346 
THE TERTIARY VOLCANOES 
BOOK VIII 
On the west side of the Coruisk river the handing is vertical ; southward 
from that stream it inclines slightly towards the south, hut soon again 
becomes vertical, and continues conspicuously so at the junction of the 
gabbro with the Torridon sandstones and plateau-basalts on the west side of 
Loch Seavaig. 
Thus, instead of being one great eruptive boss, the gabbro of this 
district is in reality an exceedingly complicated network of sills, veins and 
dykes. While the general inclination of the bedding sometimes continues 
uniform in direction and amount from one ridge to another, it is apt to 
change rapidly, as if the complex assemblage of intruded masses had been 
disrupted and had subsided in different directions. For example, after over- 
lying the bedded basalts of the plateau all the way from Cllen Brittle to the 
west side of Locli Seavaig, the gabbro descends abruptly across these basalts 
and also across the Torridon sandstones, on which they uneonformably rest. 
These two groups of rocks are not only truncated by the gabbro, but are 
traversed by the intricate system of sills, dykes and veins already referred to. 
Where it abuts against the sandstones and basalts in Loch Scavais’, the sabbro 
is arranged in vertical bands of different mineral composition and texture. 
Much of it is remarkably coarse, some bands displaying pyroxene crystals 
more than an inch in length. There is no fine-grained selvage here, indica- 
tive of more rapid cooling. So coarse, indeed, is the rock close up against 
the sandstone, that the junction-line can hardly be supposed to be the normal 
contact of the intrusive rock. This inference is confirmed by the existence 
of a singular kind of breccia between the gabbro and the sandstones. It is 
a tumultuous mass of fragments of coarse and fine gabbro, Torridon sand- 
stone and shale, and plateau-basalts, embedded in a pale crystalline matrix 
of fine granular granophyre ; veins from this acid intrusion run off’ into the 
gabbro on the one side as well as into the Torridon sandstones on the other. 
It would seem that this junction-line has been one of great movement, that 
the gabbro-sheets have subsided against a fault-wall of plateau-basalt and 
Torridon sandstone, and that subsequently an intrusion of finely granular 
granophyre has come up the fissure, involving in its ascent fragments of all 
the materials around. 
The rocks for a considerable distance to the south of the gabbro are 
intensely altered. The Torridon sandstone has been so indurated as to pass into 
a bleached white quartzite, while the shales interstra tiffed with it have been 
converted into a kind of porcellanite. But the most interesting alterations 
are those to be observed in the plateau-basalts, which at a height of about 
300 feet above the sea, are to be seen in nearly horizontal sheets that lie 
immediately on the upturned edges of the Torridon sandstones. These lavas 
have suffered great metamorphism, to which more particular reference will 
be made in Chapter xlvi. in connection with the action of the granophyre. 
Whether this alteration lias been produced by the intrusion of the gabbro or 
of some concealed mass of granophyre underneath, of which only projecting 
dykes and veins reach the surface, must remain a matter of doubt. On the 
whole, as the gabbro is here undoubtedly thrown against the basalts and 
