G. H. Williams — Peridotites near Peekskill, JV. Y. 37 



These have the characteristic color and dichroism of biotite and 

 sometimes contain acicnlar inclusions resembling the needles of 

 rutile described by the writer in biotite from the Black Forest.* 



Feldspar though frequently accessory, is never an important 

 constituent of these rocks. 



Magnetite, aside from composing the inclusions in the olivine 

 above referred to, forms small grains which line the cracks in 

 this mineral, and are especially abundant about its edge, where 

 it is in contact with the brown hornblende. The large opaque 

 grains scattered through the rock are almost all pyrrhotile, 

 (magnetic pyrites (Fe 7 S 8 )) ; chromite or picotite were not ob- 

 served ; another form of spinel however, pleonast, recognized 

 by its dark grer.n color and isotropic character, is not uncom- 

 mon. This mineral is filled with thin opaque plates almost 

 exactly like the inclusions in the well-known hercynite from 

 Eonsperg in Bohemia. Apatite was hardly ever observed. 



2. Augite Peridotite. 

 Picrite (Tschermak).f 



The true picrites of the Cortlandt Series are very closely 

 related to the hornblende- peridotites just described. They are 

 connected by a complete series of transitional stages in which 

 the amount of hornblende becomes less and less, while a non- 

 pleochroic augite, which under the microscope appears nearly 

 colorless, is developed in proportion. 



The best locality for the most typical picrite is near the east- 

 ern bank of the river on the south side of Montrose Point. 

 (No. 62). In this rock the brown hornblende, although still 

 present in the form above described, is reduced both in amount 

 and in the size of the individuals, while the augite reaches its 

 maximum development. Montrose Point rises on the western 

 side of the basin in which most of the Crugers brick-yards are 

 situated, as a rather abrupt rocky wall. This is generally 

 covered with a reddish, earthy deposit, due to the oxidation of 

 the iron in the basic minerals of which it is composed. The 

 rock, of which very fresh and unaltered specimens may be ob- 

 tained near the river bank, is of a dark green color and of an 

 even grain of medium coarseness. When examined with the 

 unaided eye its most prominent feature is the glistening, black 

 cleavage surfaces of hornblende. Small grains of magnetic 

 pyrites and reddish spots indicating the former presence of an 

 olivine crystal are abundant. 



In a thin section under the microscope the hornblende, in 

 spite of its prominence in the hand-specimen, is seen to be sub- 



*Neues Jahrbuch fur Min., Geol. mid Pal., II. Beilage— Bd., p. 617. 

 f Sitzungsber. d. k. Akad. in "Wien., 1866. Bd. xl, p. 113. 



