416 W. J. MILLER ADIRONDACK ANORTHOSITE 



certainly to be regarded as a border development of the great anorthosite 

 body^ as shown by its finer grained textnre, perfect gradation into the 

 Marcy anorthosite^ and its very close similarity to definitely kno-vvn border 

 facies in other quadrangles. 



I have also observed Whiteface anorthosite as a border facies of the 

 Marcy anorthosite in both the Newcomb and Ansable quadrangles, but 

 detailed surveys of these have not yet been made. 



Kemp's map of the Elizabethtown and Port Henry quadrangles shows 

 a very definite border development about the large mass of anorthosite 

 there exposed. This border development he maps as ^^asic anorthosite 

 and related types.'^ Judging by his description, much of it is Whiteface 

 anorthosite, though more gabbroid than that of the Lake Placid quad- 

 rangle. This border zone is from one-half to 2 miles wide. Locally it 

 has been cut into or completely cut out by syenite. In the western part 

 of the town of Moriah the geologic map shows two small areas of the 

 border facies completely surrounded by syenite and separated 1 and 2 

 miles, respectively, from the continuous border. I interpret these as 

 simply remnants of the once much more extensive border facies w^hich 

 became enveloped in the invading syenite magma. It is therefore not 

 unreasonable to consider a good many square miles of the border facies 

 to have been cut out or displaced by the big body of syenite shown on the 

 map. It is very clear from Kemp's description and map that no syenite 

 exists as a rock intermediate between the border facies and the typical 

 anorthosite, but rather the border rock grades into the Marcy anorthosite. 



Dr. Ogilvie^^ says, regarding the anorthosite of the Paradox Lake 

 quadrangle, that "the dark silicates are more abundant in the peripheral 

 portion of the intrusion," and that on one hill near the border a very 

 white variety of anorthosite occurs, though no details are given and such 

 rocks are not separately represented on the geologic map. But, judging 

 by the prominence of the border facies in both the adjacent Schroon Lake 

 and Elizabethtown quadrangles, such border types are probably well rep- 

 resented in the Paradox Lake quadrangle. 



Daly,^^ after considering a number of anorthosite areas, states that 

 "the greater anorthosite bodies usually have a well developed contact 

 phase of more or less typical gabbro or norite. In the interior each mass 

 becomes rapidly more feldspathic, and for most of the outcrop the rock is 

 monotonous anorthosite." But he makes no mention of any development 

 of syenite between any border (contact) facies and true anorthosite, such 

 as is required by Bowen's hypothesis. 



25 1. H. Ogilvie : N. Y. State Mus. Bull. 96, 1905, p. 499. 



26 R, A. Daly : Igneous Rocks and Their Origin, 1914, p. 327. 



