406 AV. J. MILLER ADIRONDACK AXORTHOSITE 



facies which is distinctl}' more gabbroid, more gneissoid, and finer grained 

 than the typical Marcy anortliosite, and wliich usually is characterized by 

 light gray or white instead of dark bluish gray feldspar. Further, both 

 the border (Whiteface) and Marcy types of the anorthosite are quite cer- 

 tainly differentiation phases of the same cooling magma, the former no 

 doubt representing a chilled border portion. The one type grades into 

 the other and nowhere has one l)een found to definitely intrude the other. 



CHEMICAL COMPOSITtOy 



Analyses of both the Marcy and Whitef ace types of anorthosite have 

 been made for and described by Professor Kemp. They are as follows : 



SiO, Al,03 Fe^Og FeO MgO CaO Na,0 K.O YiS>+ CO^ TiOo S r,0- MnO 



1.. 54.47 26.45 1.30 .67 .69 10.80 4.37 .92 .53 



2.. 53.18 23.25 1.53 1.82 2.60 1L18 3.97 .86 1.13 .34 .45 tr. .09 .11 



Xumber 1 represents an analysis by A. E. Leeds of the rock from the 

 summit of Mount Marcy. In the quantitative classification it belongs in 

 Class 1 (Persalane), order 5 (Canadare), rang 4 (Labradorase), sub- 

 rang 3 (Labradorose). Xumber 2 represents an analysis by George 

 Steiger of the rock from the summit of Mount ^Vliiteface. In the quan- 

 titative classification it falls in Class 2 (Dosalane), order 5 (Germanare), 

 rang 4 (Hessase), subrang 3 (Hessose). These two analyses are doubt- 

 less very representative of the more common ^larcy and Whiteface types 

 of anorthosite. They show close similarity in chemical composition. 

 Order, rang, and subrang are the same for both, the difference in class no 

 doubt being due to the somewhat greater percentage of femic minerals in 

 the Whiteface rock. 



The rather high percentage of potash in rocks of this character calls 

 for explanation. Lack of such dark minerals as would furnish enough 

 potash causes Kemp° to think that orthoclase must be present up to 5 per 

 cent or more as untwinned feldspar. Gushing,^" however, says : "The 

 potash is in the labradorite (or other plagioclase), replacing a certain 

 amount of soda,^" and "that analyses of this feldspar always show it.'' If 

 potash feldspar is present I have been unable to demonstrate it in the 

 thin sections which I have examined. 



BowEx's Hypothesis regardixg Origix of Axorthosite 



Since Bowen has elaborated an hypothesis to account for the structure 

 and origin of anorthosites in general, but with considerable attention to 



9 .1. F. Kemp : X. Y. State Mus. BuU. 138, 1910. p. 80. 



10 H. r. dishing: X. Y. State Mus. Bull. 95, 1905, p. 335. 



