Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/487

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Dr. Mac Culloch on Quartz Rock.

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 remarkable 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 conclusions from which its place in the system may be assigned, however in different situations its aspect and composition may vary.

It is often composed of mere grains of quartz, of an aspect extremely 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 contains 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 whole 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 fragments 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 mechanical and of a crystallized deposit, a mixture of character, which,