Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 53 



gular crystals of felspar may be observed here and there, but 

 they are of rare occurrence : its junction with the slate is not 

 exposed. 



At Kernick, about three miles from Bodmin, there are 

 several beds of granitic rocks: the mica is in larger scales 

 than in the last, and small portions of hornblende are dis- 

 tinctly developed ; and here also their junction with the slate 

 on either side is concealed, so that their size and bearings 

 cannot be ascertained. 



About a mile and a half from Bodmin, on the road to 

 Truro, an interesting kind of eurite occurs, which we have 

 termed porcelainous eurite, as indicating its nature. It is of 

 a pale greenish yellow colour, rather uniform and compact, 

 but containing grains of limpid quartz : it decomposes to a 

 considerable extent, and then resembles china-clay, which has 

 been already described as a disintegrated protogine, a rock 

 distinguished from granite by containing talc instead of mica, 

 and which abounds in the mass of granite about three 

 miles off. If this bed of porcelainous eurite were well ex- 

 posed, its constituent minerals would probably be distinctly 

 exhibited, as is generally the case in the middle of these 

 elvan-courses, where the rock becomes porphyritic and even 

 granitic. 



Granular and compact shorl rocks also occur in this part 

 of Cornwall imbedded in proteolite, and although at first 

 sight these rocks appear to have a very dissimilar composition, 

 yet a careful examination will show that they are nearly 

 related. The proteolite is frequently coloured by or even 

 intermixed with shorl, in various states ; and when the compact 

 felspar basis becomes siliceous, the shorl is present in greater 

 abundance, till at last it graduates into compact shorl-rock, 

 and the constituents becoming distinctly crystallised, die 

 granular shorl-rock is produced. The celebrated Roach rock 

 is composed of the last-mentioned variety : it rises out of the 

 slate in the form of an immense tor, about a mile distant 

 from the granite. 



E 3 



