40 



G. II WiUiams — Peridotites near PeekskiU, N. Y. 



and St. 6 (D) belong to this class of picrites. A chemical analy- 

 sis of the rock (No. 62) from the south side of Montrose Point 

 was made bj' Mr. W. H. Emerson in the laboratory of the Johns 

 Hopkins University with the following result : — 



SiO„ 47-41 



A1 2 3 -.. 6-39 



Fe,Q 3 V'06 



FeO 4-80 



CaO 14-32 



MgO 15-34 



Na 2 -69 (determined by difference) 



K„0 1-40 



h;o 2-io 



S J -49 



Specific gravity = 3-30 at 15' 



100-00 

 C. 



T The sulphur may be referred to pyrrhotite. The high specific 

 gravity and the percentage of ferric iron present indicate a con- 

 siderable amount of magnetite. The almost equal quantities of 

 lime and magnesia prove that neither hypersthene nor olivine 

 can be largely represented among the constituents. The silica 

 will be seen at once to be too high for a typical peridotite. In 

 fact all the most basic rocks of the Cortlandt Series which have 

 come under the writer's notice are too acid to be classed as 

 representative oli vine-rocks. This manifests itself, as in the 

 rocks here described, in the relatively small amount of olivine 

 associated with the pyroxenic constituents or in their tendency 

 to develop feldspar and so form transitions to olivine-gabbros 

 and norites. 



A rock, which is quite common on the northern side of Stony 

 Point and also to be met with at several localities in the town- 

 ship of Cortlandt, possesses a composition intermediate between 



the hornblende-peridotite and the 

 picrite. The groundmass of this 

 rock is a moderately fine-grained, 

 gray colored aggregate composed 

 principally of diallage with some 

 hypersthene and olivine. It corre- 

 sponds quite closely to the picrite. 

 Imbedded in this are sharply formed 

 crystals of black hornblende from 

 one to two centimeters in diameter. 

 These are short prismatic in shape, 

 being terminated at both ends by low 

 dome faces so as to give cross-sections as shown in the figure. 

 They are porphyritically scattered through the gray groundmass, 



