[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.
[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]
[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.
[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.
[174] Mr. J. S. Grant Wilson, Proc. Roy. Soc. Phys. Edin. v. (1880) p. 253.
[175] Unpublished analyses made by the late Professor Dittmar of Glasgow, and communicated to me by Mr. Clough.