20 GRANITIC ROCKS OF CORNWALL. [Ch. II. 



and capable of being wrought with a chisel, as shown by its 

 having been found to form the arches of the old chapel of 

 Penzance, when it was lately pulled down ; and the same 

 stone may be also seen in the mother church at Madron, 

 which is much more ancient. It is, therefore, very probable 

 (since the granite, a few miles north of Penzance, contains 

 protogine), that this talcose quartz-rock was obtained from 

 erratic blocks, which have long since been consumed for 

 building purposes. 



The quartzose varieties of felsparite and eurite have been 

 already noticed, as arising from the gradual predominance of 

 silica over their felspathic ingredient; giving rise to horn- 

 stone, iron-flint, and many kinds of compact quartz-rock, 

 known to the miner under the name of capel. This subject 

 will be again reverted to, when it will be seen, that these, 

 and other binary compounds, are of sufficient importance to 

 require distinct appellations : for the present, we shall only 

 farther observe, that, besides the evidence afforded by the 

 cliff-sections, mining operations have also shown, that these 

 quartzose compounds are not superficial, but are often per- 

 sistent to great depths. 



Thus we learn, that the granitic masses of Cornwall are 

 not, as some have supposed them to be, composed of one kind 

 of rock, uniform in its constitution, and uninteresting in its 

 varieties, but are as complex in their composition as the 

 stratified rocks; and, in their mineral transition into each 

 other, afford as curious and instructive subjects for investi- 

 gation : on this account, the utility of mineralogical dis- 

 tinctions is obvious; and it will be still more apparent, 

 when we consider these granitic rocks hereafter, not only 

 in their relations towards each other, but also towards the 

 crystalline schists with which they are associated. 



The granitic rocks are variously arranged among them- 

 selves. Sometimes each kind occupies a considerable space ; 

 or they alternate together in smaller masses, which are dis- 

 posed in distinct and regular beds, highly inclined, and main- 



