CHAP. XLVII! 
THE ACID SILLS 
435 
to have been in connection with the dykes having been removed, or are only 
very partially exhibited in direct connection with sills still remaining. 
Where they can be examined in detail they are seen to be dykes varying up 
to about 100 feet in width, but of no great longitudinal extent. Between 
Suisnish and Cnoc Carnach they hear E.X.E., that is, at right angles to the 
ordinary basic dykes of the district and parallel to the general direction of 
the axes of folding, though further north they change this trend, but still 
remain parallel to the strike of the Lias. 
“ These dykes are composed essentially of granophyre, identical with that 
ot the sills. In some cases, they are flanked with basalt-dykes on one or 
both sides, or the former existence of such lateral dykes is indicated by 
partly-destroyed inclusions of the basic rock in the granophyre. The basalt 
found in these cases is identical with that of the basic sills, and shows the 
same relation to the granophyre. Discontinuity and failure of the basalt 
are commoner, however, in the dykes than in the sills — a difference presum- 
ably attributable to more energetic destructive action of the acid magma 
when it was hotter and fresher. These supposed feeders of the granophyre 
sills are certainly in some cases, and have possibly been in all, double or 
triple dykes. The acid magma thus appears not only to have spread later- 
ally along the same platforms as the earlier basalts, but to have reached these 
levels by rising through the same fissures which had already given passage 
to the basic magma.” 1 
The granophyre sills which, as already stated, can be followed as an 
interrupted band from Suisnish Point to the Sound of Scalpa, emerge again 
beyond Loch Sligachan and also in the, island of Eaasay, where a great sheet 
of the acid rock covers an area of about five square miles. This tract has 
recently been mapped for the Geological Survey by Mr. H. B. Woodward, 
who has found it to have been intruded across the Jurassic series, a large 
part of its mass coming in irregularly about the top of the thick white sand- 
stones of the Inferior Oolite, But it descends beneath the Secondary rocks 
altogether, and in some places intervenes between the base of the Infra-liassic 
conglomerates and the Torridon sandstone. Its irregular coui’se transires- 
sively across the Mesozoic formations is probably to he regarded as another 
example of the intrusion of the acid material preferentially along the line of 
unconformability between the older rocks and the Tertiary basalts, now 
nearly all removed from Eaasay by denudation, though the intrusion does 
not rigidly follow that line of division, but sometimes descends below it. 
The central portions of this Eaasay granophyre possess the ordinary 
structures of the corresponding rocks in Skye. They show a finely crystalline- 
granular, micropegmatitie base, through which large felspars and quartzes 
are dispersed. But at the upper and under junction with the sedimentary 
rocks, beautiful spherulitic structures are developed. This is well seen on 
the shore near the Point of Suisnish (Eaasay), where, below the Lias Lime- 
stones, the top of the granophyre appears, and where its bottom is seen to lie 
on the Torridon sandstone, 
1 MS. notes supplied by Mr. Harker. 
