18 GRANITIC ROCKS OF CORNWALL. [Ch. II. 



as in the case of eurite. Of these, the porphyritic variety, 

 arising from the presence of crystals of felspar, is of common 

 occurrence, and very characteristic of this rock : it is the 

 felspar porphyry of Macculloch and other geologists ; but, as 

 in this work the word porphyry is not used in a generic but 

 a specific sense, it will be called porphyritic felsparite. This 

 series of rocks not only passes at its centre into a fine-grained 

 granite, like eurite, but also into a large-grained, and even a 

 prophyritic granite. But the most interesting, and indeed 

 the most conspicuous, circumstance to be noted concerning 

 felsparite is the constant change in the nature of its basis, 

 caused by the fluctuations in the proportions of the felspar 

 and quartz which enter into its composition ; for it assumes 

 different characters according as one or other of these 

 minerals predominates, exhibiting all the intermediate shades 

 between a crystalline compact felspar, jasper, hornstone, iron- 

 flint, and even quartz itself. 



The description of the various kinds of Cornish granite 

 must not be terminated without a few remarks on those 

 varieties which are very quartzose, passing, indeed, very com- 

 monly into perfect quartz. These are confined, in a great 

 measure, to the immediate vicinity of quartz-veins, which 

 are sometimes metalliferous ; and, in these instances, if the 

 vein be of moderate dimensions, they would attract little 

 attention ; but when the veins are of a large size, or when an 

 entire layer of granite is quartzose, then they would be pro- 

 nounced by most geologists to be kinds of quartz-rock. 

 When these quartzose varieties have been observed in other 

 granitic countries, they have been regarded as superficial 

 remains of strata which once covered the granite : but, as they 

 are clearly not of this nature in Cornwall, it is desirable to 

 bring them into one point of view, in order to justify our 

 considering them, not as stratified rocks, but merely as 

 varieties of granite. 



In the case of common granite, the mass sometimes be- 

 comes so exceedingly quartzose, at the same time retaining its 



