368 J. F. KEMP — GRANITES OP ATLANTIC COAST 



grain and does not difror essentially^ from the type, except, perhaps, in 

 the greater abundance of niicrocline. Quartz, microcline, orthoclase, 

 oligoclase, biotite, pyrite, and the usual accessories make it up. 



The granites from Groton and from the immediate neighborhood of 

 New London all vary but slightly from the type of the Westerly gray. 

 In one from a quarry a mile south of the little station of Waterford, on 

 the Shore Line railroad, just east of New London, the quartzes are charged 

 with twisted blades of some unknown mineral, having the form of Archi- 

 medes screws, of very long pitch. Rutile needles, fluid inclusions, and 

 the usual microscopic phenomena of the quartz of granites are universal, 

 but these spirals have not been seen elsewhere. 



The Westerly red granite. — The Westerly red granite does not differ 

 essentially in mineralogy from the gray t3^pe, although it bears no resem- 

 blance to it in the hand specimen. The former is much more coarsely 

 crystalline, the anhedra ranging from L5 to 5 millimeters. The orthoclase 

 has a pronounced pink color, which, however, disappears in thin-section. 

 Microcline, oligoclase, biotite, a little muscovite, magnetite, zircons, and 

 apatite are all present. 



Accessory minerals in the Westerly granites. — Accessory minerals of a less 

 common character have been recorded alread\^ in the Westerly granites 

 by several observers. Iddings and Cross* mentioned allanite in 1885. 

 The writer has also noted it in a hand specimen that attracted his atten- 

 tion because it was spotted with apparent blemishes or stains. The 

 discolorations spread from a dark nucleus, and in thin-section are a yel- 

 lowish decomposition product that has stained the neighbori-ng quartz 

 and feldspar. The dark nucleus was in one case 3'ellow and afforded^ 

 aggregate polarization ; in another it was pleochroic, dark brown to light 

 brown, biaxial, and of feeble polarization. All these colors correspond 

 with allanite, but as all petrographers familiar with allanite know, in its 

 altered condition it is a very unsatisfactory subject. Li a slide from Mill- 

 stone point zonal allanite was noted exactly like that figured by Keyes,t 

 but not associated with epidote. In 1891 0. A. Derby ;{: described mona- 

 zite in heavy concentrates from a specimen of the Westerly red variety 

 that was given him by the writer. In a thin-section of a specimen of 

 gray granite from the Miller quarry, Westerly, and near its contact with 

 the gneiss, the writer has met a yellow, doubly refracting mineral which 

 gives a positive uniaxial figure and which is intimately associated with 



*J. p. Iddings and Wliitman Cross: Widespread oecurrenoe of allanite as an accessory con- 

 stituent in many roclcs. Amer. Jour. Sci., Aug., 1885, p. 108. 



fC. R. Keyes : The origin and relations of central Maryland granites. XV .\nn. Rep. Director 

 U. S. Geo!. Survey, 1895, plate xxxviii, fig. 4. 



J O. A. Derby: Occurrence of xenotime as an accessory element in rocks. Amer. Jour. Sci., 

 April, 1891, p. 311. 



