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Show CLASSIFICATION OF THE BEUPTIVE EOCKS. 99 rence of the rock masses. The true porphyries are eminently intrusive rocks. Into the detailed classification of the granitoid or wholly crystalline rocks it is not intended to enter. It will suffice to say that they have been regarded by almost all geologists and petrographers as separated from the volcanics by wide barriers, resting upon wide differences in their geological relations, in their modes of occurrence, their genesis, and geological history. I have endeavored to show that the distinction is well founded. It seems right that they should be placed in different classes, not because the mere lithological fact that they differ in respect to their degrees of crystallization is such a great thing in itself, but rather because it implies a totally distinct category of relations. Whether a third class should be admitted, viz, the porphyritic rocks, is not so clear. For my own part, I incline to the admission of only two classes of igneous rocks, the volcanic and plutonicâ€"the former eruptive, the latter non-eruptive. I recognize, however, that those who are disposed to regard the porphyries as coordinate in value with the granitoids or eruptives, may have much to say in support of their tenets. Passing now to the consideration of the volcanic rocks as a class, the principles upon which it is believed they ought to be subdivided have, in general terms, already been indicated. We ought not to endeavor to take account of anything more than their chemical and physical properties, since we should otherwise run the risk of serious error. And it has been pointed out that a decided correlation exists among these properties; so that if we take a rational system, based upon one set of properties, we shall at the same time express the other properties. The broader basis I believe to be the chemical one, and I regard it also as the most convenient. It has long been recognized that lavas are easily distinguished into two principal groups, contrasting with each other not only in the superficial aspects and in the minerals they contain, but also in their composition. One of these groups was ordinarily a coarse-grained, light-colored rock, of rather low specific gravity. It contained crystals of monoclinic feldspar, sometimes abundant free quartz, and also hornblende and mica. The other group was usually fine-grained, compact, very dark colored, and very |