162 
THE TERTIARY VOLCANOES 
BOOK VIII 
In some instances the more basic rock has been first injected, and has 
subsequently been disrupted, by the more acid pitchstone. In other cases 
the order has been the reverse. The successive ruptures have taken place 
sometimes along the centre, sometimes at the margins, and sometimes 
irregularly along the breadth of the dykes. Professor Judd has recently 
studied these rocks, and lias given descriptions of their chemical composition 
and microscopic characters. He regards them as having been successively 
injected into the fissures from the same subterranean reservoir, in which two 
magmas of very different chemical constitution were simultaneously present. 1 
Nowhere in the Tertiary volcanic regions of Britain do compound dykes 
appear to be so abundant as in the centre and southern part of the island of 
Skye. During the progress of the Geological Survey in that district, Mr. 
Clough and Mr. Harker have mapped a large number in the ground between 
the Sound of Sleat and the Bed Hills. With regard to these dykes Mr. 
Harker observes that the several members are generally petrographically 
different, some being basic, others intermediate, and others acid. “ There is 
usually,” he remarks, “a symmetrical disposition, two similar and more 
basic dykes being divided by a more acid one ; for example, two andesites 
separated by a pitchstone. Thus at the mouth of the little stream which 
runs from Torran into the bay east from Dun Beag a dyke, apparently 18 
feet wide, is found on examination to consist of a central dyke (specific 
gravity 2\S6) flanked by two more basic dykes (specific gravity 3'02).” 
In the great majority of examples hitherto observed in Skye the two 
lateral dykes consist of some basic rock (diabase or basalt), while the central 
and thickest band is of some acid material (granophyre or quartz-felsite). 
This triple arrangement occurs both in dykes and sills. 
As an illustration of the association of the two kinds of rock in dykes 
I may cite an example which appears on the southern edge of the Market 
Stance of Broadford (Fig. 254). Here the characteristic triple arrangement 
is typically developed. A central light- 
coloured band, about eight to ten feet 
broad, consists of a spherulitic grano- 
phyre in which the spherulites are 
crowded together and project from the 
weathered surface like peas, though they 
do not here show the curious rod-like 
aggregation so marked in some other 
dykes. On either side of this acid centre 
a narrow basalt dyke intervenes as a wall 
next to the Torridon sandstone which 
here forms the country-rock. Such compound dykes have sometimes a 
total width of 100 feet or more. 
Fig. 254. — Compound dyke, Market 
Stance, Broadford, Skye. 
a, Granophyre; i>h, Basalt; cc, Torridon 
sandstone. 
1 Quart. Jour. Gcol. Soe. vol. xlix. (1893), p. 536. Full details of tlie compound dykes of 
Tormore and Cir Mhor in Arran, and references to previous writers will be found in this paper. 
The probable age of the youngest eruptive rocks of this island will be discussed in Chapter xlvii. 
p. 418. 
