112 Dr. B B R G E R o« //><? physical Structure 



impression of organic bodies, and I am not aware that it has ever 

 been found to contain them. It is divided into two species, common 

 grauwacke, and grauwacke slate. 



The structure of the first is compact, dividing by natural joints into 

 rhomboids or parallelipipeds : it is harder, not so fine in the grain 

 and of a darker colour than the other variety. This last quality de- 

 pends, no doubt, on its containing a greater proportion of iron. 1 

 believe that common grauwacke is a corneenne trap of the French 

 mineralogists. In Cornwall it is always found higher than the 

 grauwacke slate : it may be supposed to have been precipitated more 

 slowly, and under a less powerful pressure, whereby the mass has 

 been allowed to contract, and assume a kind of crystallisation. It 

 rests immediately upon the granite,* and, at its junction with that 

 fock, it is very frequently accompanied by veins or beds of quartz, 

 which is also often found in detached pieces or blocks, j" It is much 

 less rich in ores than the grauwacke slate. 



The structure of grauwacke slate is schistose, and the lamlnse be- 

 come thinner, as they are further removed from the junction with 

 the granite. The base is exceedingly fins, smooth to the touch 

 without being unctuous; the colour of the mass varies from dark grey 

 to white; its lustre is silvery, sometimes that of satin, especially when 

 the fractures are fresh, but that fine lustre soon goes off when it is 

 exposed to the air. It is to this variety exclusively that the Cornish 



* The rock which Ramond calls Corneenne^ and describes as interposed between the 

 fundamental granite of the chnin, the primitive limestone and the porphyritic rooks, in 

 the Pyrenees, is very likely grauwacke. Voyages au Mont Perdu, p. 4, 25, 206, 265, &c. 



+ It is probably also found in the mountains of Wales, according to the description 

 given by Arthur Aikin — " All the mountains from Bala to Aberystwith are primitive 

 " schistus, sometimes intersected by large'veins of -quartz, and of a coarse texture, somc- 

 " times forming slate." Journal of a Tour through North Wales, p. 42. 



