Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1050

920 dark ruddy-brown iron peroxide. There is a steatitic band containing enclosures of a doubly refracting variety, some of which, are more like the pseudo-organic forms of Messrs. King and Rowney than I have elsewhere seen, and many augitic crystals.

Serpentine from Quarry behind Coverack Village (no. 13).—This rock, as might be expected from its appearance, is, when examined microscopically, a peculiar one: rather clear strings of pale gold chrysotile, including not unfrequently small granules of magnetite, divide the field into a number of irregular grains; each of these is subdivided by a network of yellower strings of the same mineral; these are crowded with an exceedingly fine black dust composed of microliths of various forms, mostly rather granular; and the interspaces, often roughly quadrangular, are occupied by a colourless mineral also partly filled with dust, probably magnetite. This, commonly, is aggregated, as described above, on the outer edge of the interspace; also, by bisecting the string, it often shows the position of the original crack. There is no enstatite or diallage recognizable; but some fair-sized scales of a very dark brown and all but opaque mineral, whose cracks are filled by colourless serpentine, may have been some such mineral. With crossed prisms the larger strings vary from opaque to a pale milky grey; the smaller, commonly, have a distinct golden tinge, and the interspaces are dark or very pale dull milky grey.

Dyke of Serpentine, base of Cliffs north of Kynance (no. 3).—This may bo regarded as an exceptional condition of the serpentine of this district; it is very compact in structure, streaky, and red. On placing it under the microscope we find that the slide consists partly of roughly parallel wavy branching bands of nearly clear serpentine, with a number of roundish grains, something like the eyes in a piece of knotted wood, of a ruddy brown tint, inclining now and then to purple; there appears to be little or no unaltered olivine, but a fair sprinkling of magnetite. On examining it with the two Nicols, we find that the clearer serpentine is the feebly doubly refracting variety described above, and that in most cases the browner spots are slightly doubly refracting. Hardly any of the olivine has escaped alteration; but any one who has made himself familiar with the other slides will have no difficulty in recognizing the characteristic structure in the replacing serpentine. I havo no doubt the rock shows a true flow structure.

Specimen from Junction near George Cove (no. 6).—This is also an exceptional variety, characterized by its exceeding compactness and parallel bands of chrysotile. Under the microscope both this mineral and the serpentine appear nearly colourless; but the latter contains many streaks and fibre-like aggregates of magnetite, while the former has only occasional minute microliths included between the fibres of chrysotile, and so roughly at right angles to the sides of the bands. Some dark brown subtransluccnt grains are probably picotite. In one band of the slide these are rather abundant. The banded structure, very conspicuous throughout this specimen, is