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.
[160] Trans. Roy. Soc. Edin. xiv. (1840), p. 677.
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.
[161] Trans. Geol. Soc. iii. (1815), p. 79. This locality is further noticed on p. 164.
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]
[162] Mr. Clough is inclined to suspect that the E. and W. dykes are older than the Tertiary series and may be later Palæozoic.
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.