm-burrows (see Fig. 2).

It may often be noticed that, even where the basalt is most perfectly prismatic, it presents a cellular and even slaggy structure at the bottom. The rock that forms the Giant's Causeway, for instance, is distinctly vesicular, the vesicles being drawn out in a general east and west direction. The beautiful columnar bed of Staffa is likewise slaggy and amygdaloidal for a foot or so upwards from its base, and portions of this lower layer have here and there been caught up and involved in the more compact material above it. Even the bottom of the confusedly prismatic bed above the columnar one on that island also presents a cellular texture. A similar rock at Ardtun, in Mull, passes upward into a rugged slag and confused mass of basalt blocks, over which the leaf-beds lie.

Amygdaloidal structure is more or less developed throughout the whole series of basalts. But it is especially marked in certain abundant sheets, which, for the sake of distinction, are called amygdaloids. These beds, which form a considerable proportion of the materials of every one of the plateaux, are distinguished by the abundance and large size of their vesicles. In some places, the cavities occupy at least as much of the rock as the solid matrix in which they lie. They have generally been filled up with some infiltrated mineral—calcite, chalcedony, zeolites, etc. The amygdales of the west of Skye and of Antrim have long been noted for their zeolites. As a consequence of their cellular texture and the action of infiltrating water upon them, these amygdaloidal sheets are always more or less decomposed. Their dull, lumpy, amorphous aspect contrasts well with the sharply-defined columnar sheets above and below them, and as they crumble down they are apt to be covered over with vegetation. Hence, on a sea-cliff or escarpment, the green declivities between the prominent columnar basalts usually mark the place of such less durable bands.

Exceedingly slag-like lavas are to be seen among the amygdaloids, immediately preceded and followed by beds of compact black basalt with few or no vesicles. From the manner in which such rocks yield to the weather, they often assume a singularly deceptive resemblance to agglomerates. One of the best examples of this resemblance which have come under my notice is that of the rock on which stands Dunluce Castle, on the north coast of Antrim. Huge rounded blocks of a harder consistency than the rest of the rock project from the surface of the cliffs, like the bombs of a true volcanic agglomerate, while the matrix in which they are wrapped has decayed from around them. But an examination of this matrix will soon convince the observer that it is strongly amygdaloidal, and that the apparent "bombs" are only harder and less cellular portions of it. The contrast between the weathering of the two parts of the rocks seems to have arisen from an original variety in the relative abundance of steam-cavities. The origin of such nodular or pillow-like blocks has been already referred to at pp. 26 and 193. Another singular instance occurs at the foot of the outlier of Fionn Chro (Fig. 360), in the island of Rum. A conspicuous band underlying the basalts there might readily be taken for a basalt-conglomerate. But in this case, also, the apparent matrix is found to be amygdaloidal, and the rounded blocks are really amygdales, sometimes a foot in length, filled or lined with quartz, chalcedony, &c.

A somewhat different structure, in which, however, the appearance of volcanic breccia or agglomerate due to explosion from a vent is simulated, may be alluded to here. The best instance which I have observed of it occurs at the south end of Loch-na-Mna, in the island of Eigg, within a basalt which is remarkable for a streaky flow-structure. On the weathered faces the streaky layers may be observed to have been broken up, and their disconnected fragments have been involved in ordinary basalt wherein this flow-structure is not developed, while large blocks and irregular masses are wrapped round in a more decomposing matrix. There can be no doubt that in such cases we see the effects of the disruption of chilled crusts, and the entanglement of the broken pieces in the still fluid lava.

It is a common belief that the filling in of the steam-cavities has taken place long subsequent to the volcanic period, by the slow percolation of meteoric water through the rock. I believe, however, that at least in some cases, if not in all, the conversion of the vesicular lavas into amygdaloids was effected during the volcanic period. Thus it can be shown that the basalts which have been disrupted by the gabbros and granophyres were already amygdaloids before these basic intrusions disturbed them, for the kernels of calcite, zeolite, etc., have shared in the general metamorphism induced in the enclosing rock. Again, the blocks of amygdaloid contained in the agglomerates of the volcanic series are in every respect like the amygdaloidal lavas of the plateaux. It would thus seem that the infilling of the cavities with mineral secretions was not merely a long secular process of infiltration from the cool atmosphere, but was more rapidly completed by the operation of warmer water, either supplied from volcanic sources or heated by the still high temperature of the cellular lavas into which it descended from the surface.[223]

4th. Banded or stratiform lavas, consisting of successive parallel layers or bands which weather into projecting ribs and flutings. The deceptive resemblance to sedimentary rocks thus produced has no doubt frequently led to these lavas being mistaken for tuffs. As I have recently found them to be much more plentiful than I had supposed, a more detailed description of them seems to be required.

The banded character arises from marked distinctions in the texture of different layers of a lava-sheet. In some cases (a) these distinctions arise from differences in the size of the crystals or in the disposition of the component minerals of the rock; in others (b) from the varying number and size of the vesicles, which may be large or abundantly crowded together in some layers, and small or only sparsely developed in others. The structure thus points to original conditions of the lava at the time of its emission and may be regarded as, to some extent, a kind of flow-structure on a large scale.

(a) Where the banding is due to differences of crystalline texture, the constituent felspars, augites, and iron-ores may be seen even with the naked eye as well-defined minerals along the prominent surfaces of the harder ribs, while the broader intervening flutings of finer material show the same minerals in minuter forms. The alternating layers of coarser and finer crystallization lie, on the whole, parallel with the upper and under surfaces of the sheets in which they occur. But they likewise undulate like the streaky lines in ordinary flow-structure.

Banded structure of this type may be seen well developed in the lower parts of the basalt-plateaux throughout the Inner Hebrides and the Faroe Islands. A specimen taken from the west end of the island of Sanday, near Canna, which showed the structure by a conspicuous parallel fluting on weathered surfaces, was sliced for microscopical examination. Mr. Harker has been kind enough to supply me with the following observations regarding this slice:—

"In the slice [6660][224] the banding becomes less conspicuous under the microscope. The rock is of basaltic composition, and, with reference to its micro-structure, might be styled a fine-grained olivine-diabase or olivine-dolerite in some parts of the slice, an olivine-basalt in others. It consists of abundant grains of olivine, imperfect octahedra and shapeless granules of magnetite, little simple or twinned prisms of labradorite, and a pale brown augite. The last-named mineral is always the latest product of consolidation, but it varies in habit, being sometimes in ophitic patches moulded upon or enclosing the other minerals, sometimes in small granules occupying the interstices between the felspars and other crystals. The ophitic habit predominates in the slice, while the granulitic comes in especially along certain bands. If the former be taken as indicative of tranquil conditions, the latter of a certain amount of movement in the rock during the latest stages of its consolidation, the banding, though not strictly a flow-structure, may be ascribed in some degree to a flowing movement of the nearly solidified rock. There is, however, more than this merely structural difference between the several bands. They differ to some extent in the relative proportions of the minerals, especially of olivine and augite; which points to a considerable flowing movement at an early stage in a magma which was initially not homogeneous."

(b) Where the banding arises from the distribution of the vesicles, somewhat similar weathered surfaces are produced. In some instances, while the basalt is throughout finely cellular, interposed bands of harder, rather finer-grained and less thoroughly vesicular character serve to give the stratified appearance. Instances may be observed where the vesicles have been crowded together in certain bands, which consequently weather out differently from the layers above and below them. An excellent illustration of this arrangement occurs in the lowest lava but one of the largest of the three picturesque stacks known as Macleod's Maidens on the west coast of Skye (Figs. 260, 283, 284 and 287). This lava is thoroughly amygdaloidal, but the vesicles are specially crowded together in certain parallel bands from an inch to three or four inches thick. Some of these layers lie close to each other, while elsewhere there may be a band of more close-grained, less vesicular material between them. But the most singular feature of the rock is to be seen in the shape and position of the vesicles that are crowded together in the cellular bands. Instead of being drawn out into flattened forms in the general direction of banding, they are placed together at high angles. Each layer remains parallel to the general bedding, but its vesicles are steeply inclined in one direction, which was doubtless that of the flow of the still unconsolidated lava.[225] Weathering along these bands, the lava might easily be mistaken at a little distance for a tuff or other stratified intercalation.

Banded lavas possessing the characters now described are of frequent occurrence among the Inner Hebrides. Many striking examples of them may be seen along the west coast of Skye. Still more abundant in Faroe, they form one of the most conspicuous features in the geology of that group of islands. Along the whole of its western seaboard, on island after island, they are particularly prominent in the lower parts of the precipices, while the upper parts consist largely of amorphous or prismatic sheets. So much do they resemble stratified rocks that it was not until I had landed at various points that I could satisfy myself that they are really banded lavas.[226]

5th. Ordinary flow-structure, save in these banded lavas, is rather rare among the plateaux. It may, however, be occasionally observed, where there is no distinct banding. On a weathered surface it appears in fine, widely parallel streaks, which are sometimes wavy, puckered and broken up, as in rhyolites and felsites, while the porphyritic felspars are arranged with their long axes in the direction of flow. A good example of these characters may be seen on the summit of the Dùn Can—the remarkable truncated cone which forms the highest point on the Island of Raasay. The rock is a black olivine-basalt, partly amygdaloidal, with zeolites filling up the cavities, and its flow-lines are prominent on the weathered faces where they lie parallel to the general bedding of the lavas. Another illustration may be observed in the basalt already cited from Loch-na-Mna, in the island of Eigg, where the rock presents in places a remarkable streaky structure which, though hardly visible on a fresh fracture, reveals itself on a weathered face in thin nearly parallel ribs coincident in direction with the upper and under surfaces of the mass.

Great variety is to be found in the thickness of different sheets of lava in the plateaux. Some of them are not more than 6 or 8 feet; others reach to 80 or 100 feet, and sometimes, though rarely, to even greater dimensions. In Antrim, the average thickness of the flows is probably from 15 to 20 feet.[227] In the fine coast-sections at the Giant's Causeway, however, some bands may be seen far in excess of that measurement. The bed that forms the Causeway, for instance, is about 60 or 70 feet thick, and seems to become even thicker further east. Along the great escarpment, 700 feet high, which rises from the shores of Gribon, on the west coast of Mull, there are twenty separate beds, which give an average of 35 feet for the thickness of each flow. On the great range of sea-precipices along the west coast of Skye, which present the most stupendous section of the basalts anywhere to be seen within the limits of the British Islands, the average thickness of the beds can be conveniently measured. At the Talisker cliffs some of the flows are not more than 6 or 8 feet; others are 30 or 40 feet. The chief precipice, 957 feet high (Fig. 286), contains at least 18 or 20 separate lava-sheets, which thus average of from 47 to 53 feet in thickness. In the cliffs that form the seaward margin of the tableland of Macleod's Tables (Fig. 283) fourteen successive beds of basalt can be counted in a vertical section of 400 feet, which is equal to an average thickness of about 28 feet. But some of the basalts are only about 6 feet thick, while others are 50 or 60. The Hoe of Duirinish, 759 feet high, is composed of about sixteen distinct beds, which thus have a mean thickness of 46 feet. The average thickness of the successive flows on Dunvegan Head, which is 1000 feet high and contains at least twenty-five separate sheets, is about 40 feet. Still further north, the cliffs, 800 feet high, comprise sixteen successive flows, which have thus an average of 50 feet each. Among the Faroe Islands the average thickness of the basalt-sheets seems to be nearly the same as in Britain. Thus in the magnificent ranges of precipices of Kalsö, Kunö and Borö, forty or more sheets may be counted in the vast walls of rock some 2000 feet high, giving a mean of about 50 feet.

Each bed appears, on a cursory inspection, to retain its average thickness, and to be continuous for a long distance. But I believe that this persistence is in great measure deceptive. We can seldom follow the same bed with absolutely unbroken continuity for more than a mile or two. Even in the most favourable conditions, such as are afforded by a bare sea-cliff on which every sheet can be seen, there occur small faults, gullies where the rocks are for the time concealed, slopes of debris, and other failures of continuity; while the rocks are generally so like each other, that on the further side of any such interruption, it is not always possible to make sure that we are still tracing the same bed of basalt which we may have been previously following. On the other hand, a careful examination of one of these great natural sections will usually supply us with proofs that, while the bedded character may continue well marked, the individual sheets die out, and are replaced by others of similar character. Cases may not infrequently be observed where the basalt of one sheet abruptly wedges out, and is replaced by that of another. Where both are of the same variety of rock, it requires close inspection to make out the difference between them; but where one is a green, dull, earthy, amorphous amygdaloid, and the other is a compact, black, prismatic basalt, the contrast between the two beds can be recognized from a distance (Fig. 261). In the basaltic cliffs of the west coast of Skye, the really lenticular character of the flows can be well seen. I may especially cite the great headland south of Talisker Bay, already referred to, where, in the pile of nearly horizontal sheets, two beds may be seen to die out, one towards the north, the other towards the south. Further north, in the cliff of the Hoe of Duirinish, a similar structure presents itself. Along the coast-cliffs of Mull, Morven and Canna the same fact is clearly displayed. Thus on the west side of the Sound of Mull the slopes above Fishnish Bay show a group of basalts, which die out southward, and are overlapped by a younger group that has been poured over their ends. Such sections are best seen in the evening, when the grass-covered lavas show their successive sheets by their respective shadows, their individuality being lost in the full light of day. A more striking example occurs beyond the west end of Glen More in Mull, where one series of basalts has been tilted up, probably during some volcanic episode, and has had a younger series banked up against its edges.

In Antrim also, remarkable evidence is presented of the rapid attenuation not of single beds only, but of a whole series of basalts. Thus, at Ballycastle, the group of lavas known as the Lower Basalts, which underlie the well-known horizon of iron-ore, are at least 350 feet thick. But, as we trace them westwards, bed after bed thins out until, a little to the west of Ballintoy, in a distance of only about 6 miles, the whole depth of the group has diminished to somewhere about 40 feet. A decrease of more than 300 feet in six miles or 50 feet per mile points to considerable inequalities in the accumulation of the lavas. If the next series of flows came from another vent and accumulated against such a gentle slope, it would be marked by a slight unconformability. Structures of this kind are much rarer than we should expect them to be, considering the great extent to which the plateaux have been dissected and laid open in cliff-sections.

The basalt-plateau of the Faroe Islands exhibits with remarkable clearness the lenticular character of the basalt-sheets, and a number of examples will be cited in the description of that region to be given in Chapter xxxix. In these northern climes vegetation spreads less widely over rock and slope than it does in the milder air of the Inner Hebrides. Hence the escarpments sweep in precipices of almost bare rock from the level of the sea up to the serrated crests of the islands, some 2000 feet in height. Each individual bed of basalt can thus be followed continuously along the fjords, and its variation or disappearance can be readily observed. Coasting along these vast natural sections, we readily perceive that, as among the Western Isles, the successive sheets of basalt have proceeded from no one common centre of eruption. They die out now towards one quarter, now towards another, yet everywhere retain the universal regularity and gentle inclinations of the whole volcanic series.

ii. FRAGMENTAL ROCKS

While the plateaux are built up mainly of successive flows of basaltic lavas, they include various intercalations of fragmental materials, which, though of trifling thickness, are of great interest and importance in regard to the light which they cast on the history of the different regions during the volcanic period. I shall enumerate the chief varieties of these rocks here, and afterwards give fuller details regarding their stratigraphical relations and mode of occurrence in connection with the succession of beds in each of the plateaux.

(a) Volcanic Agglomerates.—In the tumultuous unstratified masses of fragmentary materials which fill eruptive vents in and around the plateaux, the stones, which vary in size up to blocks several feet in diameter, consist for the most part of basalts, often highly slaggy and scoriaceous. They include also fragments of different acid eruptive rocks (generally felsitic or rhyolitic in texture), with pieces of the non-volcanic rocks through which the volcanic pipes have been drilled. The paste is granular, dirty-green or brown in colour, and seems generally to consist chiefly of comminuted basalt. As in the Carboniferous and Permian necks, the Tertiary agglomerates contain abundant detritus of a basic minutely cellular pumice.

(b) Volcanic Conglomerates and Breccias in beds intercalated between the flows of Basalt.—These are of at least three kinds. (a) Basalt-conglomerates, composed mainly of rounded and subangular blocks of basalt (or allied basic lava), sometimes a yard or more in diameter, not unfrequently in the form of pieces of rough slag or even of true bombs, imbedded in a granular matrix of comminuted basalt-debris. In some cases, the stones form by far the most abundant constituents of the rock, which then resembles some of the coarse agglomerates just described. Perhaps the most remarkable accumulations of this kind are those intercalated among the basalts in the islands of Canna and Sanday, of which a detailed account will be given in Chapter xxxviii. These conglomerates, besides their volcanic materials, contain rounded blocks of Torridon sandstone and other rocks, which must have been carried from the east by some tolerably powerful river that flowed across the basalt-plains during the volcanic period. Again, on the east side of Mull, the slaggy basalts of Beinn Chreagach Mhor are occasionally separated by volcanic conglomerates. As a rule, however, such intercalations are seldom more than a few feet or yards in thickness. Their coarseness and repetition on successive horizons indicate that they probably accumulated in the near neighbourhood of one or more small vents, from which discharges of fragmentary materials took place at the beginning or at the close of an outflow of lava, and that the stones were sometimes swept away from the cones and rolled about by streams before being buried under the succeeding lava-sheets. More commonly the dirty-green or dark-brown granular matrix exceeds in bulk the stones embedded in it. It has obviously been derived mainly from the trituration of already cooled basalt—masses, and probably also from explosions of the still molten rock in the vents. A striking illustration of this type of rock may be seen on the south side of Portree Harbour, where a mass of dark-green basalt-conglomerate, with a coaly layer above it, lies near the base of the bedded basalts, and attains at one part of its course a thickness of about 200 feet. This rock will be again referred to in connection with the vent from which its materials were probably derived. As in the case of the agglomerates of the vents, pieces of older acid lavas, and still more of the non-volcanic rocks that underlie the plateaux, are found in the bedded conglomerates and breccias. In Antrim and Mull, for instance, fragments of flint and chalk are of common occurrence. A characteristic example of this kind of rock forms the platform of the columnar bed out of which Fingal's Cave, Staffa, has been excavated (Fig. 266a).

(β) Felsitic Breccia.—This variety, though of rare occurrence, is to be seen in a number of localities in the island of Mull. It is composed in great measure of angular fragments of close-grained flinty felsitic or rhyolitic rocks, sometimes showing beautiful flow-structure, together with pieces of quartzite and amygdaloidal basalt, the dull dirty-green matrix appearing to be made up chiefly of basalt-dust.

(γ) Rhyolitic Conglomerate.—Between the upper and lower group of basalts in the Antrim plateau there occur bands of a pale fawn-coloured conglomerate largely made up of more or less rounded fragments of rhyolite, like some of the varieties of the rock which occur in place on the plateau. The rhyolitic debris is often mixed with pebbles of basalt. Sometimes it becomes so fine as to pass into pale clays.

(δ) Breccias of non-volcanic materials.—These, the most exceptional of all the fragmentary intercalations in the plateaux, consist almost wholly of angular blocks of rocks which are known to underlie the basalts, but with a variable admixture of basalt fragments. They are due to volcanic explosions which shattered the subjacent older crust of rocks, and discharged fragments of these from the vents or allowed them to be borne upwards on an ascending column of lava. Pieces of the non-volcanic platform are of common occurrence among the fragmentary accumulations, especially in the lower parts of the plateaux basalts. But I have never seen so remarkable an example of a breccia of this kind as that which occurs near the summit of Sgurr Dearg, in the south-east of Mull. The bedded basalt encloses a lenticular band of exceedingly coarse breccia, consisting mainly of angular pieces of quartzite, with fragments of amygdaloidal basalt. In the midst of the breccia lies a huge mass or cake of erupted mica-schist, at least 100 yards long by 30 yards wide, as measured across the strike up the slope of the hill. To the west, owing to the thinning out of the breccia, this piece of schist comes to lie between two beds of basalt. A little higher up, other smaller but still large blocks of similar schist are involved in the basalt, as shown in Fig. 262. As the huge cake of mica-schist plunges into the hill, its whole dimensions cannot be seen; but there are visible, at least, 15,000 cubic yards, which must weigh more than 30,000 tons. Blocks of quartzite of less dimensions occur in the basalts on Loch Spelve, in the same district. There can be no doubt, I think, that these enormous fragments were torn off from the underlying crystalline schists which form the framework of the Western Highlands, and were floated upward in an ascending flow of molten basalt. Had the largest mass occurred at or near the base of the volcanic series, its size and position would have been less remarkable. But it lies more than 2000 feet up in the basalts, and hence must have been borne upward for more than that height. A similar but less striking breccia occurs on the south coast of the same island, near Carsaig, made up chiefly of pieces of quartzite and quartz.[228]