CHAP. XXXIV 
PETROGRAPHY OF THE DYKES 
137 
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 
porpliyritic felspars of earlier consolidation, and then present most of the 
characters of andesites. Professor Kosenbusch has extended the name of 
“ Tholeiites ” to rocks of this group in the North of England. 1 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 trans- 
parent and sometimes even colourless in thin slices. 
(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 per- 
centage of silica as to link them with the acid rocks. The average propor- 
tions 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 '70 to 2'96. They comprise the tholeiites, andesites and other 
more coarsely crystalline rocks of the great eastern and south-eastern dykes. 2 
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. 3 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. 4 I 11 the 
Dunoon dyke observed by Mr. Clough the siliceous finer-grained veins con- 
tain no less than 6 8 '05 per cent of silica, while the mass of the dyke itself 
shows on analysis only 4 7 ‘3 6 per cent. 5 Similar red strings have been 
noticed by the same careful observer in an east and west dyke near Loch- 
goilhead. From Mr. Teall’s examination a large part of the felspar in 
1 Mikroskopische Physiographic, 3rd edit. 1071 et scq. 
■ For analyses of dykes, see Sir I. L. Bell, Proc. lioy. Soc. xxiii. p. 546 ; Mr. J. S. Grant Wilson, 
cue. Roy. Phys. Soc. Edin. v. p. 253 ; Mr. Teall, Quart. Joum. Geol. Soc. xl. p. 209 ; Professors 
• add and Cole, Quart. Jour. Geol. Soc. xxxix. p. 444. 
On this subject see a paper by Dr. A. Lagorio, “Uber die Natur der Glasbasis sowie der 
vr ystallisationsvorgange ini eruptiven Magma,” Taehermak’s Minemlog. Mittlieil. viii. (1887), 
P- 421. 
4 Mr. J- S. Grant Wilson, Proc. Roy. Soc. Phys. Edin. v. (1880) p. 253. 
Unpublished analyses made by the late Professor Dittmar of Glasgow, and communicated 
to me by Mr. Clough. 
