Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1048

918 is still more conspicuous in two specimens lent to me by Mr. Allport, one of which is cut transversely to the streaky structure. From these, especially the latter, it is quite clear that the structure is produced, not only by a general parallelism in the longer diameters of crystals of a prismatic outline, but also by a partial separation of the constituent minerals—the augitic constituent, the olivine with strings of serpentine, and the magnetite being each, more abundant in particular layers.

Gue Graze (no. 5).—The greater part of the slide closely resembles that of no. 8, except that there is a slightly larger proportion of serpentine, and the rock has not been quite so coarsely crystalline. The augitic constituent, however, is less conspicuous, and there are a number of small patches of a semitransparent, rather earthy-looking mineral from about ⋅01 to ⋅03 inch in greatest length, most of which seem to be coated externally by a reddish, film of iron peroxide. The augitic constituent appears, as at Coverack, to be partly normal augite, with some diallage, and partly enstatite. The crystals, however, are much smaller here; so that it is more difficult to determine them. The granular mineral, with polarized light, very closely resembles the pseudomorphic product after the felspars of the gabbros described above; and on close examination, some indications of an irregularly oblong crystal-like form may be detected. This rock, therefore, probably differs from the others in containing a small quantity of felspar; and I may state that it is the only case in which I have detected that mineral in the serpentines of the Lizard.

Lower Pradanack Quarry (no. 7).—The slide from this quarry, as might be expected from the general resemblance of the rock, corresponds very closely with that described above from Mullion Cove; the olivine is in much the same condition of conversion into serpentine. It contains also little dark brown and greenish brown semitransparent grains, which I feel certain are picotite. Here also a banded structure is indicated by a partial separation of the component minerals; enstatite, however, in this rock, rather predominates over the augitic constituent.

Rill Quarry (no. 4).—This slide shows a number of subangular transparent grains, generally edged with black, of clear, colourless, and rather fibrous crystals of longish oblong outline, and of scattered granules of magnetite, set in a very pale yellowish green serpentine, now and then stained brown. A few larger, irregular, imperfectly transparent granules seem to be picotite. On applying polarized light the subangular grains prove to be olivine, and the fibrous crystals enstatite (lengthened in the direction of the macrodiagonal). There is also a crystal about 0⋅2 inch long, of rather rhomboidal shape, which proves to be this mineral; it exhibits the usual