454 WARREN AND POWERS DIAMOND HILL-CUMBERLAND DISTRICT 



section it is seen to consist of plagioclase (probably andesine), much 

 altered green hornblende, biotite with abundant magnetite (much of it 

 in the hornblende), sericite, and apatite. The presence of hornblende 

 suggests that this rock is much like that at Grants Mills. 



The quartz diorites in the four localities named are apparently very 

 closely related rocks and doubtless are identical as to origin and age. 

 West of AA'est AVrentham the diorite clearly cuts the pre-Cambrian Ashton 

 schist and probably cuts also the gabbro. It is itself cut by dikes of the 

 ^lilford type of granite. Although somewhat metamorphosed, as is also 

 the Milford type of granite in the region about the diorite, neither are 

 anywhere nearly so strongly metamorphosed as are the pre-Cambriau 

 rocks which the}^ cut. 



At Lime Rock, 3 miles south of Albion, in the limestone quarry, 

 Emerson and Perry found diorite pebbles in a conglomerate belonging to 

 the Ashton schist series. N'ear by they found a diorite ledge with several 

 types of rock, some of which resembled those shown in the pebbles. On 

 this evidence they conclude that the diorite is older than the Ashton 

 schist. There is no good reason why there may not have been diorites 

 of pre- Ashton schist age in the area, exposed during the deposition of the 

 schist and from which the diorite pebbles may have been derived ; but the 

 diorite found near the pel)bles has not necessarily any connection with 

 the latter. The other masses of diorite described above are quite clearly 

 intrusive into the pre-Cambrian schist. Moreover, at a locality west of 

 West Wrentham, as described above, pebbles of granite were found in 

 the Ashton schist conglomerate, the pebbles resembling the ^lilford type 

 of granite and its fine-grained porphyritic phases ; and yet the Milford 

 granite cuts the schist only a few hundred feet from the conglomerate 

 outcrop. From what has been said it appears that the quartz diorite ox 

 this area is at least later than the pre-Cambrian schist, although it ap- 

 pears to have come into place before the granites. 



Milford granite. — The Milford granite extends from its host known 

 locality at Milford, Massachusetts, southward througli the area mapped 

 in this paper to a point below Providence, Ehode Island. The typical 

 granite, like that from the well known quarries at Milford. ^[assachu- 

 setts, is a pale pinkish to cream colored rock of medium grain and dis- 

 tinctly gneissoid structure, marked by irregular streaks or linearly ar- 

 ranged patches of black mica, and to a less extent by a banded arrange- 

 ment of the quartz and feldspar. The quartz is in large part ^'sugary.'' 

 Emerson and Perry-^ state that the quartz is blue. So far as our ex- 



" Loc, clt.. p. 45. 



