342 
THE TERTIARY VOLCANOES 
BOOK VIII 
through it, and both having been subsequently disrupted by still later 
protrusions. 
But if by chance lie should begin his examination of the ground upon 
some of the more typically banded varieties of rock, he may for a time 
almost refuse to admit that these can be either of volcanic origin or of 
Tertiary age. 1 He will find among them such startling counterparts of the 
structure of the ancient Lewisian gneiss of the North-West of Scotland that 
he may well be pardoned if for a time he seeks for evidence that they really 
do belong to that primeval formation, and have only been accidentally 
involved among the Tertiary volcanic rocks. If, for instance, he should land 
in Loch Scavaig, and first set foot upon the gabbros as they appear around 
Loch Coruisk, he would find himself upon masses of grey coarsely crystalline, 
rudely banded rock, like much of the old gneiss of Sutherland and Boss. 
Ascending over the ice-worn domes, he would notice that the banding 
becomes here and there more definitely marked by strong differences in 
texture and colour, while elsewhere it disappears and is replaced by a granitoid 
arrangement of the crystals, which are often as large as walnuts. 
Nowhere is the gneissoid banding more beautifully developed than on 
Fig. 335. — Banded and puckered gabbro, Druim an Eidlme, Glen Sligachau, Skye. 
the east side of the Cuillin group near the head of Glen Sligachan along the 
ridge of Druim an Eidhne. It was at this locality that the four typical 
structures were observed which have already been referred to (p. 329). The 
varieties of colour and composition depend upon the exceedingly irregular 
distribution of the component minerals. The paler bands, rich in felspar, 
lie parallel with dark brown bands full of pyroxene, olivine and magnetite, 
in which, moreover, thin ribs of glistening black consist in large part of the 
iron ore. These layers vary in thickness from mere pasteboard-like laminae 
to beds a yard or more in thickness. Within a space of a few square yards 
their parallelism reminds one of stratified deposits (Fig. 336), but traced 
over a wider space they are found to be more or less irregular in thickness 
and lenticular in form. 
The resemblance to gneisses, and sometimes to the flow-structure of 
coarse rhyolites, is still further sustained by occasional undulations or minute 
puckerings (Fig. 335). Still more extraordinary are the examples of the 
1 See Quart. Journ. Geol. Soc. vol. 1. pp. 217, 657, and a paper by tlie author, “Sur la 
Structure rubannee des plus anciens Gneiss et des Gabbros Tertiaires,” Compt. rend. Cong. Geol. 
Internat. 1894, p. 139. 
