CLASSIFICATION OF THE ERUPTIVE 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 geologi- 

 cal 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 crys- 

 tallization 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 plutonic — the former eruptive, the latter non-eruptive. I recognize, 

 however, that those who are disposed to regard the porphyries as coordi- 

 nate 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 superfi- 

 cial 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 



