372 . J. F. KEMP — GRANITES OF ATLANTIC COAST 



generation on the significance of foliation in metamorphic rocks. The 

 widespF.ead foliation of the country rocks in the region under consider- 

 ation antedates the intrusion of the granites, which, although not en- 

 tirely, yet in most instances, escaped notable dynamic metamorphism. 

 As ah-eady stated in the introductory sketch, granites of post-Caml)rian, 

 if not of post-Carboniferous, age are recognizable at Newport and Conan- 

 icut island, which have penetrated wall-rocks already highly metamor- 

 phosed. The granites have themselves, then, suffered the effects of 

 pressure, and in the absence of positive evidence to the contrary we may 

 not be certain that the more western intrusions in Rhode Island and 

 Connecticut are not of the same age — that is, at least post-Cambrian. 

 They assuredly are pre-Triassic. 



The Aplites and the Pegmatites 



Aplite dikes of a small thickness are frequently met and are plainly 

 of an intrusive nature. They cut the gneisses regardless of their folia- 

 tion, and are to be considered as minor border phenomena around the 

 granites. The chief constituent minerals are quartz and orthoclase. 

 Biotite, muscovite, microcline, and acid plagioclase are all in subordi- 

 nate amount. 



The pegmatites are present in great numbers and at times attain very 

 considerable size. They cut the gneisses as dikes of varying width, and 

 are beyond question genetically connected with the granites. The peg- 

 matites vary from those of granitic mineralogy to those that are practi- 

 cally pure quartz.* They are specially abundant along the west coast 

 of Narragansett bay, as shown in plate 40, figures 2 and 3. 



The commonest kind is a very coarse aggregate of red microcline, white 

 natron-orthoclase, albite, and quartz, together with a little black, brittle 

 biotite and occasional thin plates of ilmenite and masses of magnetite. 

 The microcline is flesh-red and exhibits crystals that almost always have 

 some recognizable faces and that may be beautifully developed. An ex- 

 posure of one of the pegmatites is illustrated by plate 41, figure 1. The 

 simplest and commonest combination contains 1 (110), M (010), P (001), 

 X (101), or y (201), and z (130). The crystals are either equall}^ devel- 

 oped along each axis or else are drawn out on the vertical axis into a 

 prismatic elongation. One specially perfect one which was obtained by 

 chiseling away the surrounding quartz at the Smith quarry. Westerly, 

 exhibits the above faces, and in addition o (111), and n (021). Carlsbad 

 twins have been found, but are not particularly common. Manebacher 

 twins are quite as frequently seen, and in several instances have been 



* Professor J. D. Dana states in " The four rocks," p. 116, that tlie quartz veins are later than the 

 granite veins ; i. e., than the pegmatites, as the former cut the latter when both occur together. 



