230 PROCEEDINGS OF THE AMERICAN ACADEMY. 



with the microscopic evidence the chemical analyses show conclu- 

 sively that there is a small, but noteworthy variation in composition 

 in the rock from different points, though such variations seem to be 

 vicarious in character. 



The granite from Rattlesnake Hill is a little lower in silica and higher 

 in alkalies, thus leaning toward the granite-porphyry and fine-granite, 

 (see beyond Nos. 10 and 13) but in the ferrous and ferric iron it is like 

 the normal granite. It thus stands in an intermediate position 

 chemically l)etween the normal granite an<l its peripheral phases as it 

 does texturally and in structural position. 



The granite, No. 9, from the Stony Brook Reservation, West Rox- 

 bury, Mass., only a few miles distant (N. W.) from the area under 

 discussion and described by Professor Bascom, differs from the 

 Quincy granite chemically in higher lime, also in having lower silica; 

 soda predominates over potash and there is much less total iron. 

 Mineralogically the two are strongly contrasted, the Quincy is a mi- 

 croperthite granite, the Stony Brook granite, like an enormous pre- 

 ponderance of the granite of Eastern New England, is a two feldspar 

 granite with a strong leaning toward monzonitic types. The former 

 is characterized by soda-iron hornblendes and pyroxenes and never 

 shows epidotic alteration; the latter is characteristically biotitic, 

 perhaps carries an arfvedsonitic amphibole, and is epidotic. Though 

 the Stony Brook granite is perhaps a nearer relative to the Quincy 

 than some of the biotite granite of the Atlantic seaboard, they are still, 

 mineralogically and chemically, sharply contrasted types. 



Fine Granite. 



It should be noted at the outset in describing this rock that the 

 term "fine-granite" is here used in a more restricted sense than that 

 in which it has been used by Professor Crosby. He used it to include 

 a part of what is here termed "granite-porphyry" and he has also not 

 distinguished between the fine-granite of the alkaline type and that 

 associated in Weymouth with the biotite, subalkaline granite. Pro- 

 fessor Crosby held that the fine-granite of the Blue Hills was an inter- 

 mediate textural phase between the c^uartz or granite-porphyry and 

 the coarser granite. With the possible exceptions of one or two 

 points, there is no rock that can be properly termed fine-granite, having 

 such relations. The phase of the granite-porphyry as the granite is 

 approached is not readily recognized as a porphyry megascopically. 



