316 PROCEEDINGS OF THE AMERICAN ACADEMY. 



ture was due to their more basic (than the granite) character, or to a 

 tendency of the earher formed minerals to form smaller crystals.''^ 

 The same objection raised above, that the feldspar phenocrysts are 

 not recrystallized, would operate strongly against this view, nor is 

 there any evidence that because of a more basic or different chemical 

 composition there should result such a texture as that described: 

 nor is it the earlier formed minerals that are the smaller in size, but 

 exactly the reverse. In this whole group of xenoliths (those with a 

 marked porphyritic texture) there is little, if any, evidence that there 

 has been any addition of material from the enclosing magma, and 

 little, that there has even been any noteworthy recrystallization of 

 the xenolith since its first formation. 



In the case of the feebly or non-porphyritic xenoliths, particularly 

 those of finely granitic texture, there is less positive evidence as to 

 their origin. It is true that many of them contain jihenocrysts of 

 microperthite which show traces of an earlier formed core. But this 

 is, in character, like the margin. It may be that this type represents 

 spots in the granite which for some unknown reason developed a 

 finer texture. But why they should have sharp contacts or why, in 

 the case of those so near the granite in chemical composition as to 

 possess, at best, but very slight differences effective in modifying 

 the texture, they should show a contrasted texture, is not at all clear. 

 It has been pointed out that, allowing for possible changes in chemical 

 composition effected perhaps by the enclosing granite magma, the 

 xenoliths as a whole correspond rather closely, and in part almost 

 exactly, to the various types of rock found as peripheral phases of 

 the batholith. For reasons above set forth, the writer believes that 

 the porphyritic xenoliths are simply fragments of the peripheral phases 

 of the batholith, broken up and more or less moulded in form by 

 movements in the hot magma, and that these fragments have in part 

 sunk in the magma or been moved from their original positions by the 

 same movements. He is also strongly inclined to believe that all of 

 the patches of contrasted texture are the result of the same process. 



The above statements regarding the origin of the xenoliths appear 

 to be in substantial accord with the views of Professor Crosby. In- 

 asmuch as he relied on Dr. White's descriptions of their microscopic 

 characters some of his statements regarding them differ materially 

 from the writer's. We are, however, agreed that the porphyritic 

 texture is a plain indication that the crystallization followed, and did 



75 See Harker, op. cit., p. 348. 



