316 
THE TERTIARY VOLCANOES 
BOOK VIII 
rock as a “ phonolite.” 1 Iu 1894, during an excursion with my colleague 
Mr. C. T. Clough, I had an opportunity of examining the rocks and collect- 
ing notes for the following account of them. 
At Rudh’ an Iasgaich, about two miles from the Point of Sleat, a small 
outlier ol conglomerate lies on the edges of the Torridon Sandstone. This 
deposit has been correctly identified by Professor Judd with the similar 
strata which, in Skye and elsewhere on the west coast of Scotland, underlie 
the Liassic series. It is here about 10 or 12 feet thick, reddish and 
yellowish in colour, and distinctly calcareous. Its component pebbles con- 
sist largely of Cambrian (Durness) limestone, quartzite, and Torridon Sand- 
stone — rocks which all occur in situ, in Sleat. It may be compared with 
the limestone conglomerates of Strath and those which underlie the Lias at 
Heast on Loch Eishort. 2 That here, as elsewhere in this region, the base- 
ment conglomerate was followed by the rest of the Lias and Oolites may 
be interred with some confidence from the copious development of the 
Jurassic series a few miles off, both to north and south. But the whole 
ol this overlying succession of formations has here been swept away, and, but 
for the protection afforded by the eruptive rocks of Rudh’ an Iasgaich, the 
conglomerate would likewise have disappeared. 
Above the conglomeratic band lies a sheet of intrusive rock, which in 
Fig. 324.— Section of Dolerite Sill cut by another sill, both being traversed by dykes, Rudh’ an 
Iasgaich, western side of Sleat, Skye. 
one place has apparently cut it out, so as to rest directly upon the 
Torridon Sandstone (a, Fig. 324). The decay of the softer detrital rock 
underneath has caused the sill to break off in slices, which have left behind 
them a bold mural escarpment ( b b). 
1 he rock of this sill is a rather coarsely crystalline porphyritic olivine- 
dolerite, which towards the north attains a thickness of about TO feet. It 
exhibits the usual prismatic jointing, but less perfectly than some of the 
Trotternish sills already referred to. Besides these vertical joints, it is also 
traversed by a system of horizontal divisional planes which, though some- 
what irregular in their course, run, in a general sense, parallel to the upper 
and under surfaces of the sill. 
It seems to have been along this transverse series of joints that a 
second sill (c), five or six feet thick, has been injected. The material of this 
1 Quart. Jour. Geol,. Soc. vol. xxxiv. (1878) p. 692. 
- Op. cit. vol. xiv. (1857), p. 9 ; vol. xliv. (1888), p. 71. 
