50 Loughlin and Hechinger — Unconformity 



ite-porphyry and quartz felsite-porphyry with the same char- 

 acteristic minerals border the main mass. Besides the principal 

 occurrence, Emerson and Perry* have mapped granitic rocks 

 of similar character along the west border of the Narragansett 

 Basin at Cumberland, R. I., and Woodworthf mentions limited 

 occurrences near Hanover Four Corners, Easton, and Mans- 

 field, Mass. Recent field studies by C. H. Warren and Sidney 

 Powers in Cumberland, R. L, and by F. H. Lahee in Sharon, 

 Mass., have discovered new areas of the same alkaline type of 

 granite and have produced undoubted proof that it is intrusive 

 into the Milford and Dedham granites, the alkaline type 

 having chilled contacts against the others.j The main area of 

 Quincv granite is older than rocks which will be shown to be 

 of probable Permian age. 



Comparison of granites. — Comparison of the foregoing 

 descriptions shows that the East Greenwich granite group has 

 textural features very distinct from those of all the other gran- 

 ites. The Quincy granite also is distinctly different from the 

 others in mineral and chemical composition as well as in certain 

 textural details, and is clearly intrusive into the Milford and 

 Dedham granites. The other granites are similar to one another 

 in mineral composition save for local variations, such as the 

 contact phase of the Sterling granite gneiss and the alaskitic 

 phases of the Dedham granite. They present certain textural 

 variations, but these are not reliable criteria for their distinction, 

 as the Sterling alone varies from eugranitic to highly gneissoid, 

 and variations in the texture of the Milford granite render 

 a sharp distinction between it and the Northbridge gneiss 

 impossible. The Dedham granite, owing to its general lack of 

 gneissoid structure and its more pronounced degree of chemical 

 alteration, may appear distinct from the other granites, but the 

 results of field work thus far have not proved any sharp struc- 

 tural or textural differences between it and the Milford gran- 

 ite,§ and its greater degree of alteration, as will be shown later, 

 appears to coincide in areal extent with the area most severely 

 affected by the upheaval of Permian rocks. ~No published 

 results of a systematic study of all these granite areas is avail- 



*Op. cit., p. 51 et seq. 



f Wood worth, J. B., U. S. Geol. Survey. Mon. 33, p. 116. 



\ Personal communication by C. H. Warren. Warren and Powers are 

 publishing their results in Geol. Soc. America Bull., vol. xxv, 1914. 



£ The writers, during their limited observations in the granite areas, were 

 unable to find any reliable textural criteria for distinguishing the Deri ham. 

 Milford. and Sterling granites from one another. C. H. Warren, in a 

 personal communication, states that the results of field and laboratory studies 

 by Mr. Sidney Powers and himself in various parts of the region, and espe- 

 cially in the vicinity of Diamond Hill, R. I., lead him to believe that differ- 

 ent medium- to coarse-grained biotite granites are in reality parts of one 

 extensive batholith. 



