Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1049

Rh, and is rather cracked and separated along those parallel to ∞ P. This rock contains small quantities also of an augitic mineral, but much less than the other.

Helston Road (no. 14).—This slide shows a very pale greenish serpentine, here and there a little clouded with a pale olive tint, with colourless granules of olivine, a good deal of a rather fibrous mineral in irregular aggregated grains, granules and dust of magnetite, and larger grains of dark picotite. One or two pale brownish grains show dichroism and a cleavage like that of hornblende.

On applying polarized light the serpentine shows the usual arrangement of doubly refracting meshes on an isotropic ground, not much of the unchanged olivine remaining to show colours. By testing the fibrous-looking mineral I have found some to be enstatite; but with the greater part the optic axial plane seems certainly not to coincide with the plane of principal cleavage, but to make an angle of about 10° with it; I believe it to be hornblende, not diallage. The bedded structure is indicated by a tendency to a banded arrangement in the component minerals and a frequent approach to parallelism in the longer diameters of prismatic-shaped crystals, and of the principal cleavage-planes. Part of the slide is traversed by an irregular vein filled with a steatitic mineral, which is almost transparent with ordinary light, and with crossed prisms shows a feebly doubly refracting granular structure. I regard it as a secondary product.

Goomhilly Downs (no. 15).—Another dull-coloured serpentine with a decided banded structure. Groundmass of very pale light-greenish serpentine, with numerous small angular or subangular grains scattered in it rather irregularly, many aggregated dusky clots, streaks and grains of magnetite, several clear brown crystals with a prismatic cleavage rather like hornblende, in parts somewhat dusky, with several semiopaque dusky patches of greyish and also greenish colour. Small grains of picotite occur, dull olive-brown and subtranslucent. With crossed prisms the field appears partly dark, partly occupied by a slightly fibrous, feebly doubly refracting variety of serpentine, of a dull bluish-grey colour,—chrysotile or some allied variety. The grains of olivine show the usual clear bright colours; the hornblendic mineral is not brilliant; and enstatite is seen. The dusky spots show a granular structure, something like the felspar pseudomorphs described above; but of their true nature I cannot be certain. The streaky structure is indicated both by a tendency to grouping in parallel bands on the part of the minerals, and by a parallelism in the longer diameters of the prismatic crystals and the streaks of magnetite.

Kynance Cove (no. 2).—A fine specimen kindly lent to me by Mr. Allport, intermediate in character between that of Gue Graze and Mullion. Some unaltered olivine, a good deal of augite or diallage (it seems generally, as at Mullion, nearer the former) and perhaps a little enstatite; there are also a few semi-opaque patches which may be altered felspar as at Gue Graze. With ordinary light the serpentine is of a yellowish colour; and there is a good deal of staining with a