131 CHAP. X. On the Characters and Disposition of unstratified Iioc/cs and th ins. The unstratified rocks are far less numerous than the stratified, and they have all been comprised, in the present arrangement, under two general heads. Granite forms one of these; and, to the other, the term Trap has been applied; with some latitude, it must be owned, but to avoid the necessity of invent ing a new one. Under these two heads, in the twenty- fourth and thirty-ninth chapters, and in the Classifica tion of Rocks, will be found all the minuter particulars which it was not necessary to introduce into this very general view. O Of the Extent and Places of the unstratified Rocks. Though the unstratified rocks form but a small part of those which are visible to us, there is reason to imagine, that at depths to which we have not pene trated, they occupy a much greater extent than would be suspected on a superficial view. Conjectures have been offered respecting the proportion of space which granite fills at the surface; but no measurement has been given; nor has any thing been done towards answering this cjuestion as it relates to the later unstra tified substances. In fact, these vary so much in different countries, that if any worthy object were to be gained by that knowledge, it could still only be done after the whole surface of the globe should have K 2