28 G. II. Williams — Peridotites near Peekskill, JV. T. 



of the little altered, ancient eruptive rocks of the United 

 States ; a knowledge necessary for any intelligent work on the 

 very interesting question regarding the metamorphism of erup- 

 tive masses which is now beginning to attract the attention of 

 geologists. 



In the present paper only such of the Cortlandt rocks will 

 be described as contain the mineral olivine. These are, for the 

 most part, destitute of any considerable quantity of feldspar 

 and belong therefore to the family of Peridotites ; in some 

 cases, however, by an increase in the amount of this constit- 

 uent, they pass gradually into olivine-norites, olivine-gabbros 

 and olivine-diorites. 



Cheysolitic Rocks. 

 Class I. — Peridotite (Rosenbusch.) 



This class includes all massive, noncrystalline rocks of a 

 granular structure, which are free from feldspar and contain, as 

 their most characteristic constituent, the mineral olivine. The 

 peridotites have been variously subdivided according to the 

 other minerals which they contain, but for the present purpose 

 it will be necessary to distinguish only two groups, in one of 

 which hornblende, in the other pyroxene, is the most important 

 ingredient. It is not to be understood that the presence of one 

 of these minerals in the least excludes the other; both are al- 

 ways present, but in such varying proportions that it will be 

 advantageous to designate which of the two, in a given case, 

 plays the principal role. Both of these groups of peridotite 

 occurring in the Cortlandt Series, are, in some respects, quite 

 different from any which have been heretofore described. 

 They everywhere grade into one another, and into their corre- 

 sponding feldspathic equivalents. 



Of all the rocks occurring in the earth's crust none are so 

 subject to alteration as those composed largely of olivine. 

 Nothing, therefore, can be better calculated to give an idea of 

 the wonderful freshness of all the rocks near Peekskill than 

 the fact that this very mineral is here frequently in an almost 

 unchanged condition. The peridotites weather superficially 

 into a reddish brown soil, but specimens. taken from a short dis- 

 tance below the surface show hardly more than the beginnings 

 of alteration. The frequent polished and striated rock surfaces 

 met with indicate that the great glacier was probably instru- 

 mental in ploughing off and removing the more decomposed 

 material, thus exposing the fresher rock below.* 



The distribution of the peridotites within the Cortlandt area 

 is not a very extensive one. They are principally confined to 



*The freshness of these rocks is doubtless largely due to the relatively small 

 amount of olivine which they contain. This, as will be shown in the sequel, is 

 much less than that usually found in typical peridotites. 



