CHAP. XLII 
THE BASIC SILLS OF SKYE 
317 
younger intrusion is a black, finely crystalline dolerite or basalt, with rudely 
prismatic jointing. Its most striking feature, besides its regularity of 
position and persistency for several hundred yards as a platform along the 
shore, is the basalt-glass which marks both its under and upper surfaces of 
contact, and which is here developed upon a scale to which I have not met 
with an equal among the Tertiary sills of this country. 
The selvage of glass appears as a black tar-like layer, varying from a 
mere film to two or three inches in thickness. It is found not only on the 
upper and under surfaces, but descends along abrupt step-like interruptions 
of the upper surface, a foot or more in height, as if the sill had been broken 
by a series of subsidences. The apparent fracture, however, is probably due 
to the irregularities of the passage forced for itself by the molten rock as it 
passed from one line of horizontal joint to another through the heart of the 
older sheet. 
The exposed surface of black glass on the top of the younger sill ex- 
hibits long parallel lines, probably marking flow-structure, which are made 
conspicuous by a pale yellow ferruginous weathered crust. Portions of the 
larger intrusive sheet have been broken off and involved in the later rock. 
The younger sill disappears to the north, and is not found in the cliff of Eudha 
Churn nan Cearc, where the thick sill, lying once more 011 the band of con- 
glomerate, forms a fine escarpment above the shore. Dykes of fine-grained 
basalt (d d ) with compact chilled margins rise through both sills, together 
with veins which pursue a wavy upward path like strips of black ribbon. 
This example, and that of the Shiant Isles already described, cannot but 
impress the observer with the prodigious force with which the material of 
the sills was injected. In these instances solid sheets of intrusive rock have 
subsequently been rent open, doubtless under a superincumbent pressure of 
many hundreds of feet of the terrestrial crust, and a new injection of molten 
magma has made its way into the rents thus caused. In each case, the 
position of the rents was obviously determined by structural lines in the 
older sills, but we are lost in astonishment at the energy required to split 
open, even along these lines, such solid crystalline masses as the thick 
sills, and to overcome the superincumbent pressure of so deep a pile of rock. 
The isolation of a relic of the Tertiary sills on the west side of the pro- 
montory of Sleat presents some interesting problems to the mind of the 
geologist. The locality lies about midway between the basalt-plateau of 
Strathaird and that of Eigg, and some eight or nine miles in a direct line from 
either. The basalts cannot be proved to have once stretched continuously 
between Eigg and Strathaird, and to have covered this part of Sleat ; but 
the position of the Sleat sills makes it probable that this continuation did 
formerly exist. The denudation of the West of Scotland since early Tertiary 
time has been so stupendous that I am prepared for almost any seemingly 
incredible evidence of its effects. There can hardly be any doubt, however, 
that the sills here described belong to the great platform of intrusive sheets, 
and that they were injected under a pile of Secondary strata, if not also of 
Tertiary basalts, which has here been entirely removed. 
