Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/92

82 as an accidentally connecting medium of the former, than as an essentially constituent part of the whole rock. The crystals of the hornblende, though generally small, sometimes exceed two or three inches in length; in which instances they are not of a proportional breadth: in general also they are closely compacted with the body of the rock; but occasionally, and especially when larger than usual, they are easily separable from the mass, leaving a smooth impression of their surfaces. These impressions, as well as the crystals themselves, have commonly a dull iridescent semi-metallic lustre; arising, perhaps, from an increased oxidation of the iron of the hornblende, which by loosening the attachment of the crystals to the mass in which they are imbedded, has disposed the compound to assume that regularity in its fracture. A similar appearance often presents itself in parts of the Malvern rock; and it is probable that the kind of lustre here noticed is very characteristic of peculiar states of hornblende, and may serve to ascertain its presence in a compound rock where no traces of its crystalline form are evident.

The hornblende of the summit of this hill is indistinctly crystallized, and of a dark and dull olive green colour; generally very uniform in its character; and so closely compacted with the felspar that the fracture passes indiscriminately through both. The rock itself is remarkably hard, and has that degree of toughness which is characteristic of the class of rocks called by Wallerius Saxa Cornea, and Corneus Trapezius; which rocks, as may be collected from the volcanic dissertations of Dolomieu and Ferrara, contain hornblende as a principally constituent part. It occasionally contains particles of pyrites; and insensibly passes into a greyish green coarse and soft slate, which in