40 ANNALS NEW YORK ACADEMY OF SCIENCES 



complete gradation from a very feldspathic diorite to one in which the 

 feldspar is distinctly subordinate to the ferromagnesian minerals; the 

 hornblendites, moreover, resemble diorites in their sudden changes in 

 grain, so that on the whole these two areas may best be regarded as basic 

 segregations. 



Pyroxenite 



Pyroxenites form the most basic group found in the Cortlandt area, 

 with the exception of an occasional specimen in which olivine runs over 

 one-third and which may thus be classed as peridotite. Williams classifies 

 all rocks containing olivine as peridotite and devotes the whole of his first 

 paper to this group; most of them are considered by the present writer 

 under the heading "Olivine Pyroxenite/' below. The hornblende pyrox- 

 enites form a distinct group which Williams does not consider at all, 

 per se. He merely mentions, moreover, the pyroxenites proper, which, as 

 will be seen from the map, constitute nearly one-fourth of the whole 

 series, and are thus the most important single rock type of all. They 

 may be considered first. 



Under this head are included all those rocks which consist almost en- 

 tirely of pyroxene, whether it be monoclinic or orthorhombic. As a 

 matter of fact, nearly all of them contain both of these minerals, and only 

 occasionally would there be any warrant for calling a -specimen augite 

 rock or hypersthenite. Enstatite is quite common, moreover, and in one 

 case what is probably hedenbergite was found, so that a multiplicity of 

 names may be avoided by considering the pyroxenites as one group ; and 

 the field distribution seems to indicate that this is a rational procedure. 

 The pyroxenites may be conveniently described under two heads, accord- 

 ing as the color of the monoclinic pyroxene in thin section is pink or 

 white, or green. 



White pyroxenite, in which the augite is a light pinkish or greenish 

 white, is by far the commoner variety. It occurs as mapped on Montrose 

 Point (although the numerous other rocks which interpenetrate there are 

 not indicated) and constitutes the bulk of the large area to the east. The 

 patches of pyroxenite at Buchanan and just west and south of Montrose 

 are also of this variety, 31 and it was only in the first two of these that the 

 pinkish augite was noticed. By the entrance of olivine, it becomes the 

 olivine pyroxenite of the series; and it may be noted here that while the 

 areas in which the olivine is segregated are probably fairly definite, they 

 are at the same time probably not as well defined as depicted on the map. 



11 The "fine black rock" which Dana describes as usually associated with the "limestone 

 areas" is merely a fine pyroxenite. 



