48 Loughlin and Hecliinger — Unconformity 



the principal feldspar, and in the one illustration given* is 

 characterized by prominent albite, but inconspicuous pericline, 

 twinning. 



Sterling granite-gneiss. — The Sterling granite-gneiss forms 

 the southwestern border of the Narragansett Basin and extends 

 westward into Connecticut. It includes two principal varieties: 

 a normal phase, of pink to gray color, of strongly gneissoid to 

 eugranitic texture, and composed of basic oligoclase, strongly 

 twinned microcline, quartz, and biotite, with little or no mus- 

 covite; a contact phase of light gray color, eugranitic to gneissic 

 texture, and composed of basic oligoclase, strongly twinned 

 microcline, quartz, muscovite with biotite subordinate or incon- 

 spicuous, and, in some places, garnet. Local porphyritic varia- 

 tions of the eugranitic parts, and " augen gneiss " variations of 

 the gneissoid parts, are present in the normal phase. The con- 

 tact phase is accompanied by a large amount of pegmatite of 

 similar composition, and pegmatitic apophyses from it are intru- 

 sive into the " Coal Measures " of the southwestern part of the 

 Narragansett Basin. The microcline in both phases has the 

 typical "gitter" structure, and may show some, but in no case 

 much, perthitic structure. Chemical alteration in both phases 

 is, as a rule, not conspicuous, but small amounts of epidote, 

 sericite, and chlorite may be seen in thin section. 



Northbridge gneiss and Milford granite. — Northward the 

 Sterling granite gneiss area overlaps that of the Northbridge 

 gneiss, and the two appear to be parts of the same batholith.f 

 Emerson and Perry,:); however, have mapped the Northbridge 

 gneiss as pre-Cambrian. North of Natick, R. I., the Milford 

 granite, intrusive through Cambrian strata, is the most exten- 

 sive granite immediately west of the Narragansett Basin. This 

 granite is generally similar to the Northbridge gneiss and Ster- 

 ling granite gneiss. According to Emerson and Perry, § its 

 most gneissoid parts can not be sharply distinguished in tha 

 Held from the Northbridge gneiss, but in thin section its alkalic 

 feldspar is a microcline microperthite, whereas that of the 

 Northbridge gneiss is strongly twinned microcline without con- 

 spicuous perthitic intergrowth. The plagioclase member of 

 the microperthite is dusted with epidote, and in thin sections 

 studied by the present writers, plagioclase grains show a con- 

 spicuous microscopic alteration to epidote and sericite, and the 

 biotite grains to chlorite. The authors above cited[ regard the 

 Milford granite as older than the " Coal Measures " of the 

 Narragansett Basin, but their descriptions, cited on a later page, 

 suggest that this interpretation is open to question. 



* Idem fig. 6. 



+ Loughlin, G. F., this Journal, (4)., vol. xxix, pp. 450, 451, and 454, 1910. 



JU. S. Geol. Survey Bull. 311, p. 10, 1907. 



§Op. cit., p. 10. || Op. cit., pp. 39-40. 



