362 
THE TERTIARY VOLCANOES 
BOOK VIII 
Stac, where they dip rapidly towards south-east and south (Fig. 334). This 
steep dip away from the central mass of gabbro, is repeated in the hills to the 
north, where the beds are inclined to north-east, the angle gradually lessening 
northwards till they are truncated by the granophyre of Strathmore. The 
mass of Blath Bheinn thus occupies the centre of the dome or anticline. The 
theoretical structure of one of the gabbro bosses is represented in Fig. 343. 
It will be understood, however, that what for the sake of clearness is here 
represented in one uniform tint of black in reality consists of an exceedingly 
complex network of sheets and dykes differing from each other in texture 
and structure, as well as in the relative dates of their intrusion. 
8. The injection of so much igneous material among the bedded basalts 
has induced in these rocks a certain amount of contact metamorphism. I 
have referred to it as showing itself in the field as a marked induration, the 
rocks becoming closer grained, dull, splintery, and weathering, with a grey 
Fig. 343. — Theoretical representation of the structure of one of the Gabbro Bosses of the Inner 
Hebrides. 
a a, platform of older rock on wliicli the bedded basalts (b &) have been poured out ; c, gabbro. 
or white crust, while their amygdales lose their definite outlines, and epidote 
and calcite run in strings, veins and patches through many parts of the 
rocks. As already remarked, it is difficult to determine how much of this 
change should be referred to the influence of the gabbro, and how much to 
that of the numerous intrusions of granophyre which may be apophyses 
of much larger bodies of that rock lying not far underneath. On account of 
this difficulty, the more detailed description of the metamorphism of the 
plateau-basalts is reserved for Chapter xlvi., where it will find a place in con- 
nection with the effects produced by the intruded gran ophy res, which have 
undoubtedly been more extensive than those effected by the gabbros. 
The structure and history of the gabbro bosses of the Inner Hebrides 
find a close parallel in those of the Henry Mountains of Southern Utah, so 
well described by Mr. G. K Gilbert of the United States Geological Survey. 1 
In that fine group of mountains, rising to an extreme height of 5000 feet 
above the surrounding plateau, and 11,000 feet above the level of the sea, 
1 See the remarks and diagram, ante. p. 86. 
