CHAP. XLVII 
THE ACID ROCKS OF ST. KILDA 
411 
westward of where it must lie, the beach is cumbered with large blocks of 
rock broken up from the mass, which can be _ seen in situ -a little further 
south forming a line of low cliff with a rugged foreshore. These rocks 
consist of various gabbros and basalts of rather fine grain, profusely traversed 
with veins of white granophyre. Some of these veins are two feet or more 
in breadth, and, when of that size, show the distinctive granular texture and 
(busy structure of the main part ot the acid rock. But from these dimen- 
sions they can be traced through every stage of diminution until they 
become mere threads. When they are only an inch or two broad, they 
assume a finely granular texture like that of the veins that run through the 
body of the granophyre. 
The amount of injected material in the dark basic rocks is here and 
there so great as to form a kind of breccia (Big. 364), which, from the con- 
trast of tone between its two constituents, makes a conspicuous object on the 
shore. Here, as in the example already cited from Rum, the basic rocks seem 
to have been shattered into fragments, and the acid material to have been 
injected into the minutest interstices between them. The enclosed frag- 
ments are of all sizes from mere grains up to blocks a foot or more in length. 
They are generally angular, like rock-cliips from a quarry. Moreover, they 
are not all of the same kind of material. While at this locality most of 
them consist of basalt, they include also a few large and small pieces of 
rather coarse gabbro. There has evidently been a certain amount of trans- 
port of material, as well as an extensive disruption of the rocks in situ. 
fhe granophyre here and there assumes a darker or greener tint, as if it 
had dissolved and absorbed some portion of the older rock. 
Still more astonishing are the sections to be seen on the western cliffs 
and rocky declivities of the ridge to the north of the Dune, at a distance of 
perhaps 500 or 600 yards westwards from those of the South Bay. Here 
the gabbro-sheets are traversed by a number of conspicuous white bands, 
which on examination prove to be veins or dykes of granophyre. As viewed 
from the sea, the general disposition of the two groups of rocks is re- 
presented in Fig. 366. The broadest mass of granophyre breaks out 
towards the bottom of the precipice, and slants- upward as a sheet 
intercalated between the gabbro sills, with a breadth of about 40 or 
50 feet, but rapidly thinning away in its ascent. One of the bands below 
it has a breadth of about 15 feet. The material of these intrusions is a 
