Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1047

Rh polarized light, the clear mineral filling the interspaces of the serpentine network proves, as we should anticipate, to be olivine, often very well preserved, and showing brilliant colours, while the serpentine (with crossed prisms) is dull milky white, with an indistinct fibrous structure in the strings, and often almost or quite dark in the larger patches. The other mineral, which is colourless with ordinary light, is now seen to be of more than one species. One part exhibits the characteristic cleavage of augite; this occurs in somewhat rounded grains; very thin veins of serpentine frequently traverse the crystal, following the lines of its cleavage-planes. This mineral does not seem the result of change of the augite, but to have formed in the cracks; and the generally open condition of the cleavage-planes rather bears out the idea. The colours of the augite are generally dull; but parts of a crystal occasionally show the usual rich colours, as if the present low tints were due to some subsequent chemical change. The other mineral, also colourless, and showing much the same tints with polarized light, has a peculiar silky aspect, and one well-developed set of close and slightly wavy cleavage- planes. I was at once struck with the resemblance of this mineral to the enstatite in my specimens of lherzolite, and on testing it find it to be orthorhombic, and so true enstatite. I found, however, a crystal or two of ordinary diallage. Besides the above ferruginous microliths, doubtless secondary products, there are a few larger opaque grains of iron peroxide, probably original constituents. I searched the slide for picotite, but could not be certain of any specimen, though one or two grains resembled an opaque variety of this mineral. One or two cracks, filled with fibrous serpentine, traverse the slide.

Mullion Cove (no. 8).—With transmitted light the slide appears to be composed of a great number of small subangular grains of a clear mineral, often associated with aggregated black or brownish dust, and generally clear, rather irregularly oblong crystals, about ⋅02 inch in greatest length, showing a prismatic cleavage, one set of planes being rather strongly developed, all lying in a base of yellowish green serpentine. On applying polarized light we find that, as before, the grains are olivine, only that the process of conversion has here advanced a stage further than in the last slide. The other crystals show moderately bright colours: many of them are rather dusky in parts, as if somewhat decomposed; and the patchy change of the colours with polarized light confirms this. As in the last slide, they seem to have been cracked after crystallization, as though they had been subjected to a strain; and serpentine has been deposited in the cracks. Some specimens resemble normal augite; others are nearer to diallage; other small crystals are enstatite. This quite bears out the macroscopic appearance of the rock, which is full of a mineral with a rather silvery lustre, but not exactly like ordinary diallage. Here, too, a few small grains very much resemble picotite. In the augitic mineral the planes of principal cleavage are approximately parallel in many of the crystals, and there are other indications of a flow or pressure structure. This