ROGERS, GEOLOGY OF THE CORTLANDT SERIES 43 



Some of the larger grains show almost colorless centers. The cleavage is 

 always coarse, and there is little suggestion of a diallage parting ; in basal 

 section, the cleavages intersect at 87°. p > v. It appears from these 

 characters that the mineral is a member of the diopside series, probably 

 saiite or hedenbergite, or possibly (from the pleochroism) a lime dial- 

 lage. Titanite is scattered through the rock rather plentifully in small 

 wedge-shaped grains. Very small grains of what is probably wernerite 

 are not uncommon, and there is a very little quartz. Opaque minerals 

 are almost lacking. 



The peculiar combination of minerals in this rock indicates that its 

 relation with the associated limestone is very close. All of them, except 

 the very small amount of quartz, are lime minerals, and all of them may 

 be contact minerals. There is little doubt that the rock represents a clear 

 case of contact between limestone and probably diorite; and similar, 

 though entirely abnormal, mixtures are not uncommon in the district and 

 will be described below. 



Hornblende Pyroxenite 



Pyroxenites containing sufficient amounts of hornblende to place them 

 in this class are most common on Montrose Point, although two other 

 small areas exist : one just south of Salt Hill, and the other two miles 

 north of the first. AYilliams describes this class only in their peridotitic 

 phase ; they are then most abundant on Stony Point, though known in the 

 extreme eastern end of the main area. The hornblende pyroxenites have 

 originated in all the cases examined by local increase of hornblende in the 

 typical pyroxenite described above, although conceivably a partial para- 

 morphism of the pyroxene would give rise to a similar rock. 



These rocks cannot be distinguished in the field unless, as is often the 

 case, the hornblende occurs in very large individuals with the other min- 

 erals imbedded in it. This is the type development of the poikilitic struc- 

 ture which led Williams to coin the term. The hornblendes may reach 

 three or four inches in width, and their glistening black surfaces, inter- 

 rupted by small included grains, is an unmistakable feature. On Mon- 

 trose Point, however, this structure is often lacking entirely, the horn- 

 blende having crystallized simultaneously with the other minerals. 



Under the microscope, the proportions of the various minerals are seen 

 to vary considerably, according to the degree of completeness which the 

 segregation of hornblende has attained. The typical greenish gray augite 

 is always present and may constitute as much as two thirds of the rock. 



Hypersthene. often in the beautifully pleochroic clear rounded grains, 



