Dr. Mac Culloch on Qiiartz Rock. 477 



the vertical position, and that this angle is similar to that held by 

 the schistus with which it is accompanied. 



The composition of this rock offers through different parts of its 

 mass different varieties, and it will not be useless to enumerate the 

 most leading ones, merely for the purpose of proving in this, as in 

 'some of the other instances described, that many of the most re- 

 markable mineralogical varieties exist under the same geological 

 position. Thus the identity of the rocks as an order or division is 

 established, and thus we may legitimately deduce those general con- 

 clusions from which its place in the system may be assigned, how- 

 ever in different situations its aspect and composition may vary. 



It is often composed of mere grains of quartz, of an aspect ex^ 

 tremely various, sometimes highly crystalline, though never defined' 

 by geometrical forms; sometimes shapeless and opaque. These are 

 more or less strongly agglutinated, and are occasionally compacted to 

 such a degree that the granular appearance is nearly lost. It con- 

 tains now and then grains of felspar, imbedded in a compact quartzy 

 basis, thus bearing a general resemblance to porphyry. These 

 sometimes appear to be crystallized in an obscure manner, but more 

 frequently they exhibit no regular figure. At times the felspar and 

 quartz are so nearly equal in quantity, that the w^hole forms a pink- 

 coloured granular mass, often so loose as to crumble in the hand, 

 and in this case the grains seldom bear any mark of crystallization^ 

 but are rounded as if they had undergone attrition previously to 

 their aggregation : occasionally, but rarely, it contains imbedded frag- 

 ments of quartz. It is not often traversed by quartz veins, but 

 wherever these occur they are much confounded at their edges with 

 the mass of the stone. Here then, as in other instances^ we see that 

 it exhibits the ambiguous or rather double appearance of a mecha- 

 nical and of a crystallized deposit, a mixture of character, which,, 



