Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/307

 alternated, and that quartz rock was, as it has too often been considered, a modification of granite. This supposition I have elsewhere shewn to be unfounded, and that this rock is a member of those schistose formations which are found reposing on granite. I shall prove hereafter, that in this instance also the apparent alternation of the quartz rock is a deception, and that it is, like the limestone and the schist with which it really does alternate, superimposed on the granite.

I have in former papers presented to the Society, shown that the polarity of granite, or its power of affecting the magnet, is by no means so rare an occurrence as it was supposed. I may add to these observations, that almost all the granite found here possesses the same properties. But as this is a question involving some important considerations which would interrupt the chain of the present investigation, I shall consider it in a separate paragraph at the end of this paper.

As I reserved the mineralogical description of the granite which is found in the bed of the river in contact with the schistose rocks, till I had described the larger masses with which it is connected, I shall now describe it, as well as such other varieties as appear worthy of particular notice. I may here also say, that if a term be required for the purpose of distinguishing it from that granite which does not contain hornblende, it may be called syenitic granite; in treating however of its geological relations, I do not think it necessary to adopt so long a term, which it will be sufficient to use whenever it is requisite to distinguish its peculiar character as a compound rock. One advantage to be gained by the use of such a term will be, that the term granite will thus become a sort of generic term, including species, or varieties, which differ from the compound of mica, quartz, and felspar, and of which there are