476 WARREN AND POWERS — -DIAMOND HILL-CUMBERLAND DISTRICT 



scopic description of the biotite granites found in the area is given. On 

 the basis of field relations and of the general mineralogical and textural 

 similarity of the unsheared phases of these granites, they are all believed 

 to form parts of one batholith — the Mil ford batholith. Local facies. 

 quartz diorites, and fine granite or fine granite porphyries are briefly 

 referred to. 



The riebeckite-a?girite granites and porphyries are described and shown 

 to be of strikingly different texture and mineral composition, as well as 

 of later age, than the biotite granite series. They form a single intrusion 

 and the porphyries appear as the marginal chilled phase of the invading 

 mass. They are correlated with the closely similar granites and porphy- 

 ries found ill larger development in the Blue Hills and Quincy, and form, 

 so far as known, the southernmost extension of the great series of rocks 

 characterized by the presence of microperthitic feldspars and highly 

 alkaline hornblendes or pyroxenes, or both. The occurrence of a smaller 

 intermediate mass of the same type at Rattlesnake Hill, in Sharon, re- 

 cently discovered by Dr. F. H. Lahee, is noted as fonning still another 

 upward protuberance of the alkaline magma. 



