226 N. L. BOW EN 



geology by thrusting the locus of differentiation ever backward into 

 unseen depths. The Adirondack intrusives would, it is felt, be 

 interpreted by many in the conventional manner, and for this 

 reason some pains have been taken to present the alternative 

 view. 



Daly regards the Adirondack anorthosite-syenite complex as 

 probably a laccolith. According to his views anorthosites are 

 formed in laccoliths because those masses suffer little contamina- 

 tion from wall-rock material and anorthosite is a pure differentiate 

 of gabbro magma. A certain amount of assimilation of wall rock 

 can occur, however, without eliminating the possibility of the for- 

 mation of anorthosite, and under such circumstances the syntectic 

 magma differentiates in such a manner that syenite is formed. In 

 batholiths, on the contrary, whose emplacement takes place by 

 stoping, the consequent assimilation has so important an effect on 

 the gabbroic magma that no anorthosite is formed, according to 

 Daly. The writer's interpretation of the Adirondack complex as a 

 stratified, sheetlike mass with a lower layer of anorthosite and an 

 upper layer of syenite intimately associated with the Grenville 

 sediments is therefore in striking agreement with Daly's conception. 

 However, a consideration of crystallizing magmas in the light of 

 experimental study compels the writer to believe that there would 

 exist in association with the anorthosite a mass of syenite, even if 

 the invaded rocks were of infinitely refractory and inert materials. 

 The mass of syenite was probably augmented by assimilation of 

 foreign rocks, but that is a different matter. Apparently this 

 opinion is in accord with the conclusions of those best acquainted 

 with the Adirondack rocks in the field who, while demonstrating 

 that assimilation takes place, consider it rather as an incident than 

 as a fundamental factor controlling the genesis of rock types. And, 

 again, the writer's interpretation of the igneous mass as sheetlike 

 is offered merely because of the difficulty of picturing the general 

 relations otherwise. Indeed, it is not considered that the Adiron- 

 dack batholith or laccolith or whatever it may be called, is excep- 

 tional in this respect. Most batholiths are regarded by the writer 

 as just such masses. Consequently it is believed that the shape of 

 the intrusive is not the determining factor in the formation of 



