446 Hawkins — Notes on the Geology of Rhode Island. 



clearly marked variation of the Putnam formation ;" 

 which invades Rhode Island from the west in a semicircu- 

 lar area not more than a mile long and half a mile wide. 

 "All igneous rocks of the district occur as intrusions in 

 the Putnam gneiss, and this formation is therefore older 

 than the various dikes, sheets, pegmatite veins, and igne- 

 ous masses found associated with it" (op. cit.). "The 

 abundance of sheets of Sterling granite gneiss intruded 

 into the Putnam formation" is especially emphasized; 

 the same relationships obtain at Preston, Connecticut, 

 farther south. The Putnam gneiss and associated quartz- 

 ites of the Plainfield series are hence older than the Ster- 

 ling and Northbridge members wiiich here compose the 

 great granite batholith by which they are surrounded; 

 and for this reason they must be pre-Carboniferous in 

 age ; in fact the writer believes that this whole sedimen- 

 tary series, including quartzite and gneiss, belongs among 

 the rocks of vastly more ancient origin (which may be of 

 Cambrian age or may possibly belong to a still older 

 series like the Grenville, 12 but whose age, in the 

 absence of fossils or other conclusive evidence, remains 

 unknown), represented by the quartzites, limestones, 

 green schists, and gabbros (the Blackstone series), pres- 

 ently to be described. Through intense metamorphism 

 the basic rocks and the Carboniferous sediments have 

 locally been altered so as to approach each other in 

 appearance and mineral composition; by intrusion the 

 Carboniferous shales of Wakefield, R. I., have been 

 changed into hornblende schists, and by shearing in the 

 Woonsocket Basin they have become mica schists ; fun- 

 damental changes of this kind are such as to render 

 correlation difficult by lithologic characteristics alone. 

 Thus, for instance, there is little evidence to show 

 whether the dark inclusions in the Westerly granite are 

 Carboniferous in age or much older; (the writer, on 

 account of facts observed in the field, favors the 

 latter opinion). 



A part of the southern portion of Conanicut Island is 

 underlain by a granite which bears a strong resemblance 

 to the Sterling granite gneiss to the west 13 ; phenocrysts 

 derived from it are found in the adjacent Carboniferous 



12 All classified together as " Algonkian ? " by Emerson (idem). 



13 Loughlin, G. F., U. S. Geol. Surv., Bull. 492, 134, 1912. 



