Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/497

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487
Dr. Mac Culloch on Quartz Rock.

ancient division of primary and secondary, as far at least as relates to those rocks which bear marks, however obscure, of stratification. It will be a separate consideration by what means we can divide those of the former class which contain organic remains, from those which are without them; a distinction, perhaps not less requisite in the one than in the other of these two leading classes, since, however many members of the flœtz or secondary division may be characterized by the existence of these remains, a certain set are as invariably free from them.

It is perhaps beyond the bounds of the present notice, to suggest the propriety of separating the unstratified rocks, such as granite, porphyry, and trap, from this two-fold arrangement. Should such a measure be ultimately adopted, it will be further necessary to consider how far any one of the several unstratified rocks is peculiar to the one set of stratified ones, and how far it is common to both. A more accurate knowledge of the various rocks designated under the loose name of porphyry, will be particularly requisite to the forming of this arrangement.