458 WARREN AND POWERS DIAMOND HILL-CUMBERLAND DISTRICT 



At Hoppin Hill, south of Attleboro and 2i/^ miles southeast of Arnolds 

 Mills, is an exposure of coarse biotite granite similar to that of Joes 

 Rock; for convenience, it may be called the Hoppin Hill granite. The 

 large potash feldspar crystals are the most conspicuous feature of the 

 rock and give it something of a porphyritic habit and also its character- 

 istic pinkish color. These feldspars may reach a length of one inch and 

 have a thickness of one-third of an inch. The proportions of the two 

 feldspars to the quartz is about the same as in the Joes Eock granite, but 

 biotite or its chloritic alteration appear to be more abundant. 



Thin-sections show that the feldspars are oligoclase and microcline con- 

 taining the usual perthitically intergrown lamella of albite or oligoclase. 

 The oligoclase and microcline microperthite are about equal in amount, 

 and the former contains the usual minute inclusions of secondary min- 

 erals. The biotite is largely altered to chlorite. As the rock appears to 

 have been only slightly metamorphosed, the quartz retains its original 

 large grains with the usual allotriomorphic outlines. The general tex- 

 tural relations of the minerals are those found for the two previously 

 described granites. 



Grants Mills granite. — In a narrow north-south belt passing through 

 Grants Mills a coarse biotite granite is exposed which will be called here 

 the Grants Mills granite. There is a small quarry in this rock near the 

 State line, at the side of the railroad running through Grants Mills. It 

 is also well exposed on the hill west of there ; other good outcrops occur 

 along the north-south road a mile north of Hunting Hill. This granite 

 is closely associated with the Milford granite, but the boundaries between 

 the two can not be accurately drawn because of lack of outcrops. A fine- 

 grained a])litic to feebly porphyritic granite appears northwest of Grants 

 Mills cutting the other granites. 



Megascopically this granite is a coarse, rather unevenly grained rock 

 of cream, pale greenish or even pinkish color. The rather abundant 

 larger feldspars are unstriated and attain a length of an inch or over; 

 it is these that are sometimes greenish or pink and give color to the rock. 

 The remainder of the rock is a rather coarse mixture of quartz and feld- 

 spar in wliich are quite numerous specks and platy aggregates of dark 

 green to black biotite or chlorite. Locally the granite has been strongly 

 sheared. 



Tlie feldspars are microcline or orthoclase, with some interwoven plagi- 

 f>clasc and albite or albite oligoclase. In amount they are probably about 

 equal. The plagioclase is the more automorphic of the two; it contains 

 many minute secondary crystallizations, and in the run of sections, which 

 do not contain the larger potash feldspar crystals, seems to predominate. 



