CHAP. XLVI 
THE ACID BOSSES OF SKYE 
393 
“ The gabbro-debris,” he remarks, “ has been for the most part completely dis- 
integrated by the caustic or solvent action of the acid magma on some of its 
minerals. Those constituents which resisted such action have been set free 
and now figure as xenocrysts [foreign crystals], either intact or more or less 
perfectly transformed into other substances. At the same time the material 
absorbed has modified the composition of the magma, in the general sense 
of rendering it less acid.” Mr. Harker has traced the fate of each of the 
minerals of the gabbro in the process of solution and isolation in the acid 
magma, which, where this process has been most developed, is believed by 
him to have taken up foreign material amounting to fully one-fourth of its 
own bulk, derived not from the rocks immediately around, but from a gabbro 
probably at a considerable depth beneath. 1 
(4) Relation of the Granophyre to the Basic Dykes and Veins . — Reference 
has already been made to the fact that the “ syenite ” bosses of Skye cut off 
most of the basalt-dykes, but are themselves traversed by a few others. 2 
The locality that furnished me with the evidence on which this statement 
was originally made nearly forty years ago affords in small compass a clearer 
presentation of the facts than I have elsewhere met with. The sections 
described by me are visible at the eastern end of the boss of Beinu an 
Dubhaich, Strath ; but similar and even better examples may be cited from 
the whole northern and southern margins of that eruptive mass. On the 
north side an extraordinary number of dykes may be traced in the Cambrian 
limestone from the shores of Loch Slapin eastwards. They have a general 
north-westerly trend, but one after another, as I have already remarked, is 
abruptly cut off by the granophyre. As an example of the way in which 
this truncation takes place, I may site a single illustration from the northern 
margin of the eruptive mass, near Torrin. It might perhaps be contended 
that the numerous dykes which traverse the 
limestone and stop short at the edge of the acid 
rock, are not necessarily older than the grano- 
phyre, but may actually lie younger, their sudden 
termination at the edge of the acid boss being due 
to their inability to traverse that rock. That this 
explanation is untenable is readily proved by 
such sections as that given in Big. 350, where 
a basic dyke (&) 9 or 10 feet broad running 
through the Cambrian Limestone ( a a) is abruptly 
cut off by the edge of the great granophyre boss. 
Hot only is the dyke sharply truncated, but 
numerous pieces of it, from an inch to more than 
a foot in length, are enclosed in the granophyre. 
The latter is well exposed along the shore of 
Loch Slapin in an almost continuous section of nearly a mile in length. 
The contrast therefore between the development of dykes within and beyond 
1 Quart. Journ. Gcol. Soc . vol. lii. (1896) p. 320. 
2 Ante, p. 173, and Quart. Journ. Geol. Soc. vol. xiv. (1867) p. 16. 
Fig. 350. — Ground-plan of basic 
dyke in Cambrian Limestones 
truncated by granophyre which 
encloses large blocks of the dyke, 
Torrin, Skye. 
