38 G. H. Williams — Peridotites near Peekskill, iV. Y. 



ordinate to the -pyroxene. This mineral is chiefly represented 

 by a nearly colorless augite, which frequently assumes the 

 habit of diallage by the development of a pronounced part- 

 ing parallel to the orthopinacoid. In spite of this, however, 

 the inclusions so characteristic of diallage are, in all of this 

 monoclinic pyroxene, almost wholly absent. This mineral can 

 of course show no pleochroism when it is so nearly colorless. 

 Twinning lamellae parallel to the orthopinacoid are frequent, 

 their boundary being often visible in ordinary light as a sharp 

 line. The extinction angle in prismatic sections is in some 

 instances as great as 40°. Cleavage fragments parallel to the 

 orthopinacoid show in converged polarized light a single opti- 

 cal axis which remains nearly stationary in the field when the 

 stage of the microscope is revolved. 



Hypersthene, in all respects identical with that described as 

 occurring in the hornblende peridotites, is common, but in no 

 instance is it as abundant as the diallage. Yery interesting in-, 

 stances of the parallel growth of these two minerals were ob- 

 served where the orthopinacoids of both lie in the same plane. 

 In some cases no line of demarcation could be seen between 

 them, although the pleochroism of the hypersthene easily dis- 

 tinguished it from the diallage. A crystal of the latter mineral 

 looks as though it had become red and pleochroic at one extrem- 

 ity or the other; but between crossed nicols the orthorhombic 

 character of this pleochroic portion is very apparent and in 

 great contrast with the high extinction angle of the diallage. 



The hornblende is likewise quite identical with that already 

 described. It is, however, much reduced in amount and in the 

 size of the individuals. In the abundance of its inclusions it 

 forms a contrast to both of the pyroxenic constituents. 



The olivine shows no peculiarities which have not been 

 already noted. It is not present in large amount but forms 

 comparatively sharply defined crystals, scattered at intervals 

 among the other constituents. It is often extremely fresh but 

 in other cases completely changed to a yellow, isotropic ser- 

 pentine. 



Considerable magnetite is readily extracted from the powder 

 of this rock with a magnet. Neither apatite nor feldspar were 

 observed in the sections of the Montrose Point rock. 



Another rock almost identical with the one just described 

 from Montrose Point was collected about 135 yards west of 

 Munger's near Montrose station on theK Y. C. P. P. (No. 46.)* 



Still another specimen (No. 54), which belongs to this type 

 was obtained near the house of Mr. Butler, on the road leading 

 from Montrose Point to Montrose Station. It represents a 



* Mentioned by Prof. Dana in this Journal, Sept., 1880, at the foot of page 217. 



