If a geologist were asked to select that feature in the volcanic geology of the British Isles which, more than any other, marks this region off from the rest of the European area, he would probably choose the remarkable system of wall-like masses of erupted igneous rock, to which the old Saxon word "dykes" has been affixed. From the moors of eastern Yorkshire to the Perthshire Highlands, and from the basins of the Forth and Tay to the west of Donegal and the far headlands of the Hebrides, the country is ribbed across with these singular protrusions to such an extent that it may be regarded as a typical region for the study of the phenomena of dykes. That all the dykes in this wide tract of country are of Tertiary age cannot be maintained. It has been shown in previous Chapters that each of the great volcanic periods has had its system of dykes, even as far back as the time of the Lewisian Gneiss.

But when all the dykes which can reasonably be referred to older geological periods are excluded, there remains a large series which cannot be so referred, but which are connected together by various kinds of evidence into one great system that must be of late geological date, and can be assigned to no other than the Tertiary period in the volcanic history of Britain. As far back as the year 1861, when I first drew attention to this great system of dykes in connection with the progress of volcanic action in the country, I pointed out the grounds on which it seemed to me that these rocks belong to a comparatively recent geological period.[159] My own subsequent experience and the full details of structure collected by my colleagues of the Geological Survey in all parts of the country, have amply confirmed this view. The characters which link this great series of dykes together as one connected system of late geological date are briefly enumerated in the following list, and will be more fully discussed in later pages.

1. The prevalent tendency of the dykes to take a north-westerly course. There are exceptions to this normal trend, especially where the dykes are small and locally numerous; but it remains singularly characteristic over the whole region.

2. The increasing abundance of the dykes as they are traced to the west coast and the line of the great Tertiary volcanic plateaux of Antrim and the Inner Hebrides.

3. The rectilinear direction so characteristic of them and so different from the tortuous course of local groups of dykes. The exceptions to this normal feature are as a rule confined to the same localities where departures from the prevalent westerly trend occur.

4. The great breadth of the larger dykes of the system and their persistence for long distances. This is one of their most remarkable and distinctive characters.

5. The posteriority of the dykes to the rest of the geological structure of the regions which they traverse. They are not only younger than the other rocks, but younger than nearly all the folds and faults by which the rocks are affected.

6. The manner in which they cut the Jurassic, Cretaceous and older Tertiary rocks in the districts through which they run. At the south-eastern end of the region they rise through the Lias and Oolite formations, in the west they intersect the Chalk and also the Tertiary volcanic plateaux together with their later eruptive bosses.

7. Their petrographical characters, among which perhaps the most distinctive is the frequent appearance of the original glass of the plagioclase-pyroxene-magnetite (olivine) rock, of which they mostly consist. This glass, or its more or less completely devitrified representative, often still recognizable with the microscope among the individualized microlites and crystals throughout the body of a dyke, is also not infrequent as a black vitreous varnish-like coating on the outer walls, and occasionally appears in strings and veins even in the centre.

It is the assemblage of dykes presenting these features which I propose to describe. Obviously, the age of each particular dyke can only be fixed relatively for itself. But when this remarkable community of characters is considered, and when the post-Mesozoic age of at least a very large number of the dykes can be demonstrated, the inference is reasonable that one great system of dykes was extravasated during a time of marked volcanic disturbance, which could not have been earlier than the beginning of the Tertiary period. And this inference may be maintained even when we frankly admit that every dyke within the region is by no means claimed as belonging to the Tertiary series.

In spite of their number and the extraordinary volcanic activity to which they bear witness, the dykes form a much less prominent feature in the landscape than might have been anticipated. In the lowlands of the interior, they have for the most part been concealed under a cover of superficial accumulations, though in the water-courses they not infrequently project as hard rocky barriers across the channels, and occasionally form picturesque waterfalls. On the barer uplands, they protrude in lines of broken crag and scattered boulders, which by their decay give rise to a better soil covered by a greener vegetation than that of the surrounding brown moorland. Among the Highland hills, they are often traceable from a distance as long black ribs that project from the naked faces of crag and corry. Along the sea-coast, their peculiarities of scenery are effectively displayed. Where they consist of a close-grained rock, they often rise from the beach as straight walls which, with a strangely artificial look, mount into the face of the cliffs on the one side, and project in long black reefs into the sea on the other (Fig. 233). Every visitor to the islands of the Clyde will remember how conspicuous such features are there. But it is among the Inner Hebrides that this kind of scenery is to be found in greatest perfection. The soft dark Lias shales of the island of Pabba, for example, are ribbed across with scores of dykes which strike boldly out to sea. Where, on the other hand, the material of the dykes is coarse in grain, or is otherwise more susceptible to the disintegrating influences of the weather, it has often rotted away and left yawning clefts behind, the vertical walls of which are those of the fissures up which the molten rock ascended (Fig. 234). Some good instances of this kind are well known to summer visitors on the eastern shores of Arran. Others, on a large scale, may be seen in the interior of the same island along the crests of the granite ridges, and still more conspicuously on the jagged summits of Blath Beinn and the Cuillin Hills (Fig. 333), and intersecting the Jurassic strata along the cliffs of Strathaird in Skye.

1. GEOGRAPHICAL DISTRIBUTION

The limits of the region within which the dykes occur cannot be very precisely fixed. There can be no doubt, however, that on their southern side they reach to the Cleveland Hills of Yorkshire and the southern borders of Lancashire, perhaps even as far as North Staffordshire (p. 106), and on the northern side to the farther shores of the island of Lewis—a direct distance of 360 miles. They stretch across the basin of the Irish Sea, including the Isle of Man, and appear in Ireland north of a line drawn from Dundalk Bay to the Bays of Sligo and Donegal. Dykes are of frequent occurrence over the north of England and south of Scotland, at least as far north as a line drawn from the coast of Kincardineshire along the southern flank of the Grampian Hills, by the head of Glen Shee and Loch Tay, to the north-western coast of Argyleshire. They abound all along the line of the Inner Hebrides and on parts of the adjacent coasts of the mainland, from the remoter headlands of Skye to the shores of County Louth. They traverse also the chain of the Long Island in the Outer Hebrides. So far as I am aware, they are either absent or extremely rare in the Highlands north of the line I have indicated. But a good many have been found by my colleagues in the course of the Geological Survey of the northern lowlands of Aberdeenshire and Banffshire. The longest of these has been traced by Mr. L. Hinxman for rather more than two miles running in a nearly east and west direction through the Old Red Sandstone of Strathbogie, with an average width of about 35 feet. Another in the same district has a width of from 45 to 90 feet, and has been followed for a third of a mile. But far beyond these northern examples, I have found a number of narrow basalt-veins traversing the Old Red flagstones of the Mainland of Orkney, which I have little doubt are also a prolongation of the same late series. Taking, however, only those western and southern districts in which the younger dykes form a notable feature in the geology, we find that the dyke-region embraces an area of upwards of 40,000 square miles—that is, a territory greater than either Scotland or Ireland, and equal to more than a third of the total land-surface of the British Isles (Map I.).

Of this extensive region the greater portion has now been mapped in detail by the Geological Survey. Every known dyke has been traced, and the appearances it presents at the surface have been recorded. We are accordingly now in possession of a larger body of evidence than has ever before been available for the discussion of this remarkable feature in the geology of the British Isles. I have made use of this detailed information, and besides the data accumulated in my own note-books, I have availed myself of those of my colleagues in the Survey, for which due acknowledgment is made where they are cited.

The Tertiary basalt-plateaux of Britain have their counterpart in the Faroe Islands and in Iceland, and whether or not the lava-fields stretched throughout North-western Europe from Antrim to the farthest headlands of Ultima Thule, there can hardly be any doubt that, if not continuous, these volcanic areas were at least geologically contemporaneous in their activity. Their characteristic scenery and structure are prolonged throughout the whole region, reappearing with all their familiar aspects alike in Faroe and in Iceland. I have not seen the latter island, but in the Faroe archipelago I have found the dykes to be sufficiently common, and to cut the basalt-plateaux there in the same way as they do those of the Inner Hebrides. On the whole, however, dykes do not play, in these northern isles, the important part which they take in the geology and scenery of the West of Scotland. I have not had sufficient opportunity to ascertain whether there is a general direction or system among the Faroe dykes. In the fjords north of Thorshaven, and again along the west side of Stromö, many of them show an E. and W. strike or one from E.N.E. to W.S.W.

2. TWO TYPES OF PROTRUSION

The dykes are far from being equally distributed over the wide region within which they occur. In certain limited areas they are crowded together, sometimes touching each other to the almost entire exclusion of the rocks through which they ascend, while elsewhere they appear only at intervals of several miles. Viewed in a broad way, they may be conveniently grouped in two types, which, though no hard line can be drawn between them, nevertheless probably point to two more or less distinct phases of volcanic action and to more than one period of intrusion. In the first, which for the sake of distinction we may term the Solitary type, there is either a single dyke separated from its nearest neighbours by miles of intervening and entirely dykeless ground, or a group of two or more running parallel to each other, but sometimes a mile or more apart. The rock of which they consist is, on the whole, less basic than in the second type; it includes the andesitic varieties. It is to this type that the great dykes of the north of England and the south and centre of Scotland belong. The Cleveland dyke, for example, at its eastern end has no known dyke near it for many miles. The coal-field of Scotland is traversed by five main dykes, which run in a general sense parallel to each other, with intervals of from half a mile to nearly five miles between them. Dykes of this type display most conspicuously the essential characters of the dyke-structure, in particular the vertical marginal walls, the parallelism of their sides, their great length, and their persistence in the same line.

In the second, or what for brevity may be called the Gregarious type, the dykes occur in great abundance within a particular district. They are on the whole narrower, shorter, less strikingly rectilinear, more frequently tortuous and vein-like, and generally more basic in composition than those of the first type. They include the true basalts and dolerites. Illustrative districts for dykes of this class are the islands of Arran, Mull, Eigg and Skye.

The great single or solitary dykes may be observed to increase in number, though very irregularly, from south to north, and also in Central Scotland from east to west. They are specially abundant in the tract stretching from the Firth of Clyde along a belt of country some thirty miles broad on either side of the Highland line, as far at least as the valley of the Tay. They form also a prominent feature in the islands of Jura and Islay.

Dykes of the gregarious type are abundantly and characteristically displayed in the basin of the Firth of Clyde. Their development in Arran formed the subject of the interesting paper by Necker, already mentioned, who catalogued and described 149 of them, and estimated their total number in the whole island to be about 1500.[160] As the area of Arran is 165 square miles, there would be, according to this computation, about nine dykes to every square mile. But they are far from being uniformly distributed. While appearing only rarely in many inland tracts, they are crowded together along the shore, particularly at the south end of the island, where the number in each square mile must far exceed the average just given. The portion of Argyleshire, between the hollow of Loch Long and the Firth of Clyde on the east and Loch Fyne on the west, has been found by my colleague, Mr. C. T. Clough, to contain an extraordinary number of dykes (see Fig. 257). The coast line of Renfrewshire and Ayrshire shows that the same feature is prolonged into the eastern side of the basin of the Clyde estuary. But immediately to the westward of this area the crowded dykes disappear from the basin of Loch Fyne. In Cantire their scarcity is as remarkable as their abundance in Cowal.

Both in the North of Ireland and through the Inner Hebrides, dykes are singularly abundant in and around, but particularly beneath, the great plateaux of basalt. Their profusion in Skye was described early in this century by Macculloch, who called attention more especially to their extraordinary development in the district of Strathaird. "They nearly equal in some places," he says, "when collectively measured, the stratified rock through which they pass. I have counted six or eight in the space of fifty yards, of which the collective dimensions could not be less than sixty or seventy feet." He supposed that it would not be an excessive estimate to regard the igneous rock as amounting to one-tenth of the breadth of the strata which it cuts.[161] This estimate, however, falls much short of the truth in some parts of Strathaird, where the dykes are almost or quite contiguous, and the Jurassic strata, through which they rise, are hardly to be seen at all.

Among the districts where dykes of the gregarious type abound at a distance from any of the basalt-plateaux, reference should be made to the curious isolated tract of the central granite core of Western Donegal. In that area a considerable number of dykes rises through the granite, to which they are almost wholly confined. Again, far to the east another limited district, where dykes are crowded together, lies among the Mourne Mountains. These granite hills are probably to be classed with those of Arran, as portions of a series of granite protrusions belonging to a late part of the Tertiary volcanic period which will be treated of in Chapter xlvii.

Though the dykes may be conveniently grouped in two series or types, which on the whole are tolerably well marked, it is not always practicable to draw any line between them, or to say to which group a particular dyke should be assigned. In some districts, however, in which they are both developed, we can separate them without difficulty. In the Argyleshire region above referred to, for example, which Mr. Clough has mapped, he finds that the abundant dykes belonging to the gregarious type run in a general N.W. or N.N.W. direction, and distinctly intersect the much scarcer and less basic dykes of the solitary type, which here run nearly E. and W. (Fig. 257). Hence, besides their composition, distinction in number, breadth, rectilinearity and persistence, the two series in that region demonstrably belong to distinct periods of eruption.[162]

The characteristic habit in gregarious dykes of occurring in crowded groups which are separated from each other by intervals of variable dimensions, marked by the presence of comparatively few dykes, is well illustrated in the district of Strath in Skye, which indeed may be taken as a typical area for this peculiarity of distribution. While the dykes are there singularly abundant in the Cambrian Limestone and the Liassic strata, they have been found by Mr. Clough and Mr. Harker to be comparatively infrequent in the tracts of Torridon Sandstone. It is not easy to understand this peculiar arrangement. As the Torridon Sandstone is the most ancient rock of the district, it probably underlies all the Cambrian and Jurassic formations, so that the dykes which penetrate these younger strata must also rise through the Torridonian rocks. Some formations appear to have been fissured more readily than others, and thus to have provided more abundant openings for the uprise of the basaltic magma from below. To the effect of such local differences in the structure of the terrestrial crust we have to add the concentration of the volcanic foci in certain areas, though there seems no means of ascertaining what part each of these causes has played in the distribution of the dykes of any particular district.

3. NATURE OF COMPONENT ROCKS

The Tertiary dykes of Britain include representatives of four distinct groups of igneous rocks. 1st, The vast majority of them consist of plagioclase-pyroxene-magnetite rocks with or without olivine. These are the normal basalts and dolerites. 2nd, A number of large dykes have a rather more acid composition and are classed as andesites. 3rd, A few dykes of trachyte have been observed in Cowal and in Skye cutting the dykes of basalt (p. 138). 4th, In some districts large numbers of still more acid dykes occur. These are sometimes crystalline in structure (granophyre), more frequently felsitic (felsite, spherulitic quartz-porphyry), and often glassy (pitchstone). In some exceptional cases the basic and acid materials are conjoined in the same dyke. Such compound varieties are described at p. 161. The acid dykes, connected as they so generally are with the large bodies of granophyre or granite, are doubtless younger than the great majority of the basic dykes. They will be treated in connection with the acid intrusions in Chapter xlviii.

By far the greater number of the dykes of the Tertiary volcanic series belong to the first group, and it is these more especially which will be discussed in the present and the following Chapter. As, however, the andesitic group is intimately linked with the basaltic it will be here included with them.

1. Basalt, Dolerite and Andesite Dykes.—To the field-geologist, who regards merely their external features, the Tertiary dykes present a striking uniformity in general petrographical character. They vary indeed in fineness or coarseness of texture, in the presence or absence of porphyritic crystals, amygdales, glassy portions and other points of structure. But there is seldom any difficulty in perceiving that they generally belong to one or other of the types of the basalts, dolerites, diabases or andesites. This sameness of composition, traceable from Yorkshire to Skye and from Donegal to Perthshire, is one of the strongest arguments for referring this system of dykes to one geological period. At the same time, there are enough of minor variations and local peculiarities to afford abundant exercise for the observing faculties alike in the field and in the study, and to offer materials for arriving at some positive conclusions regarding the geological processes involved in the uprise of the dykes.

There appears to be reason to believe that, when the petrography of the dykes is more minutely studied, marked differences of material will be found to denote distinct periods of eruption. Already Mr. A. Harker of the Geological Survey, who is engaged in mapping the interesting and complicated district of Strath in Skye, has observed that the dykes which are older than the great granophyre bosses of that tract may be distinguished from those which are later than these protrusions. The older basic dykes are not conspicuously porphyritic, are frequently marked by a close-grained margin or even with a veneer of basalt-glass, sometimes have an inclination of as much as 45°, are occasionally discontinuous, and not infrequently branch or send out veins. The younger dykes, on the other hand, as will be more particularly noticed in the following chapter, are distinguished by the frequent and remarkable character of their porphyritic inclusions, by the presence of foreign fragments in them, by the greater perfection of their jointing, and by their seldom departing much from the vertical.[163] They are likewise often markedly acid in composition, including such rocks as granophyre, felsite and pitchstone.

(1) External Characters.—As regards the grain of the rock, every gradation may be found, from a coarsely crystalline mass, in which the component minerals are distinctly traceable with the naked eye, to a black lustrous basalt-glass. Each dyke generally preserves the same character throughout its extent. As a rule, broad and long dykes are coarser in grain than narrow and short ones. For the most part, there runs along each side of a dyke a selvage of finer grain than the rest of the mass. This marginal strip varies in breadth from an inch or less up to a foot or more, and obviously owes its origin to the more rapid chilling of the molten rock along the walls of the fissure. It usually shades away inperceptibly into the larger-grained inner portion. Even with the naked eye its component materials can be seen to be more finely crystalline than the rest of the dyke, though where dispersed porphyritic felspars occur they are as large in the marginal strip as in any other part of a dyke, for they belong to an earlier period of crystallization than the smaller felspars of the groundmass and were already floating in the magma while it was still in a molten state.

This finer-grained external band, so distinctive of an eruptive and injected rock, is of great service in enabling us to trace dykes when they traverse other dykes or masses of igneous rock of similar characters to their own. When one dyke crosses another, that which has its marginal band of finer grain unbroken must obviously be the younger of the two.

But in many examples in the south of Scotland, Argyleshire and the Inner Hebrides, the fineness of grain of the outer band culminates in a perfect volcanic glass. Where this occurs, the glass is usually jet black, more rarely greenish or bluish black in tint, and varies in thickness from about a couple of inches to a mere varnish-like film on the outer face of the dyke, the average width being probably less than a quarter of an inch (Fig. 235). On their weathered surface these external glassy layers generally present a pattern of rounded or polygonal prominences, varying up to four or five lines or even more in diameter, and separated by depressions or narrow ribs. The transition from the glass to the crystalline part of the marginal fine-grained strip is usually somewhat abrupt, insomuch that on weathered faces it is often difficult to get good specimens, owing to the tendency of the vitreous portion to fly off when struck with the hammer. The glass doubtless represents the original condition of the rock of the dyke. It was suddenly chilled and solidified by contact with the cold walls of the fissure. Inside this external glassy coating, the molten material could probably still move, and had time to assume a more or less completely crystalline condition before solidification. Not infrequently the glass shows spherulitic forms, visible to the naked eye, and likewise a more or less distinctly developed perlitic structure. These features, however, are best studied in thin sections of the rock with the aid of the microscope, as will be subsequently referred to.

In some dykes, the glass is not confined to the edges, but runs in strings or broader bands along the central portions, or has been squeezed into little cavities like steam-holes or into minute fissures. One of the most remarkable examples of this peculiarity occurs in the well-known dyke of Eskdale, which runs for so many miles across the southern uplands of Scotland.[164] This dyke throughout most of its course is a crystalline rock of the andesitic type. At Wat Carrick, in Eskdale, it presents an arrangement into three parallel bands. On either side, a zone about eight feet broad consists of the usual crystalline material. Between these two marginal portions lies an intercalated mass 16 to 18 feet broad, of a very compact and more or less vitreous rock. The demarcation between this central band and the more crystalline zones of the outside is quite sharp, and the two kinds of rock show a totally distinct system of jointing. There can, therefore, be little doubt that the glassy centre belongs to a later uprise than the outer portions, though possibly it may still have been included in the long process of solidification of one original injected mass of molten material. If the marginal parts adhered firmly to the walls, the centre, which with its band of vesicles seems often to have been a line of weakness, might be ruptured and subsequent intrusions would find their way along the rent. Examples of this splitting of dykes with the intrusion of later eruptive Material will be cited in later pages.

Mr. Clough, while mapping for the Geological Survey the extraordinarily numerous dykes in the eastern part of Argyleshire between the Firth of Clyde and Upper Loch Fyne, observed six or seven examples of dykes showing glassy bands in their centres, with characters similar to those of the Eskdale dyke. He found an absence of definite and regular joints in the central glassy band, and on the other hand, an irregular set of divisional planes by which the rock is traversed, and which he compared to those seen in true perlitic structure.

While, as a general rule, the external portions of a dyke are closer-grained than the centre, rare cases occur where the middle is the most finely crystalline part. I am disposed to regard these cases and the glassy centres as forming in reality no true exceptions to the rule, that the outer portions of a dyke consolidated first, and are therefore finest in texture. For the most part, each dyke appears to be due to a single uprise of molten matter, though considerable movements may have taken place within its mass before the whole stiffened into stone. Some particulars regarding these movements will be given in section 12 of the next Chapter. It has already been mentioned that in large dykes which have served as volcanic pipes, it is conceivable that while the material next the outside consolidated and adhered to the walls, the central portion may have remained liquid, and may even have been propelled upward and have been succeeded by a different kind of magma, as has been suggested by Mr. Iddings. In such cases, which, if they occur, are probably excessively rare, we may expect that the earlier and later material will not be sharply marked off from each other, unless we suppose that the whole of the earlier liquid magma was so entirely ejected that only its congealed marginal selvage was left as bounding walls for the newer injection.

Where, after more or less complete consolidation had taken place, the fissure opened again, or from any other cause the dyke was split along its centre, any lava which rose up the rent would tend to take a finer grain than the material of the rest of the dyke, and might even solidify as glass.

Large scattered crystals of felspar, of an earlier consolidation than that of the minuter forms of the same mineral in the general groundmass of the rock, give a porphyritic structure and andesitic character to many dykes. Occasionally such crystals attain a considerable size. Mr. Clough has observed them in some of the Argyleshire dykes reaching a length of between three and four inches, with a thickness of two inches. Sometimes they are distributed with tolerable uniformity through the substance of the dyke. But not infrequently they may be observed in more or less definite bands parallel with the boundary walls. Unlike the younger lath-shaped and much smaller felspars of the groundmass, they show no diminution either in size or abundance towards the edge of the dyke. On the contrary, as already mentioned, they are often conspicuous in the close-grained marginal strip, and may be found even in the glassy selvage, or touching the very wall of the fissure. Indeed, they are sometimes more abundant in the outer than in the inner portions of a dyke, having travelled outwards to the surfaces of earliest cooling and crystallization.

Mr. Clough has given me the details of an interesting case of this kind observed by him in Glen Tarsan, Eastern Argyleshire:—"For an inch or so from the edge of this dyke," he remarks, "porphyritic felspars giving squarish sections, and ranging up to one-third of an inch in length, are so abundant as nearly to equal in bulk the surrounding groundmass. For the next inch and a half, they are decidedly fewer, occupying perhaps hardly an eighth of the area exposed. Then for a breadth of three inches they come in again nearly as abundantly as at the sides; after which they diminish through a band 27 inches broad, where they may form from ¹/₈ to ¹/₁₂ of the rock." He found another case where, in a dyke several yards wide, porphyritic felspars, sometimes an inch long, are common along the eastern margin of the dyke in a band about two inches broad, but nearly absent from the rest of the rock. Elsewhere the crystals are grouped rather in patches than in bands. Among the dykes south of Oban some similar instances of coarsely porphyritic felspars may be observed.

Not only are these porphyritic felspars apt to occur in bands parallel with the outer margins of the dykes, but they tend to range themselves with their longer axis in the same direction, thus even on a large scale, visible at some distance, showing the flow-structure, which is so often erroneously regarded as essentially a microscopic arrangement, and as specially characteristic of superficial lava-streams.

Mr. Harker in his survey of Strath, Skye, has met with some remarkable examples of the enclosure and incorporation of foreign materials in the younger group of dykes which in that district traverse the granophyres and gabbros. He remarks that the great majority of these dykes are basic, and he has found them to be capable of convenient division into two groups. 1st, Non-porphyritic basic dykes with a specific gravity between 2·87 and 2·97, and an amygdaloidal structure affording clear indication of flowing movement, either at the sides or along a central band. These dykes do not greatly differ from those of pre-granophyre eruption. 2nd, Porphyritic basic dykes which present features of peculiar interest. The porphyritic (or pseudo-porphyritic) elements, according to Mr. Harker's observations, are constantly felspar, frequently subordinate augite, and exceptionally quartz. The felspars have for the most part rounded outlines with a bordering zone of glass cavities apparently of secondary origin. The augite, in rounded composite crystal-grains, differs from that of the groundmass and resembles the augite of the gabbros. The quartz-grains are likewise rounded, and show sometimes a distinct corroded border.

These characters, Mr. Harker observes, are those of crystals derived from some foreign source, and it can scarcely be doubted that this is the explanation of their presence. He noticed that the dykes in question frequently enclose fragments, varying up to several inches in diameter, of gabbro, granite or granophyre, bedded lava, quartzite, etc., which show clear evidence of having been rounded and corroded by an enveloping magma, and recognizable crystals from some of the fragments may be observed in the surrounding parts of the matrix of the dykes. Most of the felspar and augite crystals disseminated through these porphyritic basic dykes may be referred to the partial reabsorption of enclosed fragments of gabbro. The same observer has found that many of the dykes which rise through the basalt-plateau of Strathaird are crowded with gabbro fragments.

Another megascopic character of the material composing the dykes is the frequent presence of amygdales. It has sometimes been supposed that amygdaloidal structure may be relied upon as a test to distinguish a mass of molten rock which has reached the surface from one which has consolidated under considerable pressure below ground. That this supposition, however, is erroneous is demonstrated by hundreds of dykes in the great system which I am now describing. But the amygdales of a dyke offer certain peculiarities which serve in a general way to mark them off from those of an outflowing lava. They are usually smaller and more uniform in size than in the latter rock. They are also more regularly spherical and less frequently elongated in the direction of flow. Moreover, they are not usually distributed through the whole breadth of a dyke, but tend to arrange themselves in lines especially towards its centre (Fig. 236). In these central bands the cavities are largest and depart farthest from the regular spherical form, so that for short spaces they may equal in bulk the mass of enclosing rock. In some rare instances, a whole dyke is composed of cellular basalt, like one of the lava-sheets in the plateaux, as may be seen on the north flank of Beinn Suardal, Skye. Mr. Harker has observed that an amygdaloidal structure is more common among the earlier than among the later dykes of that district.

Besides the common arrangement of fine-grained edges and a more coarsely crystalline centre, instances are found where one of the contrasted portions of a dyke traverses the other in the form of veins. Of these, I think, there are two distinct kinds, probably originating in entirely different conditions. In the first place, they may be of coarser grain than the rest of the rock; but such a structure appears to be of extremely rare occurrence. I have noticed some examples on the coast of Renfrewshire, where strings of a more coarsely crystalline texture traverse the finer-grained body of the rock. Veins of this kind are probably of the same nature as the so-called "segregation-veins," to be afterwards referred to as of frequent occurrence among the thicker Tertiary sills. They consist of the same minerals as the rest of the rock, but in a different and more developed crystalline arrangement, and they contain no glassy or devitrified material, except such portions of that of the surrounding groundmass as may have been caught between their crystalline constituents.

The second kind of veins, which, though not common, is of much more frequent occurrence than the first, is more particularly to be met with among the broader dykes, and is distinguished by a remarkable fineness of grain, sometimes approaching the texture of felsite or jasper, and occasionally taking the form of actual glass. Such veins vary from half an inch or less, up to four or five inches in breadth. They run sometimes parallel with the walls of the dyke, but often irregularly in all directions, and for the most part avoid the marginal portions, though now and then coming up to the edge. They never extend beyond the body of the dyke itself into the surrounding rock. Though they have obviously been injected after the solidification of the rock which they traverse, they may quite possibly be extrusions of a deeper unconsolidated portion of the same rock into rents of the already stiffened overlying parts. The field-geologist cannot fail to be struck with the much greater hardness of these fine-grained veins and strings that ramify through the coarsely crystalline dolerite, andesite or other variety of the broader dykes. He can readily perceive in many cases their more siliceous composition, and the inferences he deduces from the rough observations he can make in the field are confirmed by the results of chemical analysis (see p. 137).

In connection with veins of finer material, that may belong to a late stage of the consolidation of the general body of a dyke, reference may be made here to the occasional occurrence of patches of an exceedingly compact or homogeneous texture immersed in the usual finely crystalline marginal material. They look like angular and subangular portions of the more rapidly cooled outer edge, which have been broken off and carried upward by the still moving mass in the fissure.[165]

In general, each dyke is composed of one kind of rock, and retains its chemical and mineralogical characters with singular persistence. The difference of texture between the fine-grained chilled margin, with its occasional glassy coating, and the more coarsely crystalline centre is due to cooling and crystalline segregation in what was no doubt originally one tolerably uniform molten mass. The glassy central bands, too, though they indicate a rupture of the dyke up the middle, may at the same time quite conceivably be, as I have said, extrusions from a lower portion of the dyke before the final solidification of the whole. The ramifying veins of finer grain that now and then traverse one of the large dykes are likewise explicable as parts of a stage towards entire consolidation. All these vitreous portions, whether still remaining as glass or having undergone devitrification, are more acid than the surrounding crystalline parts of the rock. They represent the siliceous "mother-liquor," so to speak, which was left after the separation from it of the crystallized minerals, and which, perhaps, entangled here and there in vesicles of the slowly cooling and consolidating rock, was ready to be forced up into cracks of the overlying mass during any renewal of terrestrial disturbance.

But examples occur where a dyke, instead of consisting of one rock, is made up of two or more bands of rock which, even if they resemble each other closely, can be shown to be the results of separate eruptions. These, which are obviously not exceptions to the general rule of the homogeneity of dykes, I will consider in the next Chapter.

Among the petrographical varieties observable in the field is the occasional envelopment of portions of the surrounding rocks in the body of a dyke. Angular fragments torn off from the fissure-walls have been carried upwards in the ascending lava, and now appear more or less metamorphosed, the amount of alteration seeming to depend chiefly upon the susceptibility of the enclosed rock to change from the effects of heat. Cases of such entanglement, however, are of less common occurrence than those already referred to, where pieces of some deep-seated rock, such as the gabbros of Skye, have been carried up in the ascending magma. Occasionally, where the enclosed fragments are oblong, they are arranged with their longer axes parallel to the walls of the dyke, showing flow-structure on a large scale. Mr. Clough has found some dykes near Dunoon which enclose fragments of schist nearly three feet in length.

One of the most interesting of the megascopic features of the dykes is the joints by which they are traversed. These divisional planes are no doubt to be regarded as consequences of the contraction of the original molten rock during cooling and consolidation between its fissure-walls. They are of considerable interest and importance, inasmuch as they furnish a ready means of tracing a dyke when it runs through rock of the same nature as itself, and also help to throw some light on the stages in the consolidation of the material of the dyke.

Two distinct systems of joints are recognizable (Fig 237). Though sometimes combined in the same dyke, they are most conspicuously displayed when each occurs, as it generally does, by itself. The first and less frequent system of joints (a) has been determined by lines of retreat, which are parallel to the walls of the dyke. The joints are then closest together at the margin, and may be few or altogether absent in the centre. They are sometimes so numerous, parallel and defined towards the borders of the dyke, as to split the rock up into thin flags. Where transverse joints are also present these flags are divided into irregular tesseræ.