in the Narragansett Basin. 49 



Dedham granite. — The Dedham granite with associated basic 

 phitonic rocks covers a large area, including the northwestern 

 corner of the Narragansett Basin and almost the entire northwest 

 side of the Norfolk County Basin. At several places it is cut 

 and in part covered by bodies of felsite. Rocks of the same 

 general descriptions surround the Boston and Norfolk County 

 basins, and probably occupy much of the area east of the Narra- 

 gansett Basin, but the writers' acquaintance with the latter held 

 is too limited to warrant a more positive statement. Granite 

 similar in all respects to the porphyritic phase of the Dedham 

 granite is exposed on Newport Neck and Conanicut Island at 

 the southern end of the Narragansett Basin. The complete 

 series of rocks probably includes all types and gradations from 

 alaskite and aplite, through common biotite granite, granodior- 

 ite, diorite, and gabbro, but only the first two types need con- 

 sideration here. The biotite granite is greenish gray to pink 

 or even red, medium- to coarse-grained and in places coarsely 

 porphyritic. Grneissic structure is almost everywhere absent. 

 Its principal minerals are basic oligoclase (to andesine), strongly 

 twinned microcline-microperthite (potassic member dominant), 

 quartz, biotite, and in some cases hornblende. Epidote and 

 chlorite are everywhere conspicuous, either in short irregular 

 veins, or replacing plagioclase and the black silicates. Micro- 

 scopic sericite is also abundant, and secondary quartz and calcite 

 may be present, especially in the veins. The aplitic phase is 

 of generally pink color and varies in texture from an even- 

 grained aplite to granite-porphyry. The mineral composition 

 and character is much the same as in the type just described, 

 save that the plagioclase is more sodic and the green alteration 

 minerals are less conspicuous. The Dedham granite is clearly 

 younger than Cambrian rocks, and older than Permian, but its 

 relations to "Coal Measures" and Milford granite have not 

 been definitely determined. 



Quincy granite* — The Quincy granite occupies a limited 

 area at the northeast end of the Norfolk County Basin, sepa- 

 rating it from the Boston Basin. The main body is bluish 

 gray, or pink where locally oxidized, and medium-grained. Its 

 feldspar is a microcline microperthite in which the two mem- 

 bers are in about equal amount, the microcline member show- 

 ing only albite twinning and the albite member frequently 

 forming a continuous rim around the main intergrown portion. 

 Plagioclase is scarce or absent. The black minerals are rie- 

 beckitic amphibole and gegerite, both in distinct grains and in 

 minute crystals impregnating the feldspar and quartz. Gran- 



* An exhaustive study of the Quincy granite and associated alkaline rocks 

 has been made by C. H. Warren. Petrology of the alkali-granites and por- 

 phyries of Quincy and the Blue Hills, Mass. , Am. Acad. Arts & Sci. . vol. xlix, 

 No. 5, pp. 203-330, Sept. 1913. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 223.— July, 1914. 

 4 



