Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/409

From Wikisource
Jump to navigation Jump to search
This page has been validated.
399
various parts of Scotland.

that the habits of this rock, and its connections, in all the cases where I have had an opportunity of seeing it, seem to point it out, as an argillaceous schist, altered and indurated in consequence of its proximity to greenstone.

The last bed is the immense mass of columnar trap already mentioned, which surmounts the whole. The character of this rock is not that of a greenstone, to use the word in its more restrained sense, as it does not contain distinct grains of hornblende and felspar, but is an obscure mixture of hornblende, felspar, and quartz.

I have therefore both here and in other places, (where I have had occasion to describe the varieties of this rock, of which the characters are not so decided as to admit of their being placed either under the heads of basalt, wacke, or greenstone,) preferred the general name of trap to the use of a specific one with which the characters of the individual cannot be made to agree. In so doing I have left out of view the confusion which must arise if the siliceous schist, and Lydian stone, which appear to be the trap of some authors, be considered as forming a part of this great natural division, and have taken the term, as I imagine, in the sense intended by the followers of Werner. It is thus used by them as the title of a family including basalt, wacke, greenstone, and clinkstone, a family well expressed by the vernacular term whin, now, I believe, and perhaps unfortunately, obsolete. The characters of basalt, wacke, greenstone, and clinkstone, are tolerably decided, and the geological relations of the trap family are so remarkable that we ought to find little difficulty in assigning proper terms of distinction to its respective members. But, independently of these four well characterized rocks, innumerable varieties occur not properly referable to either, and which cannot even be truly expressed without much circumlo-