Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/124

 112 impression of organic bodies, and I am not aware that it has even 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 parallelepipeds: it is harder, not so fine in the grain and of a darker colour than the other variety. This last quality depends, no doubt, on its containing a greater proportion of iron. I believe that common grauwacke is a cornéenne 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 rock, it is very frequently accompanied by veins or beds of quartz, which is also often found in detached pieces or blocks. It is much less rich in ores than the grauwacke slate.

The structure of grauwacke slate is schistose, and the laminæ become thinner, as they are further removed from the junction with the granite. The base is exceedingly fine, 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 of when it is exposed to the air. It is to this variety exclusively that the Cornish