FETTKE, MANHATTAN -b'C///-8T OF NEW YORK 335 



Farther south along the same railroad, just north of Springdale, an- 

 other large cut has been made in this same granite. The granite has a 

 coarse pegmatitic texture in places, although much of it remains normal, 

 medium-grained granitoid. The gradation from one into the other is a 

 gradual one. It contains several inclusions of a basic igneous rock. 

 These are usually massive and have a coarse crystalline texture and dark 

 green color. A thin section made from one of them shows a granitoid 

 texture and consists of green hornblende with good prismatic cleavage 

 and deep brown biotite. A little titanite is present as an accessory con- 

 stituent. The space formerly occupied by feldspar is now filled with an 

 aggregate of calcite, quartz and other secondary products. 



The gneissoid phase of the Thomaston granite is well shown in the 

 vicinity of the Stamford reservoir, south of North Stamford. The lo- 

 cality is near the western border of the New Canaan mass and inclusions 

 of schist are a prominent feature. It is to these that the banded structure 

 of the gneiss is partially due. 



Several smaller bosses of a similar granite occur to the west. One of 

 the largest is just west of Pelhamville in the vicinity of Mount Vernon. 

 The rock has a light gray color with a distinctly gneissoid structure and 

 medium-grained crystalline texture. Under the microscope in thin sec- 

 tion, it is granitoid and consists of microcline, quartz, orthoclase, some 

 biotite and muscovite and a little plagioclase. Apatite is the principal 

 accessory constituent. 



Aplites and Pegmatites 



Closely related genetically to the granites just described are a large 

 number of intrusive sheets and dikes varying in texture from medium 

 granitoid to very coarsely pegmatitic. Of these the intrusive sheets and 

 lenticular masses injected parallel to the foliation of the mica schist are 

 the most abundant. They appear in nearly every outcrop of the schist. 

 Sometimes the injections are so numerous that the schist takes on a 

 gneissoid appearance and becomes an injected gneiss (PL X, Fig. 1). 

 They vary greatly in size from sheets 50 feet or more in thickness to those 

 less than an inch thick. The same is true of the lenticular masses. The 

 intrusives which are parallel to the foliation of the mica schist are often 

 involved in all the intricate folds which have been developed in the latter. 



The dikes of granite and pegmatite, on the other hand, cut across the 

 foliation. They also cut the intrusive sheets and lenses and likewise each 

 other, showing that they did not all enter at one time but that some are 

 later than others. They also vary greatly in size. In some cases, as at 

 Bedford Village, they reach a width of over two hundred feet, while in 

 other cases they only have a thickness of a fraction of an inch. 



