438 Hawkins — Notes on the Geology of Rhode Island. 



westward to the Connecticut line, the Sterling granite 

 gneiss is to be seen. Their age relationships are some- 

 what obscure, since contact zones are usually either cov- 

 ered with glacial debris or occupied by streams or tidal 

 estuaries. Some evidence as to age is, however, available. 



Much of the contact zone between the Northbridge and 

 Milford granite gneisses is occupied by an acidic granite 

 phase like the " northfieldite ' ' of Emerson. 3 Yet a mile 

 northwest of Oak Valley, R. I., near the road, at a point 

 where it makes a sudden detour, just southwest of the 

 right-angled cross-roads, a granite gneiss of Milford type 

 is in contact with a very dark phase of the Northbridge 

 granite gneiss. The former appears to intrude the lat- 

 ter and to include portions of it. From these rather 

 unsatisfactory bits of evidence it has been concluded that 

 the Milford is probably at least slightly younger in age 

 than the Northbridge. 4 Between the Northbridge and 

 the Sterling, aplites are found to obscure the relations in 

 the principal places where exposures are available. On 

 Nooseneck Hill, and again just north of Summit, how- 

 ever, a coarse porphyritic pink granite gneiss is found to 

 be thoroughly interlaminated with a fine-grained gray 

 granite gneiss type. This is interpreted to represent 

 the contact zone between the Northbridge and the Ster- 

 ling and to indicate that the contact is gradational. The 

 Northbridge and Sterling granite gneisses are alike in 

 many respects, and it might be reasonable even to suggest 

 that the Northbridge type may be a modification of that 

 of the Sterling, due to excessive assimilation of basic 

 material (hereafter to be more fully discussed), perhaps 

 in conjunction with processes of differentiation which 

 were possibly set in motion by the latter process. It is 

 to be hoped that in spite of scarcity of good exposures the 

 relations of these granite gneisses may in time be discov- 

 ered in detail. ( See further discussion below. ) 



East of the Carboniferous sediments of the Narragan- 

 sett Basin a similar series of granite gneisses is exposed. 

 Exact relationships with the foregoing types are not 

 known, but it has been suggested 5 that certain of the rocks 

 closely resemble the Milford and Northbridge type ; and 



3 Emerson, B. K., this Journal, 40, 212-217, 1915. 



4 Warren, C. H., and Powers, S., Bull. Geol. Soc. Am., 25, 459, 1914. (See 

 also Emerson, op. cit.) 



5 (a) Emerson and Perry, op: cit., 10. (b) Shaler, N. S., Woodworth, 

 J. B., and Foerste, A. F., IT. S. Geol. Surv., Mon. XXXIII, 274, 1899. 



