ROGERS, GEOLOGY OF THE CORTLANDT SERIES 41 



All of the important emery developments are located in the eastern 

 pyroxenite area, although a local development of a more acid rock often 

 constitutes the immediate wall rock. 



These rocks cannot be distinguished megascopically from those carrying 

 small amounts of olivine, but their general identity as pyroxenites is at 

 once apparent. When fairly fresh, they are dull black; upon moderate 

 alteration, they assume, in many cases, a reddish brown color, and 

 strangely enough when badly altered, they become gray and look like an 

 entirely fresh rock. The reddish rock is common in the region around 

 Salt and Dickerson Hills; the gray, farther to the east. They always 

 retain their toughness, but the rock, especially when it contains olivine, 

 will often disintegrate under the hammer, rather than break. That the 

 rock is the most resistant in the series is evidenced by the fact that the 

 two highest hills in the county (Dickerson and Salt Hills) are composed 

 of it. Very perfect jointing is often developed in the pyroxenite. The 

 grain varies greatly but is generally medium fine. In the district just 

 west of Dickerson Hill, however, it becomes very coarse, containing crys- 

 tals of hypersthene and diallage one inch or more long and frequently 

 masses as large as one's fist ; and this coarse facies occurs in other places 

 also. 



Under the microscope, it is seen that the bulk of the rock is generally 

 made up of a light colored pyroxene, which is probably augite (fassaite). 

 It is a uniform light greenish gray in color, with extinction of 42°-45° 

 in the vertical zone (in diopside Zac = 38°). The alteration, moreover, 

 is usually to antigorite or to brown hornblende or chlorite, so that too 

 much FeO is indicated for a diopside. Furthermore, the analyses indi- 

 cate a mineral carrying 2 to 4 per cent. A1 2 3 , so that it seems to be an 

 augite. The mineral sometimes occurs in idiomorphic grains, but it is 

 ordinarily in an entirely allotriomorphic condition. The grains are some- 

 times clear and coarse, and sometimes exhibit the fine parting of diallage 

 (generally parallel to 100) in great perfection. They often carry a 

 notable amount of fine black rod-like inclusions, which are very different 

 from the inclusions in the green augite of the norites, although occa- 

 sionally the latter are found and constitute a typical diallage. Simple 

 and polysynthetic twinning are common, and the latter especially reaches 

 a high degree of perfection in these rocks. The other and much less 

 common variety is similar in every respect excepting color; this is a 

 light brownish pink, with very faint pleochroism. Its color is very pos- 

 sibly due to Ti0 2 , but the rock containing this mineral appears to be 

 intimately associated with the normal variety. The alteration of the 

 augite is almost always perceptible and often far advanced ; the common 



