Ciovelly, North Devon. 499 



destitute of metallic veins. It alternates with transition limestone, 

 and where it does so, occasionally contains organic remains. It alter- 

 nates also, in one instance at least, with large beds of a species of 

 culm. Its veins contain few or no extraneous substances besides 

 common quartz and, though less frequently, calcareous spar. * 



The latter (the Killas of Cornwall) has generally the silky charac- 

 ter ascribed to clay slate. f It is traversed almost throughout its 

 whole extent by frequent veins, or rather dykes, of a porphyritic 

 rock, which forms one variety of the Elvan of the Cornish miners, 

 and which does not occur either in the dunstone or shillat. It has 

 been found to contain at St. Agnes, the topaz, and in various places 

 the tourmalin and the garnet : the veins are frequently occupied by 

 chlorite, by mica, and by crystallized felspar. A considerable part of 

 the tin of Cornwall is obtained from the killas, and the grey ore of 

 cobalt has repeatedly been found in it. All these minerals have 

 usually been considered as characteristic of the earlier rocks, and 

 none of them are, I believe, stated by the writers of our most es- 

 teemed mineralogical systems to occur in that to which they assign 

 the name of grauwacke. ijl 



It is not without the greatest diffidence in my own knowledge of 

 the subject and powers of observation that I venture to dissent, even 

 on a mere point of nomenclature, from the opinion of my able and 

 experienced friend Dr. Berger, I cannot however but conceive that 



* In the neighbourhood of Bideford, a little to the west of Ilfracombe, the rock is 

 said to be much traversed by this mineral. 



+ May it not be added that its siliceous varieties are of a texture much finer and more 

 compact than that of Dunstone ? 



X The following substances also which appear (although less positively) to be restricted 

 to primitive formations by the school of Werner, occur in the killas; wolfram, n;itivc 

 silver, horn silver, schicfer spar, actinolite. I believe that the chiastolite has also been 

 found in it. 



3r 2 



