456 F. H. Lahee — Metamorphism and Geological Structure. 



— Relations of the granite phase to the Carboniferous sedi- 

 ments. — Within the Boston Neck granite are many elongate, 

 highly metamorphosed inclusions of schist, which, as inferred 

 from their structural and petrologic relations, are undoubtedly 

 parts of the Carboniferous formation of the Basin.* The 

 dimensions of these xenoliths vary from an inch to several 

 scores of feet. In general, their lengths trend nearly north- 

 south, i. e., about parallel to the strikes of the bedding of the 

 adjacent Carboniferous strata, and their schistosity runs in the 

 same direction. Their extremities are often ragged, and some- 

 times strips may be observed to have been torn away and 

 removed a short distance before having been frozen in. Apo- 

 physes project into them from the granite. 



At the contact there is more or less blending, due either to 

 solution or to interpenetration by the magma at the time of 

 injection. Near the more basic xenoliths the granite may dis- 

 play an increase in its content of biotite and garnet. Micro- 

 scopic sections show that grains of the feldspars which are char- 

 acteristic of the intrusive may occur in the inclusion, abundant 

 near the contact, but decreasing in amount away from it. 

 While the granite never becomes aphanitic adjacent to the 

 country rock, it may grow sensibly finer. Apparently the 

 schists were comparatively warm when intrusion took place. 



There can be no doubt, therefore, that the Boston Neck 

 granite is irruptive into, and consequently later than, the 

 Carboniferous strata. 



The pegmatite phase : Description. — Pegmatite is found, 

 not only at various localities in the granite area, but also nearly 

 as far northward as Hamilton (Loc. 8, B-C : 11, iig. 1), and 

 eastward, in the form of small dikelets, on Dutch Island 

 (Loc. 14, C-D : 13). 



These pegmatites are composed chiefly of n;.icrocline, micro- 

 perthite, graphic granite, and quartz, a little orthoclase, acid 

 plagioclase, and muscovite. Biotite may be present instead of, 

 or accompanying, the muscovite. Garnet may be plentiful. 

 ' Pneumatolytic minerals,' such as beryl, tourmaline, topaz, and 

 fluorite, are very rare. Tourmaline alone has been found in 

 one instance by the writer. Kemp stated that, westward, 

 tourmaline, monazite, and molybdenite have been observed. f 

 Of the principal constituents, the feldspars occurs in crystals, 

 often excellently shaped, up to thirty inches in length. Quartz 

 grains, too, may measure as much as two or three feet across. 

 Muscovite occurs either as thick plates, sometimes two inches 



, * Shaler, N. S., Foerste, A. F.. and Woodworth, J. B., Geology of the 

 Narragansett Basin, U. S. G. S., Monog. xxxiii, 1899, pp. 245, 377. 



+ Kemp, J. F., Granites of Southern Ehode Island and Connecticut. Bull. 

 Geol. Soc. Am., x, p. 361, 1899. 



