IGNEOUS ROCKS 459 



Biotite, now more or less altered to chlorite, is quite abimdant. The fine 

 granite appears to be an aplitic phase of the same granite. 



Comparison of the biotite granites. — Comparing the Grants Mills and 

 the Joes Eock granites, we find that they are substantially the same rock 

 mineralogically and texturally, or at most they show only minor differ- 

 ences. There seems to be no reason why they may not belong to the same 

 intrusion. To the northeast of the present area, at Franklin, they ap- 

 pear to blend, and further northward, in Sharon, they appear to merge 

 into the Dedham granite, which, although on the whole somewhat finer in 

 grain, possesses essentially the same mineralogical and textural features. 

 Compared with the Milford granite, they are coarser in grain and show 

 a greater tendency toward a porphyritic habit. The Milford granite is^ 

 also, as a rule, much more strongly sheared. Aside from the tendency of 

 the Grants Mills and the Joes Eock granites to develop larger crystals 

 of microcline or orthoclase, they are texturally very similar to the less 

 sheared phase of the Milford, and mineralogically they are substantially 

 the same. So far as we can see, there is no field evidence to show that 

 they are not of the same age and belong to one batholithic intrusion. It 

 has not been possible to carry out a chemical study of the various types, 

 but the microscopic evidence makes it highly improbable that there are 

 any substantial differences chemically between them. Such differences 

 as exist are not other than might be expected in different parts of a large 

 batholithic intrusion such as the Milford batholith undoubtedly is, and 

 until some positive field evidence is brought forward to show that they 

 are of different age, we hold it best to group them all together as belong- 

 ing to one period of batholithic intrusion. 



We may remark here that a careful chemical study of the various types 

 of biotite granites in eastern Massachusetts and Ehode Island is desirable. 

 It is true that three analyses of the Milford have been made and pui)- 

 lished,^'^ but study of them sliows wide variations in several respects and 

 leads to the suspicion that either they were not executed on suitably 

 chosen samples or that there are errors in the analyses. At all events, 

 they are as they stand of doubtful value from the petrographical point of 

 view. 



The age of these granites is known to be pre-Carboniferous becauso 

 pebbles of them occur abundantly in the Narragansett series. On the 

 east side of Hoppin Hill, north of the railroad track, the granite out- 

 crops within 50 feet of the red Lower Cambrian shale. South of the 

 railroad, 150 feet north of the road across the reservoir, are outcrops of 



These are quoted by Dale in BuU. U. S. Geol. Survey No. 358. 



