Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1038

908 (sometimes rendering nearly the whole opaque), or clouded granules disseminated in various ways. At last when the whole grain is converted into serpentine, the black strings marking the original cracks become less definite, being interrupted and disturbed, probably from the action of molecular forces, so that their significance might readily be overlooked. The chrysotile strings also become less conspicuous; but still they may often be traced when the prisms are crossed. It is worth noting that, as a rule, the formation of chrysotile appears to proceed from the surface of a crack inwards, and is generally arrested at a fairly constant distance from the outside; but the conversion of the remainder of the grain into non-crystalline serpentine appears to be nearly simultaneous over the whole. The process very closely corresponds with that which I have already described in the Ariége lherzolite (see fig. 9, p. 916), except that here, as the olivine is apparently ferruginous, there is a more marked discoloration. The augitic constituent is less abundant than the other two minerals. Most of it is diallage; but grains, and sometimes even parts of a crystal, seem rather to be normal augite. Occasionally the serpentine appears to have been deposited in a crack in them: but as a rule there is not the slightest advance towards a conversion of these minerals into serpentine.

The diallage occasionally assumes a dusky, finely granulated, stained aspect along the principal cleavage-planes, which I have observed in cases where a change to hornblende is commencing; and one crystal is bordered by an aggregate of minute crystals, which I fully believe to be actinolite. There are few included microliths, excluding these ferruginous clots; but there seem to be one or two grains of magnetite, and in one of the diallage crystals are two needles which I think are apatite.

Slide II. (cut from a rather duller-coloured specimen) does not materially differ, except that the iron stains in the olivine are more uniformly dark. Some of the diallage (which mineral is fairly abundant in this specimen) contains small opaque belonitic microliths lying parallel to the plane of principal cleavage.

In slide III. the felspar decidedly predominates over the other minerals, the olivine coming next. Some of the grains here are wholly converted into serpentine; one shows, with polarized light, a single speck of the original mineral alone remaining in a good-sized grain. There are both diallage and augite; and here, as in the other slides, there are appearances of a passage from one variety to the other. One crystal shows in parts the ordinary imperfect cleavages of augite parallel to ∞P, with an occasionally marked clino-diagonal cleavage; then, in places, the last cleavage dominates over the other, forming very definite parallel lines of weak cohesion; and, lastly, the inner (and major) portion of the crystal is filled by a fine parallel cleavage, giving it an almost granular-linear aspect, in which, I think, occasional cracks indicate traces of a cleavage parallel to ∞P. The absorptive powers of the two minerals seem to differ. By rotating