Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 4-5 



air, in consequence of the decomposition of iron pyrites. This 

 mineral not only coats the joints of this rock, but is also some- 

 times disseminated throughout its substance in small amyg- 

 daloidal granules. 



Each of the preceding schistose rocks, but more particu- 

 larly the greenstone, gradually passes into thick lamellar 

 slates, which exhibit various shades of blue and green, are 

 soft, and abound in metalliferous veins, particularly of copper, 

 the matrix of which is composed of quartz and dark green 

 crystalline chlorite : they also contain layers or beds, which 

 are compact and earthy, differing only from the slate in 

 wanting the fissile structure. 



Thus we find, that the rocks of the schistose group imme- 

 diately surrounding the granite of Cornwall, may be referred 

 to five distinct genera, viz. cornubianite, proteolite, green- 

 stone, actynolite-rock, and chlorite-rock. To these it is pro- 

 posed to subjoin a description of the magnesian rocks, the 

 true position of which has perhaps not been decidedly ascer- 

 tained, though it appears in Cornwall to be in the lowest part 

 of the calcareous group, the upper part of which contains 

 organic remains, and therefore it may possibly belong to the 

 intermediate or transition class. 



The magnesian rocks of this county may be divided . into 

 three genera : diallage-rock or euphotide, serpentine, and 

 talc-schist ; the last is generally received as a distinct rock, 

 and, indeed, in other countries, it is extended over large 

 tracts, with different associations ; otherwise it might in 

 Cornwall be regarded merely as a schistose species of ser- 

 pentine. 



The euphotide consists of felspar and diallage, both of 

 which are very crystalline, and are generally very distinct, in 

 the form of various sized crystals aggregated together, and 

 mutually penetrating each other, after the manner of granite : 

 its felspar does not appear to be identical with that of granite, 

 which may be owing to the presence of magnesia, just as 



