F. H. Lahee — Metamorjphism and Geological Structure. 463 



(1). The Boston Neck granite is genetically related to the 

 pegmatites, for (a) both contain essentially the same minerals ; 

 (b) both phases may alternate, schlieren-like, in the same dike ; 

 and (c) the granite may be seen to grade into the pegmatite. 



(2). The pegmatites and the quartz veins are genetically 

 related, for the pegmatites, by constant increase in their rela- 

 tive content of quartz and decrease in their relative content of 

 feldspar and other minerals, grade into true quartz veins. 



(3). The Acid Intrusive Series (Boston Neck granite, peg- 

 matites, and quartz veins) is intrusive into the Carboniferous 

 sediments of the Basin, for (a) it cuts the sediments in dike 

 form ; (b) it contains inclusions of the sediments ; (c) it shows 

 endom orphic effects in contact with the sediments ; and (d) 

 the sediments display exomorphic alteration near the intrusive 

 rocks. 



(4). Of the several members of the Acid Intrusive Series, 

 (a) the residual pegmatite represents a somewhat later stage of 

 crystallization than the main body of the granite ; (b) the mar- 

 ginal pegmatite probably crystallized about the same time as 

 the contact portions of the granite body ; (c) the offshoots from 

 the marginal pegmatite zone, and the quartz veins, represent a 

 later stage of crystallization than do the marginal pegmatites 

 proper. 



Hence, the younger members of the series are more acid 

 than the older members. 



(5). The Acid Intrusive Series is distributed as follows : 

 (a) the granite occurs only southwest and west of the south- 

 western part of the Narragansett Basin, as a border rock ; here, 

 too, are the residual pegmatites ; (b) the marginal pegmatite 

 and its offshoots occur chiefly in the southwestern portion of 

 the Basin, and have somewhat greater extent than the granite, 

 northward, northeastward, and eastward ; (c) the quartz veins 

 occur locally in all parts of the southern half of the Basin, but 

 more abundantly southwestward toward the pegmatites.* 



Hence, the series exhibits higher acidity and evidence of 

 more intense operation of mineralizers, with increasing distance 

 from the granite. 



(6). The most important catalytic agent active in the crys- 

 tallization of these intrusive rocks appears to have been water 

 gas, for there are found very few of the rarer ' pneumatolytic 

 minerals,' the occurrence of which would be a token of the 

 presence of other volatile constituents. 



(7). Evidences for solution and chemical alteration as contact 

 phenomena — slight in any casef — are most pronounced near 



* Such distribution may not be due to horizontal distance from a vertical, 

 or nearly vertical, contact, for the upper surface of the granite may slope 

 northward and eastward. 



f It is therefore questionable whether solution has contributed much toward 

 opening the spaces for these bodies, as was suggested by Crosby and Fuller 

 for similar dikes and veins (op. cit., p. 338). 



