Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1034

904 have some resemblance to certain hypersthenes; but the mineral does not belong to the orthorhrombic system. There is a general banded structure shown in the arrangement of the minerals.

The second slide is cut from a pale-coloured, apparently imperfectly crystallized rock, consisting chiefly of two minerals—the principal a whitish or pale pinkish felspathic one, the other a pale green mineral. The former, under the microscope, when examined with polarized and analyzed light, assumes the characteristic granular aspect of the saussuritic pseudomorph after plagioclase. The other mineral appears to be an almost colourless augite, containing a large number of very minute microiiths. There are but slight indications of secondary hornblende. Some small roundish white specks, just visible to the eye, in the felspathic portion of the rock, appear, under the microscope, as rather oval blotches, often opaque and brownish, but in other cases showing aggregate polarization of a somewhat fibrous mineral, with colours from a rather orange yellow to a purplish blue, probably one of the zeolites. This rock also shows a banded structure.

On the eastern side of the headland we come at once on the serpentine as the predominant rock, though it is broken through by many veins of coarse gabbro. The mineral composition of these resembles that just described. Some are of considerable thickness; but others die away as mere strings. The material of the gabbro, therefore, must have been injected in a very fluid condition, and, as even the strings are coarsely crystalline, must have made its way into rock of high temperature, and have cooled down very slowly. The larger masses more frequently exhibit a schistose structure than these finer veins, though they are sometimes quite without it. Every stage may be noticed here as on the other side of the headland. Here, also, the "eyes" of hornblende are frequently seen in the schistose varieties.

Between the headland and Lankidden Cove are several gabbro veins. The serpentine is a red variety, much resembling one presently to be described. On approaching the Cove two or three narrow greenstone dykes are seen in the cliffs, and one in a skerry projecting from the sand. These cut both the serpentine and the gabbro, and closely resemble those already described near Kennack Cove. A section from one exhibits plagioclase felspar in fair preservation (the crystals commonly five or six times as long as broad, characteristically twinned, and mostly well defined), augite (often well preserved), and some magnetite. Besides this there is in the interval of the larger plagioclase crystals, a good deal of an aggregate of a fibrous transparent mineral, changing, with polarized light, from bright blue to yellowish or occasionally pinkish colours—doubtless a pseudomorph after felspar, perhaps replacing a magma. Here and there the augite changes to pale green rather fibrous hornblende, showing marked dichroism, and rather rich colours with analyzer and polarizer. There is also a brown dichroic mineral: some of this is probably little more than a ferruginous staining; but in one case it is certainly a distinct mineral, probably an iron-mica. There is no