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.

[Footnote 163: In the Blath Bheinn group of gabbro-hills, however, it is the youngest dykes which have been found by Mr. Harker to possess the lowest hade.]

(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.

[Footnote 164: See _Proc. Roy. Phys. Soc. Edin._ v. (1880), p. 241.]

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]

[Footnote 165: See Mr. J. J. H. Teall, _Quart. Journ. Geol. Soc._ xl. (1884), p. 214.]

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_.

In the second or transverse system of joints (_b_), which is the more usual, the divisional lines pass across the breadth of the dyke, either completely from side to side, or from one wall for a longer or shorter distance towards the other. Where this series of joints is most completely developed the dyke appears to be built up of prisms piled horizontally, or nearly so, one above another. These prisms, in rare instances, are as regular as the columns of a basalt-sheet (see Fig. 166). Usually, however, they have irregularly defined faces, and merge into each other. Where the prismatic structure is not displayed, the joints, starting sharply at the wall of the dyke, strike inwards in irregular curving lines. It is such transverse joints that enable the eye, even from a distance, to distinguish readily the course of a dyke up the face of a cliff of basalt-beds, for they belong to the dyke itself, are often at right angles to those of the adjacent basalt, and by their alternate projecting and re-entering angles seam the dyke with parallel bars of light and shade (see the double dyke in Fig. 333). Where they traverse not only the general mass of a dyke, but also the "contemporaneous veins" which cross it, it may be inferred that these veins were injected before the final solidification and contraction of the whole dyke.

An interesting modification of the transverse joints may sometimes be observed, where, as in the case of the Palæozoic "Rock and Spindle," at St. Andrews (Fig. 222), the molten material has solidified in a tubular or spherical cavity. The joints then radiate inwards from the outer curved surface. The most remarkable instance of this structure which I have found among the Tertiary volcanic plateaux occurs on the east side of the island Fuglö, the most north-easterly of the group of the Faroes. It is cut in section by the face of the precipice, where it appears as a round mass about 40 or 50 feet in diameter piercing the plateau-basalts. A selvage of finer material round its outer edge shows the effect of rapid chilling, while the joints diverge from the periphery and extend in fan-shape towards the centre (Fig. 238).

One of the most remarkable exhibitions of joint-structure hitherto noticed among the Tertiary dykes is that which occurs in the central vitreous band of the Eskdale dyke already referred to. The rock is divided into nearly horizontal prisms, each of which consists of an inner more vitreous core and an outer more lithoid sheath. By the coherence of their polygonal and irregular faces, and the greater durability of their material, these sheaths project on the weathered wall of the vitreous centre of the dyke in a curiously reticulated grouping of prominent ribs each about two inches broad (Fig. 239, A), while the vitreous cores, being more readily acted on by the weather, are hollowed out into little cup-shaped depressions. Each rib is thus composed of the sheaths or outer lithoid portions of two prisms, the line of separation being marked by a suture along the centre (B). Between this median suture and the inner glassy core the rib is further cut into small segments by a set of close joints, which are placed generally at right angles to the course of the rib (C). Examined with a lens, the lithoid substance of these sheaths has a dull finely granular aspect, like that of felsitic rocks, with scattered felspars. It is obviously a more devitrified condition of the material which forms the core of each prism. This material presents on a fresh fracture a deep iron-black colour, dull resinous lustre and vitreous texture. It at once recalls the aspect of many acid pitchstones, and in the early days of petrography was naturally mistaken for one of these rocks. Through its substance numerous kernels of more glassy lustre are dispersed, each of which usually contains one or more amygdales of dull white chalcedony, but sometimes only an empty black cavity. These black glistening kernels of glass, of all sizes up to that of a small bean, scattered through the dull resinous matrix, form with the white amygdales the most prominent feature in the cores; but crystals of felspars may also be observed. Some details of the microscopic characters of this remarkable structure will be given in a subsequent page. The relation of the cores and sheaths to the prismatic jointing of the rock seems to show that devitrification had not been completed when these joints were established, and that it proceeded from the faces of each prism inwards.

(2) _Microscopic Characters._--Much information has now been obtained regarding the microscopic structure of the basaltic, doleritic and andesitic dykes. The crystalline characters of those in the North of England have been studied by Mr. Teall,[166] and some of those from the West of Scotland have been investigated by Professors Judd and Cole.[167] Taken as a whole, the rocks composing the dykes are found, when examined microscopically, to consist essentially of mixtures of a plagioclase felspar, pyroxene and iron oxide, with or without olivine, and usually with more or less interstitial matter.

[Footnote 166: _Quart. Journ. Geol. Soc._ vol. xl. (1884).]

[Footnote 167: _Op. cit._ vol. xxxix. (1883) p. 444 (basalt-glass); xlii. (1886) p. 49, where Professor Judd discusses the gabbros, dolerites and basalts as a whole.]

The felspar appears to be in some cases labradorite, in others anorthite, but there may be a mingling of several species in many of the dykes, as in the augite-andesite of the Santorin eruption in 1866, wherein Professor Fouqué found that the larger porphyritic felspars were mainly labradorite, but partly anorthite, while those of the groundmass were microlites of albite and oligoclase.[168] The large felspars scattered porphyritically through the groundmass are evidently the result of an early consolidation, unless where they are survivals from fragments of older porphyritic rocks which have been enveloped and partially dissolved in the dykes. They are often cracked, penetrated by the groundmass, or even broken into fragments, and have corroded borders. They sometimes include portions of the groundmass, and present the zonal growth structure in great perfection. The small felspars of the groundmass, on the other hand, are as obviously the result of a later crystallization, for they vary in size and crystallographic development according to their position in the dyke. Those from the centre are often in well-formed crystals, which sometimes pass round their borders into acicular microlites. Those in the marginal parts of the dyke occur chiefly in the form of these microlites, forming the felted aggregate so characteristic of the andesites. Curious skeleton forms, composed of aggregates of microlites, connect the latter with the more completely developed crystals, and illustrate the mode of crystallization of the felspathic constituents of the dykes.[169]

[Footnote 168: _Santorin et ses Éruptions_, 1879, p. 203.]

[Footnote 169: See Mr. Teall's excellent description of the Cleveland dyke, in the paper above cited.]

The pyroxene is probably in most cases monoclinic (black or common augite), but is sometimes rhombic (usually enstatite, less frequently perhaps hypersthene). It occurs in (_a_) well-developed crystals, (_b_) crystalline masses with some of the faces of the crystals developed, (_c_) granular aggregates which polarise in one plane, (_d_) separate granules and microscopic microlites, which may be spherical (globulites) or oblong (longulites).

The black iron-oxide is sometimes magnetite, sometimes ilmenite, or other titaniferous ore. Apatite not infrequently occurs among the original constituents. Olivine is entirely absent from most of the large solitary dykes, especially at a distance from the great volcanic centres, and no serpentinous matter remains to indicate that it was ever present in them. But it is to be met with in numerous basalt-dykes in the volcanic areas, either in sparsely scattered or in tolerably abundant crystals. Biotite occasionally appears. Among the secondary products, calcite and pyrites are doubtless the most common. To these must be added quartz, chalcedony and various zeolitic substances, besides the aggregates which result from the decomposition of the ferro-magnesian constituents and the oxidation of the ferrous oxides.

In many dykes there is little or no interstitial matter between the crystalline constituents of the groundmass. In others this matter amounts to a half or more of the whole composition, and from such cases a series of gradations may be traced into a complete glass containing only the rudimentary forms of crystals (globulites, longulites, etc.), with scattered porphyritic crystals of an earlier consolidation. The process of the disappearance of this original glass may be admirably studied in many dykes. At the outer wall, the glass remains nearly as it was when contact with the cold walls of the fissure solidified it. From that external vitreous layer the successive devitrification products and crystalline growths may be followed inwards until in the central parts of a broad dyke little or no trace of the interstitial matter may be left.

[Footnote 170: _Proc. Roy. Phys. Soc. Edin._ v. (1880), p. 255.]

The most instructive example of the process of devitrification which has come under my observation occurs in the Eskdale dyke. The central "cores" already referred to present a true glass, which in thin sections is perfectly transparent and almost colourless, but by streaks and curving lines of darker tint shows beautiful flow-structure. The devitrification of this glass has been accomplished by the development of crystallites and crystals, which increase in number until all the vitreous part of the rock disappears. What seems under a low power to be a structureless or slightly dusty glass can be resolved with a higher objective into an aggregate of minute globules or granules (globulites), which average perhaps 1/20,000 of an inch in diameter. Some of these bodies are elongated and even dichotomous at the ends. These granules are especially crowded upon clear yellow dart-shaped rods, which in turn are especially prominent upon crystals and crystalline grains of augite that bristle with them, while the immediately surrounding glass has become clear. There can be little doubt that these rudimentary bodies are stages in the arrested development of augite crystals. There occur also opaque grains, rods and trichites, which no doubt consist in whole of magnetite (or other iron oxide), or are crusted over with that mineral.

At least two broad types of microscopic structure may be recognized among the basic and intermediate dykes. (1) Holocrystalline, or with only a trifling proportion of interstitial matter. This type includes the dolerites and basalts, as well as rocks which German petrographers would class as diabases or diabase-porphyrites. The rocks are very generally characterized by ophitic structure, where the lath-shaped felspars penetrate the augite, and are therefore of an earlier consolidation. In such cases there is a general absence of any true interstitial matter. The rocks of this type are often rich in olivine, and appear to be on the whole considerably more basic than those of the second group. It is observable that they increase in numbers from the centre of Scotland westwards, and throughout the region of the basalt-plateaux they form the prevailing type. (2) In this type there is a marked proportion of interstitial substance, which is inserted in wedge-shaped portions among the crystallised constituents ("intersertal structure" of Rosenbusch). The ophitic structure appears to be absent, and olivine is either extremely rare or does not occur at all. The rocks of this group are obviously less basic than those of the other. They form the large dykes that rise so conspicuously through the South of Scotland and North of England, and their general characters are well described by Mr. Teall in the paper already cited. In some instances they enclose abundant porphyritic felspars of earlier consolidation, and then present most of the characters of andesites. Professor Rosenbusch has extended the name of "Tholeiites" to rocks of this group in the North of England.[171] The vitreous condition is found in both types, but is perhaps more frequent in the second. The glass of the basalts, however, even in thin slices, is characteristically opaque from its crowded inclusions; while that of the andesitic forms, though black in hand specimens, appears perfectly transparent and sometimes even colourless in thin slices.

[Footnote 171: _Mikroskopische Physiographie_, 3rd edit. 1071 _et seq._]

(3) _Chemical Characters._--The only one of these to which reference will be made here is the varying proportion of silica. While the dykes as a whole are either intermediate or basic, some of them contain so high a percentage of silica as to link them with the acid rocks. The average proportions of this ingredient range from less than 50 to nearly 60 per cent. The rocks with the lower percentage of acid are richer in the heavy bases, and have a specific gravity which sometimes rises above 3·0. They include the true dolerites and basalts. Those, on the other hand, with the higher ratio of silica, are poorer in the heavy bases, and have a specific gravity from 2·76 to 2·96. They comprise the tholeiites, andesites and other more coarsely crystalline rocks of the great eastern and south-eastern dykes.[172]

[Footnote 172: For analyses of dykes, see Sir I. L. Bell, _Proc. Roy. Soc._ xxiii. p. 546; Mr. J. S. Grant Wilson, _Proc. Roy. Phys. Soc. Edin._ v. p. 253; Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 209; Professors Judd and Cole, _Quart. Jour. Geol. Soc._ xxxix. p. 444.]

Not only do the dykes differ considerably from each other in their relative proportions of silica, but even the same dyke may sometimes be found to present a similar diversity in different parts of its mass. It has long been a familiar fact that the glassy parts of such rocks are more acid than the surrounding crystalline portions. The original magma may be regarded as a natural glass or fused silicate, in which all the elements of the rock were dissolved, and which necessarily became more acid as the various basic minerals crystallised out of it.[173] In the Eskdale dyke the silica percentage of this glassy portion is 58·67, that of the little kernels of black glass dispersed through the rock as much as 65·49.[174] In the Dunoon dyke observed by Mr. Clough the siliceous finer-grained veins contain no less than 68·05 per cent of silica, while the mass of the dyke itself shows on analysis only 47·36 per cent.[175] Similar red strings have been noticed by the same careful observer in an east and west dyke near Lochgoilhead. From Mr. Teall's examination a large part of the felspar in these veins is probably orthoclase. It forms a much larger percentage of the entire rock than the felspar does in normal dolerites.

[Footnote 173: On this subject see a paper by Dr. A. Lagorio, "Über die Natur der Glasbasis sowie der Krystallisationsvorgänge im eruptiven Magma," Tschermak's _Mineralog. Mittheil._ viii. (1887), p. 421.]

[Footnote 174: Mr. J. S. Grant Wilson, _Proc. Roy. Soc. Phys. Edin._ v. (1880) p. 253.]

[Footnote 175: Unpublished analyses made by the late Professor Dittmar of Glasgow, and communicated to me by Mr. Clough.]

2. Trachyte Dykes.--In the Cowal District of Argyleshire, and in the south of Skye, Mr. Clough has encountered a limited number of dykes of trachyte. On a hasty inspection these are not always readily distinguished from the basalt-dykes with which they agree in general external aspect and in direction. Where their relation to these dykes, however, can be determined they are found to traverse them, and thus to be on the whole later, though one case has been observed where a trachytic dyke is in turn traversed by one of the basic series. Mr. Clough has supplied me with the following notes of his observations regarding the trachytic dykes. They are all characterized by the possession of spherulitic structures near their margins. These features, easily perceptible to the naked eye, afford the readiest means of distinguishing the dykes of this group. So abundant are the spherulites that they not infrequently impinge on each other in long parallel rows forming rod-like aggregates. Thus in a dyke near Craigendavie, at the head of Loch Striven, numerous planes about a quarter of an inch apart, and composed of such close-set rods, may be observed running parallel to the marginal wall for a distance of several inches from the edge. Most of these planes show on their surfaces that the rods are always parallel to each other, but may run in different directions in the different layers, being sometimes horizontal, sometimes vertical, or at any angle between. On examination, each rod is found to be made up of polygonal bodies, the angles of which are quite sharp, but with their sides often slightly curved, as if they had assumed their forms from the mutual pressure of original spherical bulbs. Further scrutiny shows that the polygonal bodies often exhibit an internal radiate structure.

In the central parts of the dyke the spherulitic arrangement is not traceable. About a foot from the margin it begins to be recognizable. At a distance of three or four inches the spherulites are about the size of peas, and gradually diminish towards the edge until they can no longer be seen.

Another characteristic of the trachyte dykes has been found by Mr. Clough to be a useful guide in discriminating them from the basalt-group. While the amygdales in the latter are generally rudely spherical, those in the trachytes are commonly elongated in the direction of the length of the dyke, and are frequently three quarters of an inch, sometimes even an inch and a half, in length, though less than a quarter of an inch in breadth.

A good example of these trachytic dykes, which occurs at Dunans, about the head of Glendaruel, has been examined microscopically and chemically. The central better crystallised portion was found by Mr. Teall to be composed mainly of small lath-shaped crystals of orthoclase, together with scales of brown biotite, a few prismatic crystals of pale somewhat altered pyroxene and scattered granules of magnetite. The chemical analysis of this rock by Mr. J. H. Player gave the following composition:--

Silica 56·4 Alumina 19·0 Ferric oxide 3·5 Ferrous oxide 4·8 Lime 2·6 Magnesia 1·5 Soda 4·5 Potash 5·0 Loss on ignition 2·6 ---- 99·9 ====

4. HADE

In the majority of cases, especially among the great single dykes, the intrusive rock has assumed a position nearly or quite vertical. But occasionally, where one of these solitary examples crosses a deep valley, a slight hade is perceptible by the deviation of the line of the dyke from its normal course. Sedgwick long ago noticed that the Cleveland dyke has, in places, an inclination of at least 80° to its N.E. side.[176] In the coal-workings, also, a trifling deviation from the vertical is sometimes perceptible, especially where a dyke has found its way along a previously existing line of fault, as in several examples in Stirlingshire. But in those districts where the dykes are gregarious, departures from the vertical position are not infrequent, more particularly near the great basalt-plateaux. It was noticed by Necker, that even in such a dyke-filled region as Arran, almost all of the dykes are vertical, though sometimes deviating from that position to the extent of 20°.[177] Berger found that the angle of deviation among those of the north of Ireland ranges from 9° to 20°, with a mean of 13°.[178] The most oblique examples are probably those which occur in the basalt-plateaux of the Inner Hebrides, where the same dyke in some parts of its course runs horizontally between two beds, across which it also descends vertically (see Figs. 251, 252, 374). But with these minor exceptions, the verticality of the great system of dykes, pointing to the perpendicular fissure-walls between which the molten rock ascended, is one of the most notable features in their geological structure. In the Strath district of Skye Mr. Harker has noticed that while the earlier dykes have sometimes a hade of 45°, those younger than the granophyre are generally vertical or nearly so. In the Blath Bheinn group of hills, however, as already alluded to, he has observed that it is the youngest dykes which are inclined in a north-westerly direction, with a hade of as much as 40° from the horizon.

[Footnote 176: _Cambridge Phil. Trans._ ii. p. 28.]

[Footnote 177: _Trans. Roy. Soc. Edin._ xiv. p. 677.]

[Footnote 178: _Trans. Geol. Soc._ iii. p. 227.]

5. BREADTH

An obvious characteristic of most dykes is the apparent uniformity of their breadth. Many of them, as exposed along shore-sections, vary as little in dimensions as well-built walls of masonry do. Departures from such uniformity may often indeed be noted, whether a dyke is followed laterally or vertically. The largest amount of variation is, of course, to be found among the dykes of the gregarious type, the thinner examples of which may diminish to a width of only one inch or less, while their average breadth is much smaller than in the case of the great solitary dykes. In the district of Strathaird, in Skye, Macculloch estimated that the remarkably abundant dykes there developed vary from 5 to 20 feet in breadth, but with an average breadth of not more than 10 feet.[179] In the isle of Arran, according to Necker's careful measurements, most of the dykes range from 2 or 3 to 10 or 15 feet, but some diminish to a few inches, while others reach a width of 20, 30, or even 50 feet.[180] In the North of Ireland, Berger observed that the average breadth of thirty-eight dykes traversing primitive rocks (schist, granites, etc.) was 9 feet; and of twenty-four in Secondary rocks, 24 feet.[181]

[Footnote 179: _Trans. Geol. Soc._ iii. p. 80.]

[Footnote 180: _Trans. Roy. Soc. Edin._ xiv. p. 690 et seq.]

[Footnote 181: _Trans. Geol. Soc._ iii. p. 226. He believed that dykes in Secondary rocks reach a much greater thickness than in other formations. My own observations do not confirm this generalisation.]

But when we pass to the great solitary dykes, that run so far and so continuously across the country, we encounter much thicker masses of igneous rock. Most of the measurements of these dykes have been made at the surface, and the variations noted in their breadth occur along their horizontal extension. The Cleveland dyke, which is the longest in Britain, varies from 15 feet to more than 100 feet, with perhaps an average width of between 70 and 90 feet.[182] Some of the great dykes that cross Scotland are of larger dimensions. Most of them, however, like that of Cleveland, are liable to considerable variations in breadth when followed along their length. The dyke which runs from the eastern coast across the Cheviot Hills and Teviotdale to the head of the Ale Water, is in some places only 10 feet broad, but at its widest parts is probably about 100 feet. The Eskdale and Moffat dyke is in parts of its course 180 feet wide, but elsewhere it diminishes to not more than 40 feet. These variations are repeated at irregular intervals, so that the dyke alternately widens and contracts as its course is traced across the hills. Some of the dykes further to the north and west attain yet more gigantic proportions. That which crosses Cantyre opposite Ardlamont Point has been measured by Mr. J. B. Hill, of the Geological Survey, who finds it to be from 150 to 180 feet broad on the shore of Loch Fyne, and to swell out beyond the west side of Loch Tarbert to a breadth of 240 to 270 feet. A dyke near Strathmiglo, in Fife, is about 400 feet wide. The broadest dyke known to me is one which I traced near Beith, in Ayrshire, traversing the Carboniferous Limestone. Its maximum width is 640 feet.

[Footnote 182: At Cockfield, where it has long been quarried, it varies from 15 to 66 feet; at Armathwatie, in the vale of the Eden, it is about 54 feet (Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 211).]

Unfortunately, it is much less easy to get evidence of the width of dykes at different levels in their vertical extension. Yet this is obviously an important point in the theoretical discussion of their origin. Two means are available of obtaining information on the subject--(_a_) from mining operations, and (_b_) from observations at precipices and between hill-crests and valley-bottoms.

(_a_) In the Central Scottish coal-field and in that of Ayrshire, some large dykes have been cut through at depths of two or three hundred feet beneath the surface. But there does not appear to be any well-ascertained variation between their width so far below ground and at the surface. In not a few cases, indeed, dykes are met with in the lower workings of the coal-pits which do not reach the surface or even the workings in the higher coals. Such upward terminations of dykes will be afterwards considered, and it will be shown that towards its upper limit a dyke may rapidly diminish in width.

(_b_) More definite information, and often from a wider vertical range, is to be gathered on coast-cliffs and in hilly districts, where the same dyke can be followed through a vertical range of many hundred feet. But so far as my own observations go, no general rule can be established that dykes sensibly vary in width as they are traced upward. Every one who has visited the basalt-precipices of Antrim or the Inner Hebrides, where dykes are so numerous, will remember how uniform is their breadth as they run like ribbons up the faces of the escarpments.[183] Now and then one of them may be observed to die out, but in such cases (which are far from common) the normal width is usually maintained up to within a few feet of the termination.

[Footnote 183: This point did not escape the attention of that excellent observer, Berger, in his examination of the dykes in the North of Ireland. We find him expressing himself thus:--"The depth to which the dykes descend is unknown; and after having observed the sections of a great many along the coast in cliffs from 50 to 400 feet in height, I have not been able to ascertain (except in one or two cases) that their sides converge or have a wedgeform tendency" (_Trans. Geol. Soc._ iii. p. 227).]

All over the southern half of Scotland, where the dykes run along the crests of the hills and also cross the valleys, a difference of level amounting to several hundred feet may often be obtained between adjacent parts of the same dyke. But the breadth of igneous rock is not perceptibly greater in the valleys than on the ridges. The depth of boulder clay and other superficial deposits on the valley bottoms, however, too frequently conceals the dykes at their lowest levels. Perhaps the best sections in the country for the study of this interesting part of dyke-structure are to be found among the higher hills of the Inner Hebrides, such as the quartzites of Jura and the granophyres and gabbros of Skye. On these bare rocky declivities, numerous dykes may be followed from almost the sea-level up to the rugged and splintered crests, a vertical distance of between 2000 and 3000 feet. The dykes are certainly not as a rule sensibly less in width on the hill-tops than in the glens. So far, therefore, as I have been able to gather the evidence, there does not appear to me to be, as a general rule, any appreciable variation in the width of dykes for at least 2000 or 3000 feet of their descent. The fissures which they filled must obviously have had nearly parallel walls for a long way down.

6. INTERRUPTIONS OF LATERAL CONTINUITY

In tracing the great solitary dykes across the country, the geologist is often surprised to meet with gaps, varying in extent from a few hundred feet to several miles, in which no trace whatever of the igneous rock can be detected at the surface. This disappearance is not always explicable by the depth of the cover of superficial accumulations; for it may be observed over ground where the naked rocks come almost everywhere to the surface, and where, therefore, if the conspicuous material of the dykes existed, it could not fail to be found. No dyke supplies better illustrations of this discontinuity than that of Cleveland. Traced north-westward across the Carboniferous tracts that lie between the mouth of the Tees and the Yale of the Eden, this dyke disappears sometimes for a distance of six or eight miles. In the mining ground round the head of the South Tyne the rocks are bare, so that the absence of the dyke among them can only be accounted for by its not reaching the surface. Yet there can be no doubt that the various separated exposures, which have the same distinctive lithological characters and occur on the same persistent line, are all portions of one dyke which is continuous at some depth below ground. We have thus an indication of the exceedingly irregular upward limit of the dykes, as will be more particularly discussed further on.

But there are also instances where the continuity is interrupted and then resumed on a different line. One of the best illustrations of this character is supplied by the large dyke which rises through the hills about a mile south of Linlithgow and runs westward across the coal-field. At Blackbraes it ends off in a point, and is not found again to the westward in any of the coal-workings. But little more than a quarter of a mile to the south a precisely similar dyke begins, and strikes westward parallel to the line of the first one. The two separated strips of igneous rock overlap each other for about three-quarters of a mile. But that they are merely interrupted portions of what is really a single dyke can hardly be questioned. A second example is furnished by another of the great dykes of the same district, which after running for about twelve miles in a nearly east and west direction suddenly stops at Chryston, and begins again in the same direction, but on a line about a third of a mile further north. Such examples serve to mark out irregularities in the great fissures up which the materials of the dykes rose.

7. LENGTH

In those districts where the small and crowded dykes of the gregarious type are developed, one cannot usually trace them for more than a short distance. The longest examples known to me are those which have been mapped with much patience and skill by Mr. Clough in Eastern Argyleshire. Some of them he has been able to track over hill and valley for four or five miles, though the great majority are much shorter. In Arran and in the Inner Hebrides, it is seldom possible to follow what we can be sure is the same dyke for more than a few hundred yards. This difficulty arises partly, no doubt, from the frequent spread of peat or other superficial accumulation which conceals the rocks, and partly also from the great number of dykes and the want of sufficiently distinct lithological characters for the identification of any particular one. But making every allowance for these obstacles, we are compelled, I think, to regard the gregarious dykes as essentially short as well as relatively irregular.

In striking contrast to these, come the great solitary dykes. In estimating their length, as I have already remarked, we must bear in mind the fact that they occasionally undergo interruptions of continuity owing to the local failure of the igneous material to rise to the level of what is now the surface of the ground. A narrow wall-like mass of andesite or dolerite, which sinks beneath the surface for a few hundred yards, or for several miles, and reappears on the same line with the same petrographical characters, while there may be no similar rock for miles to right and left, can only be one dyke prolonged underneath in the same great line of fissure. But even if we restrict our measurements of length to those dykes or parts of dykes where no serious interruption of continuity takes place, we cannot fail to be astonished at the persistence of these strips of igneous rock through the most diverse kinds of geological structure. A few illustrative examples of this feature may be selected. It will be observed that the longest and broadest dykes are found furthest from the basalt-plateaux, while the shortest and narrowest are most abundant near these plateaux.

Not far from what I have taken provisionally as the northern boundary of the dyke region, two dykes occur which have been mapped from the head of Loch Goil by Arrochar across Lochs Lomond and Katrine by Ben Ledi to Glen Artney, whence they strike into the Old Red Sandstone of Strathmore, and run on to the Tay near Perth--a total distance of about 60 miles. If the dyke which continues in the same line on the other side of the estuary of the Tay beyond Newburgh, is a prolongation of one of these, then its entire length exceeds 70 miles. A few miles further south, one of a group of dykes can be followed from the heart of Dumbartonshire by Callander across the Braes of Doune to Auchterarder--a distance of 47 miles, with an average breadth of more than 100 feet. In the district between the Forth and Clyde a number of long parallel dykes can be traced for many miles across hill and plain, and through the coal-fields. One of these is continuous for 25 miles from the heart of Linlithgowshire into Lanarkshire. Still longer is the dyke which runs from the Firth of Forth at Grangemouth westward to the Clyde, opposite Greenock--a distance of about 36 miles. Coming southward, we encounter a striking series of single dykes on the uplands between the counties of Lanark and Ayr, whence they strike into the Silurian hills of the southern counties. One of these runs across the crest of the Haughshaw Hills, and can be followed for some 30 miles. But if, as is probable, it is prolonged in one of the dykes that traverse the moorlands of the north of Ayrshire and south of Renfrewshire to the Clyde, its actual length must be at least twice that distance. The great Moffat and Eskdale dyke strikes for more than 50 miles across the South of Scotland and North of England. The Hawick and Cheviot dyke runs for 26 miles in Scotland and for 32 miles in Northumberland.

But the most remarkable instance of persistence is furnished by the Cleveland dyke. From where it is first seen near the coast-cliffs of Yorkshire the strip of igneous rock can be followed, with frequent interruptions, during which for sometimes several miles no trace of it appears at the surface, across the North of England as far as Dalston Hall south of Carlisle, beyond which the ground onwards to the Solway Firth is deeply covered with superficial deposits. The total distance through which this dyke can be recognized is thus about 110 miles. But it probably goes further still. On the opposite side of the Solway, a dyke which runs in the same line, rises through the Permian strata a little to the east of the mouth of the Nith. Some miles further to the north-west, near Moniaive, Mr. J. Horne, in the progress of the Geological Survey, traced a dark compact dyke with kernels of basalt-glass near its margin, running in the same north-westerly direction. Still further on in the same line, another similar rock is found high on the flanks of the lofty hill known as Windy Standard. And lastly, in the Ayrshire coal-field, a dyke still continuing the same trend, runs for several miles, and strikes out to sea near Prestwick. It cannot, of course, be proved that these detached Scottish protrusions belong to one great dyke, or that if such a continuous dyke exists, it is a prolongation of that from Cleveland. At the same time, I am on the whole inclined to connect the various outcrops together as those of one prolonged subterranean wall of igneous rock. The distance from the last visible portion of the Cleveland dyke near Carlisle to the dyke that runs out into the Firth of Clyde near Prestwick, is about 80 miles. If we consider this extension as a part of the great North of England dyke, then the total length of this remarkable geological feature will be about 190 miles.

8. PERSISTENCE OF MINERAL CHARACTERS

Not less remarkable than their length is the preservation of their normal petrographical characters by some dykes for long distances. In this respect the Cleveland dyke may again be cited as a typical example. The megascopic and microscopic structures of the rock of this dyke distinguish it among the other eruptive rocks of the North of England. And these peculiarities it maintains throughout its course.[184] Similar though less prominent uniformity may be traced among the long solitary dykes of the South of Scotland, the chief variations in these arising from the greater or less extent to which the original glassy magma has been retained. The same dyke will at one part of its course show abundant glassy matter even to the naked eye, while at a short distance the vitreous groundmass has been devitrified, and its former presence can only be detected with the aid of the microscope. Where a dyke has caught up and absorbed abundant foreign materials its composition naturally varies considerably from point to point. Mr. Harker has observed some good examples of this variation in Skye.

[Footnote 184: See the careful examination of this dyke by Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 209.]