CHAP. XLVII 
THE ACID ROCKS OF ST. K1LDA 
413 
body of granophyre, which emerges to the surface about a third of a mile to 
the eastward, but may of course be at no great depth underneath. 
Besides the broader bands of acid rock, and diverging from them, a com- 
plicated network of veins ramifies in all directions through the gabbros, as 
at the South Bay. The extraordinary degree to which the basic rocks have 
been shattered into fragments is strikingly 
displayed here, likewise the extreme liquidity 
of the acid magma, whereby it was able to 
insinuate itself into every chink and cranny. 
But the observer notices that this condition of 
excessive disruption is not shared by all the 
basic sills, and is not attendant upon all the 
acid dykes. As an example of this irregular 
distribution of the structure, I give the ac- 
companying sketch (Fig. 365), where a fine- 
grained gabbro has been completely broken 
up and intersected with granophyre veins, 
while the coarser sheet overlying it has almost 
entirely escaped. The dark basaltdike sheets 
appear generally to have been much more disrupted than the more largely 
crystalline varieties. It is noticeable here, also, that the fragments en 
Flo. 365. — Veins of granophyre tra- 
versing a line-grained galibro and 
scarcely entering a coarse-grained 
sheet, west side of Rueval, St. Kilda. 
tangled in the network of granophyre veinings do not entirely belong to 
the rock that has been shattered, hut sometimes include large and small 
lumps of different gabbros, showing some transference of material with the 
inrush of the acid magma. 
Though closer in grain where it comes in contact with the gabbro, the 
granophyre never assumes any vitreous texture along its margin. A series of 
thin slices, prepared from specimens collected by me in the South Bay in the 
summer of 1895, was examined by Mr. Marker, who furnished the following 
notes regarding them : — “ The basalt traversed by the granophyre is a fine- 
textured variety with small porphyritic felspars. These latter seem to be 
usually unaltered, retaining the glass cavities which in some of the crystals 
are abundant. The groundmass, however, shows minerals of metamorphic 
origin which must be derived mainly from the original augite. A brown mica 
is the most conspicuous ; but with it are associated some brownish-green 
hornblende and certain chloritic and perhaps serpentinous substances. It is 
chiefly near the margin of a fragment of basalt that the mica gives place to 
these minerals. The basalt still retains plenty of unaltered granules of 
augite in the central parts of a fragment. It is not certain that the 
secondary minerals named come exclusively from the augite of the basalt : 
from their form and mode of occurrence they may in part have replaced 
olivine or even rhombic pyroxene. 
“ The acid rock, though styled granophyre above, belongs to a granitoid 
variety of that group of rocks, and has but little indication of micrographic 
structures. Compared with the other granophyres from St. Kilda, sliced and 
examined, these examples show a less acid composition. This is expressed 
