Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1016

886 rock varies in the hand specimen from a black, schistose, but not very fissile rock, which appears to consist mainly of hornblende, to a dark greyish granitoid rock, in which the hornblende is not conspicuous, and the prevailing minerals are quartz and felspar in variable quantities, so that the rock sometimes might almost be called a quartzite; at others it resembles a vein granite. The more schistose hornblendic varieties, on a closer examination, show, generally, fine white specks of felspar. Now and then we find porphyritic varieties, sometimes with felspar crystals about $1⁄4$ inch long, sometimes with hornblende about the same size; occasionally also the rock appears to contain a talcose or chloritic mineral in large quantities, especially when it shows signs of decomposition. I have examined microscopically three varieties of this rock—one (1) a typical specimen of the prevalent black hornblende schist, another (2) a dark grey granitoid variety, and the third (3) a greyish quartzo-felspathic rock, very difficult to distinguish from true vein granite.

1. About two thirds or more of the field is occupied by crystalline grains of hornblende of rather irregular outline, with very characteristic cleavage, of a green colour, and strongly dichroic. There are many small rounded or vermicular grains of a black mineral, probably magnetite, often included in the hornblende, occasional small rounded or subangular grains of clear quartz, and a few small acicular hexagonal crystals, probably apatite. The rest of the field is occupied by a kaolinized or altered felspathic mineral, which seems, in parts, as if it had never been perfectly crystallized.

2. Taken from a junction with a gabbro vein, and shows traces of foliation parallel to the common surface. Exhibits an imperfectly crystallized and decomposed groundmass, as in the last, though much more abundant, as well as numerous fairly defined felspar crystals, generally plagioclase; quartz rare; magnetite less abundant than in the last, as is, of course, the hornblende; the crystals of this are also more irregular, with less-distinct cleavage-planes, often somewhat acicular, platy or fibrous. There are microliths—some, probably, apatite; others, shorter and strongly dichroic, may be tourmaline.

3. A rarer variety, closely resembling a vein granite, being highly crystalline, and consisting of quartz, felspar, and a little mica. A microscopic description of one of these will be given further on.

In general the hornblende schist, except in the darkest and most compact varieties, shows distinct signs of stratification; sometimes thin felspathic (or quartzose) and hornblendic bands alternate, occasionally exhibiting current-bedding; and sometimes quartzo-felspathic strata, from an inch to a few feet thick, alternate with more hornblendic, chloritic, or earthy layers. Epidote is occasionally present in minute quantities; and veins of quartz, felspar, and ferruginous matter occur. Foliation seems generally almost, or quite, parallel with the stratification; there are many extremely beautiful small contortions. The rock is rather sharply jointed, and weathers into bold headlands, dark in the wash of the waves, grey and lichen-covered where exposed to the blasts.