400 W. J. MILLER ADIRONDACK AXORTHOSITE 



Page 



Occurrence near Keene village • -138 



Area near Upper Jay 441 



Sentinel Range area 441 



Sunrise Notch area 442 



Area west of East Kilns 442 



Some occurrences in the Schroon Lake quadrangle 443 



Keene gneiss of other Adirondack regions 445 



Significance of the distribution of the Keene gneiss 447 



Bowen's suggestion of possible origin of some Keene gneiss by assimi- 

 lation 450 



Significance of the distribution of f emic minerals 450 



Significance of the thickness of the Keene gneiss 451 



General absence of Grenville and syenite-granite fi'om the anorthosite area 451 

 Origin of anorthosite by diflrerentiatiou in a laccolith of gabbroid magma. 455 



Laccolithic structure of the anorthosite 455 



Probable origin of the anorthosite by settling of f emic constituents . . 456 



Origin of variations in the anorthosite 460 



Summary of conclusions 461 



IXTEODUCTIOX 



This paper takes wp the whole problem of the structure, relations, and 

 origin of the great body of anorthosite in the Adirondack region. Refer- 

 ences are made to anorthosites of other well known regions, and it is 

 hoped that some important light may be thrown on the problem of the 

 anorthosites in general. Particular attention is given to Bowen's recent 

 paper,^ '"The problem of the anorthosites," in which he elaborates an 

 hA-pothesis regarding the structure and origin of anorthosite with special 

 reference to that of the Adirondacks. I regard this paper as a very im- 

 portant contribution to the subject, because he has called attention to 

 many important features hitherto either overlooked or not sufficiently 

 emphasized. But, as a result of more than six months of field-work 

 within and close to the Adirondack anorthosite area and much laboratory 

 work, I find Bowen's hypothesis untenable. 



Still more recently, by way of discussion of Bowen's paper, two short 

 articles by Gushing- and one by Bowen-^ have appeared. I also consider 

 the main points of these papers. 



In the perusal of the papers Just referred to. one is repeatedly struck 

 with the meagerness of evidence based on actual field facts. Up to the 

 time of the appearance of Bowen's paper, "The problem of the anortho- 



1 X. L. Bowen : Jour. Geol.. vol. 25, 1917. pp. 209-243. 



2H. P. Gushing: .lour. Geol.. vol. 25. 1917. pp. 501-509 and 512-514. 



3X. L. Bowen: Jour. Geol.. vol, 25. 1917. pp. 509-512. 



