THE TERTIARY VOLCANOES 
BOOK VIII 
2.36 
In Eigg the fragment of the basalt-plateau which has been preserved, 
rests unconformably on successive platforms of the Jurassic formations. Its 
component sheets of lava rise in cliffs around the 
greater part of the island. As they dip gently south- 
wards their lower members are seen along the 
northern and eastern shores, while on the south-west 
side their higher portions are exposed in the lofty 
precipices which there plunge vertically into the sea. 
The total thickness of the volcanic series may here 
be about 1100 feet. The rocks consist of the usual 
types — black, fine-grained, columnar and amorphous 
basalts, more coarsely crystalline dolerites, dull earthy 
amygdaloids with red partings, and occasional thin 
bands of basalt-conglomerate or tuff. The individual 
beds range in thickness from 20 to 50 or 60 feet. 
Though they seem quite continuous when looked at 
from the sea, yet, on closer examination, they are 
found not unfrequently to die out, the place of one 
bed being taken by another, or even by more than one, 
in continuation of the same horizon. The only 
marked petrographical variety which occurs among 
them is a light -coloured band which stands out con- 
spicuously among the darker ordinary sheets of the 
escarpment on the east side of the island. The 
microscopic characters of this rock show it to belong 
to the same series of highly felspathic, andesitic, or 
trachitic lavas as the “ pale group ” of Ben More, in 
Mull. It is strongly vesicular, and the cells are in 
some parts so flattened and elongated as to impart a 
kind of fissile texture to the rock. There can be no 
doubt that this band is a true lava, and that it was 
poured out during the accumulation of the basalt- 
plateau. It supplies an interesting example of the 
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intercalation of a lighter and less basic lava among the 
ordinary heavy basic basalts and dolerites. 
That feature of the island of Eigg which renders 
it so remarkable and conspicuous an object on the 
west coast is the long ridge of the Scuir. Bising 
gently from the valley which crosses the island from 
Laig Bay to the Harbour, the basaltic plateau ascends 
south-westwards in a succession of terraces, until 
along its upper part it forms a long crest, from 900 
to 1000 feet above the sea, to which it descends on 
the other or south-west side, first by a sharp 
slope, and then by a range of precipices. Along the watershed of 
this crest runs, in a graceful double curve, the abrupt ridge of the Scuir, 
cm 
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