DISTRIBUTION AND PETROGRAPHY OP THE GRANITES 367 



tains numerous garnets. The Leets Island quarries, north of the station, 

 supply a red granite, as do those at Sachems head. An exposure of the 

 former, with basic inclusions, is shown on plate 37, figure 2. Gneissoid 

 varieties of the same are obtained on Hoadlys point, south of the station 

 at Leets island, and the workings are very extensive. The stone is, how- 

 ever, chiefly used for bridge piers. Some small outlying exposures near 

 Leets Island station yield gray granite. 



PETROGRAPHY 



General characteristics. — All the granites are biotite granites, muscovite, 

 though present, being very subordinate and hornblende failing entirely. 

 They will be described under these types : The Westerly gray, the W-est- 

 erly red, the Stony Creek red, the Stony Creek gray, the Lyme pink. A 

 brief mention will be made of the Quonochontaug orbicular variety. 



The Westerly gray granite.— The Westerly gray variety consists of an- 

 hedra of an average size of from 0.5 to 1.0 millimeter. The principal 

 minerals are quartz, orthoclase, microcline, plagioclase near oligoclase, 

 brown biotite, a little muscovite, magnetite, zircon, apatite, and some un- 

 common accessories. The ones mentioned are, except the last, the normal 

 minerals in granite, and they are so familiar that extended details would 

 be the iteration of elementary phenomena. The quartz has abundant 

 needles of rutile and cavities with bubbles. It contains zircons which 

 show the combinations I and 11. The quartz is sometimes included in 

 the feldspar and exhibits tendencies to develop micropegmatites with it. 

 The period of formation of the quartz clearly began before that of the feld- 

 spar closed. The orthoclase is sometimes kaolinized and is again water- 

 clear. Zonal growth and Carlsbad twins are both present. Microcline 

 varies in quantit}'', but ma}^ be very abundant. Tlie plagioclase is inferior 

 in amount to the potash feldspars, and as a rule affords very low ex- 

 tinction angles in basal sections. These facts would indicate oligoclase, 

 and the presence of considerable lime in the analyses tends to corroborate 

 the inference. The biotite is deep brown and strongly pleochroic. It 

 bleaches to green on alteration, and has muscovite occasionally associated 

 with it. The biotite is almost always in irregular shreds, but instances 

 have been met in which it is drawn out into elongated parallelograms 

 with good crystallographic boundaries. Its relative abundance is shown 

 in plate 38, figure 1. The magnetite and apatite present no noteworthy 

 features. The other accessories, some of which, are of special interest, 

 will be referred to after mention has been made of the red variety of the 

 Westerly granite. 



The Millstone Point granite closely resembles the Westerly gray in 

 appearance, but it has less of a bluish cast. It is of about the same 



