CHAP. XL1I 
THE BASIC SILLS OF SKYE 
3ii 
thick, and it not impossibly passes under the still thicker pile of Garbh 
Eilean. Yet it has been horizontally ruptured near its base, and into the 
rent thus produced another mass of molten matter has been thrust. This 
subject will be again referred to in connection with another remarkable 
example on the west coast of Skye. 
In contrast to such enormous thicknesses of intrusive material as those 
of Trotternish and the Shiant Isles, instances may be culled from the same 
belt of sills where the molten rock has been injected in thin leaves and mere 
threads into the Jurassic sandstones and shales, or into the shales and coals 
intercalated among the plateau-basalts. Thus, on the cliff immediately to 
the north of Ach na Hannait, between Loch Sligachan and Portree Bay, the 
section may be seen which is represented in Fig. 321. At the base lies a 
vesicular dolerite with a slaggy upper surface (a). Next comes a zone of sedi- 
Fig. 3*21. — Section of thin Intrusive Sheets and Veins in carbonaceous shales lying among the Plateau- 
basalts, cliffs north of Ach na Hannait, between Portree Bay and Lock Sligachan. 
mentary material about live or six feet thick, the lower portion consisting of an 
impure coal, which passes towards the right hand into brown and grey carbon- 
aceous shale with plant-remains. ( b ). This coaly layer has been already 
alluded to as probably lying on the same horizon with the coal of Portree 
(p. 288). Traced northward, it is found to have a bed of fine tuff beneath it, 
and sometimes a volcanic breccia or conglomerate. It fills up rents in the 
underlying slaggy lava, and was undoubtedly deposited upon the cooled sur- 
face of that rock. Immediately above this lower band the black carbon- 
aceous shale which follows has been invaded by an extraordinary number of 
thin cakes or sills and also by veins or threads of basalt. For a thickness 
of two or three feet the band (d) consists mainly of these intrusions, which, 
in the form of a fine grey basalt, vary from less than an inch to three or 
four inches in thickness. They are separated by thin partings of coaly 
shale, and as they tend to break up into detached nodule-like portions, 
