16 GRANITIC ROCKS OF CORNWALL. [Ch. II. 



proportion of its minerals, is generally associated with, and 

 gradually passes into, the different kinds of true granite. On 

 the other hand, it frequently, by the disappearance of the 

 felspar, becomes perfect shorl-rock, a crystalline compound 

 of quartz and shorl, which, like granite, exhibits many 

 varieties, according to the size or predominance of either of 

 its constituent parts. One form of this rock deserves very 

 particular notice, not only on account of its association with 

 its congeners, but also because it is of frequent occurrence 

 amongst all the other kinds of granitic rocks. It consists of 

 an intimate union of quartz and shorl, forming a compact 

 shorl-rock, which sometimes has the external appearance of a 

 hornblende-rock, but may be distinguished therefrom by its 

 great brittleness. In the compact shorl-rock the quartz 

 generally predominates, so much so that the shorl often 

 seems only to act the part of a colouring mineral ; and this 

 rock is often seen accompanying quartz in alternating stripes of 

 various sizes, from that of a line to a foot or more in thick- 

 ness. These are sometimes straight, and sometimes curved, or 

 even contorted, in as intricate figures as the agates ; and not 

 unfrequently the shorl, as the colouring ingredient, is so dis- 

 posed, both in rounded and angular forms, as to impart to 

 the mass the resemblance of a fragmentary rock. 



The granitic rocks of Cornwall also present other kinds : 

 the most important of these is that one which contains talc 

 instead of mica ; for its granitic variety is the source of the 

 china-stone and china-clay, more than twelve thousand tons of 

 which are annually exported for the use of the potteries. 



This rock is a species of protogine : its talc is commonly in 

 the state of small scales, exhibiting various tints of yellowish 

 green. It assumes most of the forms which are common to 

 the shorl-rock : all of them do not probably exist in Cornwall, 

 but they occur in countries where this rock is more exten- 

 sively developed. 



The most common kinds of Cornish protogine depend on 

 the size and proportion of their felspar, quartz, and talc : but 



