F. JET. Lahee — Metamorphism and Geological Structure. 469 



(11). The post-Carboniferous intrusives include a few 

 minette dikes, on the one hand, and an extensive, perhaps 

 related, series of granites, pegmatites, and quartz veins (Acid 

 Intrusive Series), on the other hand. 



(12). Of the Acid Intrusive Series, the granite (Boston ISTeck 

 granite) is oldest, the pegmatites are younger, and the quartz 

 veins represent the latest differentiation phase. 



(13). The Boston Neck granite is limited to Boston and 

 Little Necks, the Tower Hill ridge, and westward (Sterling 

 granite) ; the pegmatites have a wider distribution (within the 

 Basin), north to Barber's Height and east to Dutch Island ; 

 and the quartz veins, although most abundant in the south- 

 western portion of the Basin, occur throughout the area 

 investigated. 



(14). These igneous rocks (Acid Intrusive Series and prob- 

 ably minettes) were injected during, and immediately sub- 

 sequent to, the folding of the Carboniferous sediments. 



(15). More or less static and dynamic metamorphism attended 

 the intrusion of these igneous rocks, but this metamorphism is 

 local and is of a distinctly different character from the regional 

 metamorphism due to the folding. 



We conclude, then, that the Carboniferous strata of the 

 Narragansett Basin, after deep burial, were folded by forces 

 that acted with greater intensity in the south ; that, contem- 

 poraneous with, and consequent upon, this deformation, these 

 sediments were regionally metamorphosed ; that this deforma- 

 tion and this metamorphism were accompanied, in their later 

 stages, by the intrusion of certain igneous rocks — a process 

 which continued, with magmatic differentiation, for some time 

 after folding ceased ; and that, these facts being accepted, the 

 regional metamorphism, and the injection of the post-Carbon- 

 iferous igneous rocks, maybe regarded as nearly parallel effects 

 of the mountain-building forces. 



Cambridge, Mass., 

 February 5, 1912. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 197.— May, 1912. 

 31 



