Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1025

Rh a pale translucent bluish grey. Now and then there is a portion still showing the colour and striping of plagioclase felspar. This alteration is very common in the gabbros of this district. Some from Coverack Cove, which I shall presently describe, show it in an early stage; some from Karak Clews in a later; a specimen also from this very headland, cut to shew the junction of the gabbro with the hornblende schist, still retains its plagioclase in many parts unaltered. A similar alteration has taken place in a gabbro which I have collected from Mont Colon, in the Pennine Alps; and I have observed it in not a few other cases. I regard the mineral therefore as a kind of pseudomorph, the result of the alteration of labradorite or some plagioclase felspar. It is the mineral often called saussurite, and is quite as hard as, sometimes a little harder than ordinary felspar.

The other mineral is sometimes diallage, but in others a rather dark green mineral, something resembling chlorite at first sight; microscopic examination proves this to be hornblende. The larger patches are found to be composed of irregular aggregates of small prismatic crystals and grains (or possibly occasionally folia) of that mineral; these are generally pale green in colour, fairly dichroic, changing from a strong dull green to a sort of straw-green, and now and then showing very distinctly the characteristic cleavage along ∞P (fig. 3). On examining a series of specimens, both macroscopically and microscopically, this change, which we shall find to be very common in these Cornish gabbros, is seen to take place as follows:—The aggregated hornblende crystals form as a kind of border to the diallage (fig. 7, p. 912), when the latter generally becomes rather opaque under the microscope and loses its brilliant colours with polarized light, and its metallic lustre with reflected light, assuming a greenish colour and silky aspect. Small crystals of hornblende also appear here and there in the body of the crystal, inserting themselves, as it were, between