118 GEOLOGY OF THE NARRAGANSETT BASIN. 



Diamond Hill is a mass of quartz veins probably segregated as late as 

 the period of the Wamsutta formation of the northern border. The quartz, 

 as shown by Dr. Foerste's studies, is mainly developed in the southern part 

 of a mass of granite-porphyry having a superficial extension of about 2 

 square miles. 



GABBRO HILLS OF SHARON. 



Aii elongated lenticular area of gabbro with diorite occurs in the 

 Wrentham-Hingham granitic uplift, bordering the southern side of the Nor- 

 folk County Basin. The following notes are from a description of the rock 

 by Messrs. J. R. Finlay and H. I. Richmond, jr., to whom I am indebted for 

 the use of their manuscript report: 



From North Foxboro to Canton Junction there is a heretofore uudescribed range 

 of hills composed almost entirely of gabbro. The area of this rock is 7 miles long and 

 its greatest breadth about 2 miles. The highest point of this range is Moose Hill, 

 which attains an elevation of 5G0 feet. Southwest of Foxboro the gabbro extends in 

 disconnected bosses as far as Wrentham. 



At Foxboro, Mansfield, East Foxboro, and Canton there are large ledges of 

 coarse, light-colored, hornblendic grauitite, so situated as practically to surround the 

 gabbro on at least three sides. In the gabbro and along the northern side are frequent 

 dikes of aplite. The outcrops of a fine-grained grauitite along the northern side attain 

 considerable size. Exactly what is the relation of this fine-grained granitite to the 

 coarser granitite which covers so much territory to the southeast can not be determined 

 in the field, as they are nowhere in contact. 



The coarse-grained granitite is found in several places, as, for instance, just west 

 of Canton Junction, penetrating in an intricate manner patches of crystalline schist. 

 In the railroad cut just north of the station at Canton Junction there is a large 

 exposure of coarse-grained hornblendic granitite, while south of the station and just 

 east of the gabbro is a large area of biotite-granitite. Adjoining these rocks to the 

 south and east is the light-colored coarse granitite of the region. South of Canton 

 Junction, a mile east of the railroad, there is a large boss of diorite which has burst 

 through the granitite. Blocks of coarse-grained granitite are inclosed in the diorite. 

 Between the diorite and the gabbro is a strip of granitite a quarter of a mile wide. 



The coarse granitites are older than the diorite, but are younger than the schists 

 at Canton Junction. The gabbro is older than the fine-grained granitite dikes pene- 

 trating its mass, but their relation to the coarse granitites is unknown. The youngest 

 rock of the region is a diabase, which is found in dikes cutting the other rocks. 



A petrographical study of the gabbro showed variations to a dioritic phase. The 

 diorite exists in certain areas in the gabbro. 



