WARREN. — ALKALI-GRANITES AND PORPHYRIES. 307 



or less broken up and scattered, itself becoming in consequence slightly 

 more acidic.^* 



At lower levels of the contact, such as those represented by the 

 area located in eastern and southern Quincy and northern Weymouth, 

 the magma first consolidated as the fine-granite against the slate. 

 Here again the contact phase was fractured and to some extent en- 

 gulfed in the underlying magma as shown by the included blocks, the 

 dikes in the fine-granite and the sharp contacts generally existing 

 between the two granites. 



The fine-granite and the granite-porphyry characteristic of the 

 chilled cover at lower and the other at higher levels are, so far as extent 

 and thickness are concerned, by far the most important marginal 

 phases of the batholith, and although they differ sharply as to texture 

 and to some extent in the character of their dark silicates, they are, 

 as we should expect, almost identical in chemical composition. They 

 are both slightly more basic than the average of the coarse-granite. 

 Their textures and the absence of differentiation products in them 

 suggest that, being more quickly cooled, they did not dift'erentiate but 

 solidified with substantially the composition of the magma as intruded, 

 and it is only beneath them, or at deeper contacts against the slates, 

 that the magma differentiated. 



At other relatively deep contacts, such as those represented by the 

 Pine Hill and the Pine Tree Brook areas, we find the first formed 

 secretion of the magma against the slate to be the rhombenporphyry, 

 indicative of more considerable differentiation. The rhomljenpor- 

 phyry, like the first formed rocks generally, was broken through by 

 the underlying magma and portions of it are found abundantly dis- 

 tributed through the associated granite and porphyry. The magma 

 on breaking through this early phase, in part consolidated as a thin 

 zone of coarse porphyry, in part as a porphyritic phase of the fine- 

 granite, or as fine-granite. All of these were again broken through 

 by the magma, as shown by the dikes which are so prominent a feature 

 of the Pine Tree Brook area. 



An objection might here be raised, that at the granite-slate contacts 



64 The aporhyolite was originally in large part glassy and has since suffered 

 devitrification and alteration so that its exact original chemical composition 

 is in doubt. As it stands, it appears to be on the average about as siliceous 

 as the granite, perhaps shghtly more so. This might be held to favor its being 

 of later origin. On the other hand it may well represent a relatively siliceous 

 and originally more aqueous upper portion of the invading magma doubtless 

 closely similar in this respect to much of the fine-grained feldspar-quartz- 

 porphja-ies of the higher elevations. 



