234 ANNALS NEW YORK ACADEMY OF SCIENCES 



sists essentially of plagioelase feldspar and hornblende, with magnetite, 

 apatite and a very little biotite as accessory constituents. The plagio- 

 elase gives extinction angles up to 28° in sections at right angles to the 

 albite lamellae and is apparently an acid labradorite. The hornblende is 

 a green variety occasionally showing typical prismatic cleavage. It oc- 

 curs in small grains between the feldspar laths, and these are frequently 

 encroached upon by it so that the crystallographic boundaries of the 

 feldspar are not clean-cut. This would suggest that the space between 

 the latter might originally have been occupied by augite which had after- 

 wards altered to hornblende. No traces, however, of augite were noticed 

 in the section, and the hornblende in all other respects has the appearance 

 of being primary. 



ACIDIC INTRUSIVES 



In addition to the basic intrusives already discussed, there are other 

 types which are of granitic composition varying from true granites to 

 very coarse pegmatites and occurring as dikes, intrusive sheets and len- 

 ticular masses injected parallel to the foliation of the schist. The sheets 

 and lenticular masses are far more abundant than the dikes. They ap- 

 pear in one form or another in nearly every outcrop of mica schist. 

 Large masses of granite in bosses and batholiths outcrop, especially in 

 Connecticut just beyond the New York line, where they become quite 

 abundant. The Connecticut Geological Survey has given them the name 

 Thomaston*^ granite. 



Thomaston Granite 



As typically developed, the Thomaston is a light colored biotite granite 

 of medium to fine grain. It consists essentially of feldspar, quartz, 

 biotite and muscovite. At many places, it shows practically no gneissic 

 structure, but at other places is quite strongly foliated. 



The granite covers a large area in the vicinity of New Canaan. It is 

 well exposed in a railroad cut about one-half mile south of New Canaan 

 on the New Canaan branch of the New York, New Haven and Hartford 

 Eailroad. It is a light pink massive granite with medium-grained text- 

 ure. When examined in thin section under the microscope, it shows a 

 granitoid texture and consists of microcline and a little plagioelase. A 

 perthitic intergrowth of orthoclase and plagioelase may be occasionally 

 noticed. Apatite and zircon are present as accessory constituents. Some 

 of the feldspar has undergone slight alteration to kaolin and sericite, 

 while a little chlorite is developed on some of the biotite. 



*2 Preliminary geological map of ('onnecticut. Conn. Geol. and Nat. Hist. Surv. Bull. 

 No. 7. 1907. 



