GEOLOGY OF THE AREA DISCUSSED 363 



the Sound, with stretches of salt marsh and meadow between them* The 

 coast is plainly one that has been drowned by recent submergence. 

 Several of the rocky points contain intrusions of granite, which furnish 

 material for quite an important quarry industry. 



The gneisses are with few exceptions granitic in composition, and 

 mineralogically are very similar to the massive granites. They contain, 

 however, noticeabl}' more of the dark silicates than do the latter. The 

 chief component minerals are quartz, orthoclase, microcline, rather acid 

 plagioclase, and biotite. The biotite is of a greenish brown color, and 

 has the characteristic strong pleochroism. Magnetite, apatite, and zircon, 

 the usual accessories in such rocks, are not lacking. In one or two in- 

 stances garnet has been observed. Mechanical deformations are very 

 widesi)read, and are often of an extremely pronounced character. The 

 quartz and feldspars are crushed and granulated, and the biotite is rubbed 

 out into long strips and bent and bowed in a striking manner. The folia- 

 tion is always pronounced, and it may at times almost reach the extreme 

 of a schist. 



On Kingston hill, Rhode Island, in the ridge on which is located the 

 village of Kingston, the gneiss contains many augen of feldspar, and is 

 of a marked augen variety. It is so characteristic that it may be called 

 the Kingston type of gneiss in distinction from that of the remainder of 

 the area. There is little doubt that it is a sheared porphyritic granite, 

 but there may have been considerable reci'ystallization of the material 

 during metamorphism. The Kingston type of gneiss is well exposed *in 

 the quarries of the Rhode Island College of Agriculture and Mechanic 

 Arts. 



In a gneiss that forms the wall-rock of the granite in the east quarry of 

 the Smith Company, at Westerly, considerable hornblende was observed 

 along with biotite, and the same mineral has been noted in exposures 

 east of Chapman pond. Westerly, Rhode Island, and at Clinton, Madison, 

 and Rocky point just west of Niantic, all in Connecticut. Abundant 

 quartz and feldspar were also present, so that the rock is essentially 

 granitic in its mineralogy, and quite different from the basic hornblendic 

 gneisses to be presently described. 



An average hand-specimen of the biotite-granite-gneiss was selected 

 and its specific gravity, was determined to be 2.704, a value about 0.05 

 to 0.07 above the ranges of the massive granites and due to the relatively 

 greater abundance of biotite. 



At somewhat rare intervals throughout the gneissic area bands of 

 bksic hornblende-gneiss or amphibolite are found interfoliated with the 



♦Compare in this connection J. D. Dana : On the geology of the New Haven region, with special 

 reference to tlie origin of some of its topographical features. Trans. Connecticut Acad, of Arts 

 and Sciences, vol. ii, 1870, p. 45. 



