Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1019

Rh. Finally, a granite vein has cut irregularly through all these rocks.

The rock (4) is not easy to determine; its texture is coarser on the face of the crag than it is along the outcrop a few feet back; I have had slides prepared from each part. The finer variety consists of an interwoven mass of rather acicular crystals of hornblende (actinolite), in a clear base, which, with crossed prisms, either remains dark or exhibits an obscure microcrystalline structure. This in parts seems to resemble steatite; in others it is more like one of the pseudomorphic products after felspar, which will be noticed below. Granules of magnetite are scattered about; and there are some very irregular grains or plates of a brown hornblende, full in many parts of a black dust. This mineral appears, from its cleavage, to have a platy structure. In the coarser specimen there appears still more actinolite, and the brown hornblende grains are much larger; they appear in some cases to have been broken or partly destroyed after they had been formed. The mineral to the eye has a clove-brown colour; it much resembles anthophyllite, but is certainly not an orthorhombic mineral; probably it is the variety of hornblende noticed by Rosenbusch (Mikr. Phys. p. 264). I believe that this rock was originally a hornblende schist, that its entanglement with the intrusive rock affected it to some extent, and that since then it has undergone further changes, chiefly by the action of water.

Nearly above this place, at the top of the cliff (the Rill), is a landslip, or a deserted quarry, of considerable size. Here are two granite veins in the serpentine—one about 4 feet wide, forming a curved dyke running up the face of the higher cliff, the other showing in it a short distance towards the south. Both are much decomposed and of a red colour; they crack the serpentine in contact considerably; and in places it is so much altered that for a few inches it might be taken for a talcose schist. We have thus an irregular line of veins and small bosses of granite extending pretty continuously over more than half a mile. The serpentine forming the cliff just mentioned is compact in texture, of a dull purplish colour; the bronzite crystals are few and small (no. 4). When examined carefully the rock shows a sort of parallel streaky structure, indicated by darker lines; the faces of the joints are coated by films of green or whitish steatite; and old surfaces weather dark rusty brown. The parallel structure becomes much developed by the weathering, and might easily induce the supposition that the rock was really stratified. It is particularly well exhibited along the down to the north of Kynance Cove, though it may also be observed to the south of it, as in other places. In the coarser varieties the structure appears to be formed by bronzite crystals, which have resisted weathering better than the matrix in which they are imbedded.