504 H. P. CUSHING 



cover locally persist. 1 My view is that a complex of Grenville, 

 resting on orthogneiss, existed in the region at the time of the 

 intrusion of the anorthosite-syenite group, that much of this 

 orthogneiss still remains in the region, and that the later intrusives 

 broke through this complex in separate masses, instead of forming 

 one great body. 2 Obviously the presence of a great laccolith 

 constituting the entire region, such as Bowen postulates, is much 

 more possible under the former view than under the latter. 



It is quite true that many of the syenite masses are much mingled 

 with Grenville and that the anorthosite area contrasts rather sharply 

 in this respect, as Bowen contends. And I am quite in accord with 

 his view that there has been deeper erosion in the eastern Adiron- 

 dacks, where the anorthosite occurs, than there has been to the 

 west and south, where the syenite bodies occur, and have repeat- 

 edly so stated. Nor am I particularly disposed to quarrel with the 

 view that the anorthosite mass may be laccolithic instead of batho- 

 lithic in structure. I do not know which it is. The mass has 

 certainly great thickness, since the climb up Mt. Marcy furnishes 

 a 3,500-foot section of pretty clean anorthosite, with no particular 

 indication that the entire thickness may not be vastly greater; 

 then allowance must be made for at least an equal thickness of 

 gabbro and pyroxnite underneath and for an unknown thickness 

 of overlying syenite, since eroded away. Nevertheless, a sheet 

 structure is entirely possible. 



Differentiation of the anorthosite body. — If I have correctly under- 

 stood Dr. Bowen's interpretation of the structure of the region — a 

 sheetlike igneous mass, composed of probable gabbro below, then 

 anorthosite, and finally a cover of syenite and granite, the cover 

 full of fragments from the Grenville roof, and with a certain 

 amount of disturbance occurring during the freezing of the mass, 

 whereby liquid syenite is brought into lateral contact with solid 

 anorthosite — his argument seems to me to imply, or to require, that 

 this sheet was at least equal in size to the present pre-Cambrian 

 area of northern New York and that anorthosite must everywhere 

 underlie syenite. The field evidence, however, seems to me to 



1 Bull. Geol. Soc. Am., XXV, 243-64. 



2 Am. Jour. Set., XXXIX, 288-94. 



