82 Dr. KiDt) on the Mineralogy of St. DavtcTs, 



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 them- 

 selves, have commonly a dull iridescent semi-metallic lustre ; arising, 

 perhaps, from an increased oxydation of the iron of the hornblende, 

 which by loosening the attachment of the cr^'-stals 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 horn- 

 blende, and may serve to ascertain its presence in a compound rock 

 where no traces of its crystalline form are evident, 



Carnllidy, 



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 re- 

 markably hard, and has that degree of toughness which is character- 

 istic of the class of rocks called by WalleriusSaxa Cornea, andCorneus 

 Trapezius ; which rocks, as may be collected from the volcanic disser- 

 tations 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 



