300 PROCEEDINGS OF THE AMERICAN ACADEMY. 



intrusion, the space now occupied by the alkaUne rocks was formerly 

 filled b;s^ Cambrian and pre-Caml)rian sediments, and probablx- also 

 in part by an older biotite granite intrusive into at least a part of these. 

 Whatever the exact manner in which these older rocks were replaced 

 by the batholith ^^ these certainly had no appreciable effect on the 

 chemical composition of the batholith as now exposed by erosion 

 and quarrying. The Cambrian sediments now extant in contact 

 with the batholith or in its neighborhood, nor the pre-Cambrian rocks 

 existing in nearby areas, could not, by any process of assimilation, 

 produce the present highly specialized chemical character of the 

 batholith imless such assimilation were accompanied by a drastic 

 differentiation which it is quite certain has not taken place in the 

 batholith as exposed. That the magma of the present batholith was 

 originally produced at greater depths by the differentiation of a magma 

 which had during some past epoch assimilated older rocks, is not 

 denied, indeed, the writer believes that such a process did take place. 

 The assimilation of any noteworthy quantity of sediments and subse- 

 quent differentiation demands an enormous amount of heat, or a long 

 continued period of sufficient heat to ensure the necessary mobility, 

 which does not appear to have been the case in this magma. This is 

 shown by the lack of extreme metamorphism of the Cambrian slates 

 found at the contacts, even of the coarse granite on Nortli (,'ommon 

 Hill, Quincy, which must represent relatively deep portions of the 

 original contact, as has already been pointed out. There is for example 

 much less metamorphism in these sediments than that described by 

 V. M. Goklsmidt ^^ for the sediments in contact with, or included in. 



at least in Essex Co., by acid alkaline intrusive and effusive rocks which either 

 lie upon an eroded surface of the older granite, etc. or cut them. The l)ulk 

 of the evidence referred to is contained in the unpublished thesis of Dr. C. H. 

 Clapp (Thesis for the degree of Doctor of Philosophy in Geology, C. H. Clapp, 

 Mass. Inst, of Technology, Boston, Mass., 1911.) An abstract of this thesis 

 has been published setting forth the main results and a paper will appear later 

 as a bulletin of the U. S. Geological Survey. The facts regarding tlie alkaline 

 granite in Rhode Island will be published by the writer and Mr. Sidney Powers 

 in a future pa]ier now in preparation. 



55 AlUiough the mass of alkaline rocks exposed is not large, perhaps not large 

 enough to warrant the use of the word batholith according to the ideas of some 

 geologists, the writer has used it as a convenient term. In all probability 

 the mass is connected with a much greater mass of alkaline rock beneath, which 

 has sent up protuberances as it were, of which the Quincy-Blue Hill mass is 

 one, the Rockport, Mass. Granite another, and those near Diamond Hill, 

 Rhode Island another. There is no evidence whatever of anything in the 

 nature of a laccolith about the Quincy-Blue Hill mass. 



56 Videnskapsselskapeta Schrifter, 1, Mat. Naturv. Klasse, No. 1 (1911). 



