Hawkins — Notes on the Geology of Rhode Island. 461 



Near contacts with the basic rocks, and often for dis- 

 tances of several miles from visible contacts with large 

 masses of basic rock, the intrusive granite gneisses com- 

 monly, though not always, have been found to show an 

 abnormally dark color, increasing as the contact is 

 approached. With this darkening of color, small dark 

 fragments of unassimilated basic rock appear, 30 scat- 

 tered at intervals throughout the granite gneiss. At the 

 same time the granite becomes increasingly porphyritic, 

 the orthoclase phenocrysts often attaining a length of one 

 or two inches. The best example of this phenomenon is 

 found in the "Absalona" type of the Northbridge granite 

 gneiss, well shown on Absalona Hill, east of Chepachet. 31 

 That a real and not inconsiderable change in the compo- 

 sition of the granite has actually taken place is plainly 

 indicated by the analyses of the granite and green schist 

 types (to be more fully discussed; see Table III). The 

 granites also apparently have had the power to assimilate 

 a certain amount of quartzite and similar acid rocks, 

 becoming more acid thereby ; but on account of similari- 

 ties of composition of the intruding and intruded rock 

 types, the changes in a physical way are certainly not so 

 noticeable. 



The result of this assimilation process is seen in the 

 field in the widespread production of rocks which are 

 both in their appearance and in their physical and chem- 

 ical composition intermediate between the gabbro and 

 granite types. Coarse, porphyritic, dark-colored gran- 

 ites of almost dioritic aspect are in places extensively 

 developed, and appear at first to add much to the com- 

 plexity of the geologic situation. (Compare Hybrid 

 Rocks, Bowen, 32 also analyses 3, Table II, and 18, Table 

 III.) This is especially true of the Northbridge granite 

 gneiss, which is made up of a variety of abnormally basic 

 granite types throughout northwestern Rhode Island. 

 It is found in this condition, dark-colored, porphyritic, 

 and filled with basic masses, stringers and fragments, as 

 indicated oh the map, throughout an area bounded on the 

 north by the Rhode Island-Massachusetts line, its south- 



30 The orbicular Westerly granite type of Quonochontaug, E. I. (See 

 Kemp, J. F., Trans. N. Y. Acad. Sci., 13, 140-144, 1894), is probably 

 similar in origin. 



31 This is the "Carboniferous conglomerate ' ' mentioned by Emerson 

 (idem, 229). 



32 Bowen, K L., Jour. GeoL, 23, Supplement, 85, 1915. 



