218 ANNALS NEW YORK AVADEMY OF SCIENCES 



lying strata wlieii this took place, as the alteration of hornblende into 

 biotite requires rather deep-seated conditions. ^^ 



Small stringers of pegmatitic material are fairly numerous in the liorn- 

 blende schist at this place. These usually folloAV the foliation of the 

 schist, although at times they also cut across it and occasionally widen 

 out into lenticular or irregular shaped masses. Associated with these 

 stringers are occasionally found lenticular masses of epidote schist evi- 

 dently derived by alteration from the hornblende schist. These are sel- 

 dom more than six inches wide and three or four feet long (PI. YlII, 

 Fig. 2). 



In thin section, this variety is seen to consist principally of epidote 

 associated with remnants of the unaltered dark green hornblende. Some 

 calcite is also present as a secondary product. Quartz appears both in 

 little irregular shaped grains distributed through the whole mass and also 

 in little stringers. The accessory constituents present are magnetite, 

 titanite and zircon. A chemical analysis of the epidote schist will be 

 found in a later paragraph. Such a rock is known as an epidosite. 



Attention has already been called to the sharp contacts between the 

 hornblende schist and mica schist. This is very noticeable wherever such 

 contacts are exposed. Thin sections of the hornblende schist and mica 

 schist where they adjoin were examined from two parallel hornblende 

 schist sheets occurring in the mica schist about one and one-quarter miles 

 northwest of Hartsdale along the road to Elmsford. The lower of these 

 sheets is about two and a half feet thick, while the upper one is much 

 thicker but is partially covered. Two and one-half feet of mica schist 

 separate the two. They are involved in a sharp anticline. 



The mica schist is a quartzitic variety at this place which has a 

 medium-grained crystalline texture and foliated structure. It consists 

 largely of quartz in irregular grains and usually elongated parallel to the 

 foliation; of a dark greenish brown biotite showing parallel orientation 

 and a little feldspar, mostly plagioclase; of an occasional small garnet, 

 and of a few rounded grains of zircon. The hornblende schist is a dark 

 greenish black rock with a more or less foliated texture. It consists prin- 

 cipally of a dark brownish green hornblende, together with feldspar and 

 a little quartz. Accessory constituents are magnetite, zircon, titanite and 

 zoisite. The hornblende shows marked pleochroism from light brown 

 through brownish green to dark green. The feldspar is 'mostly plagio- 

 cla.se, giving extinction angles up to 23° 30' in sections at right angles to 

 the albite lamellae, thereby indicating an andesine. 



Very little difference from the normal was noticeable in these two rocks 

 in the specimens taken from near the contacts. Occasionally biotite be- 



^C. R. Van Hise : Treatise on Metamorphism. U. S. Geol. Surv., Mon, 47, p. 290. 

 1904. 



