GRANITE IN THE WEST OF ENGLAND. 229 



to be affirmed, because the dikes are there often unaccompanied by 

 dislocation. 



These instances are sufficient to show the truth of two proposi 

 tions of general application to this subject. 



When igneous rocks accompany convulsions, we can always 

 fix the minimum of their geological antiquity ; when they throw 

 off veins or intrude in the shape of dykes, or interpolated beds, 

 among stratified rocks, we are able to assign the maximum of their 

 antiquity. 



Guided by these views, and restricting our illustrations as much 

 as possible to the British Isles, we proceed to describe some details 

 of the characteristic phenomena of plutonic rocks, and to fix the eras 

 of their production. 



In referring to the works of authors who have written on granite, 

 we have sometimes followed them in using the terms eruptive granite 

 and metamorphic granite, because these terms conveniently express 

 differences in the relations of granite to the associated rocks ; and not 

 as implying that the origin of granite may be different at different 

 points in the same chain ; for we have already expressed an opinion 

 that most eruptive granites are only^ metamorphic granites which 

 have been injected into fissures or otherwise displaced by faulting 

 from the positions in which granitic structure was first developed. 



South-West of England. 



Granite of Cornwall. The granite of Cornwall, as described by 

 Mr. John Arthur Phillips, F.R.S., is usually coarse-grained, and in 

 addition to quartz, felspar, and mica, almost invariably contains schorl. 

 Sometimes the mica is partly replaced by a talc-like mineral, and the 

 granite thus becomes a kind of protogine. This rock is cited by Dr. 

 Boase 1 as occurring a few miles north of Penzance, but its existence is 

 denied by Dr. Haughton.* The mica frequently becomes almost re- 

 placed by schorl, and in many places the felspar disappears also. The 

 felspar is orthoclase, sometimes mixed with albite. The black or dark 

 brown mica is either muscovite or lepidomelane, and the pearly white 

 or pink mica is lepidolite. The quartz is usually granular, but some- 

 times in distinct crystals. It is transparent or white, occasionally 

 bluish or grey, and abounds in microscopic cavities, which may be 

 either partly full or empty. 



Mr. Phillips has found that at a temperature of 185 centigrade 

 some of the bubbles in the quartz greatly diminish in size, and infers 

 that they disappear at very varying temperatures ; and since many of 

 the cavities appear to be full, it may be concluded that the depth at 

 which the rock consolidated, as inferred by the method of Dr. Sorby, 

 is open to considerable doubt. 2 



Schorl Granite of the West of England. There are six principal 

 masses of granite in Devon and Cornwall, including the Scilly Isles, 



1 "Treatise on Primary Geology," 1834, p. 20. 



2 Q. J. G. S., vol. xxxi. p. 330, J. A. Phillips. 



