26 NEW YORK STATE MUSEUM 



1 2 



SiO. 54-47 53 -iS 



AlA 26.45 23.25 



Fe^Oa 1.30 1-53 



Feb 67 1.82 



MgO 69 2.60 



CaO " 10.80 II. 18 



Na^O 4.37 3-97 



K,0 92 .86 



H,0+ 53 1. 13 



cOo .34 



TiO„ .45 



S tr. 



P2O5 .09 



MnO .11 



No. I represents the analysis by A. R. Leeds of the rock from 

 Mt Marcy, and no. 2 represents the analysis by George Steiger 

 of the rock from Mt Whiteface. According to Kemp, no. i, in 

 the Quantitative Classification of Cross Iddings, Pirsson, and Wash- 

 ington, belongs in class i (Persalane), order 5 (Canadare), rang 

 4 (Labradorase), subrang 3 (Labradorose) ; and no. 2 falls in 

 class 2 (Dosalane), order 5 (Germanare), rang 4 (Hessase), sub- 

 rang 3 (Hessose). The mineral composition of a thin section of 

 rock from the summit of Mt Whiteface is shown by no. 14 of 

 table I on page 21. These two analyses are doubtless very repre- 

 sentative of the more common Marcy and Whiteface types of 

 anorthosites in the Lake Placid quadrangle, as judged by the micro- 

 scopic examination of various thin sections. The analyses show a 

 close similarity in the chemical composition of these two rock 

 types. Order, rang and subrang are the same for both, the dif- 

 ference in class no doubt being due to the somewhat greater per- 

 centage of ferro-n'iagnesian minerals in the Whiteface rock which 

 happened to be chosen for analysis. Such close similarity of chem- 

 ical composition strongly supports the idea that the Marcy and 

 Whiteface types of anorthosite represent differentiates of the 

 same magma. 



The rather high percentage of potash in rocks of this character 

 calls for explanation. The lack of such dark-colored minerals as 

 would furnish enough potash causes Kemp^ to think that orthoclasc 



*N. Y. State Mys. BuL 138, p. 30. 1910. 



