GEOLOGY OF THE SCHROON LAKE QUADRANGLE 



35 



The writer believes the Adirondack anorthosite (not necessarily 

 the anorthosite as such) was intruded essentially laccolithically, 

 and the syenite-granite was intruded essentially batholithically. 

 But it is not at all necessary to assume, as does Bowen, that both 

 great bodies are batholithic if they are regarded as distinctly sepa- 

 rate intrusions. 



Probabfy cjabbro or pencfoiHe. 



series 

 Line A B- present erosion surface. 



(Brenuille. strata 

 Marcy anorthosrte 

 BChil/ed border of anorfhosite 



Fig. 2 Highly generalized northwest-southwest structure section through 

 the Adirondack anorthosite 'body, showing the relation of the anorthosite to 

 the Grenville and syenite-granite series 



Positive proof for the laccolithic structure of the Adirondack 

 anorthosite can not be won from a study of its relation to the 

 intruded Grenville strata. In the first place, only a very few 

 (usually small) areas of Grenville are known to lie against the 

 borders of the anorthosite because the Grenville has been so exten- 

 sively cut out by the syenite-granite, and these few contacts are 

 almost all concealed under Pleistocene deposits. In the second 

 place, such Grenville strata were more or less disturbed a second 

 time by the later syenite-granite intrusion. 



Many of the most important field facts best harmonize with the 

 conception of a laccolithic structure of the Adirondack anortho- 

 site. Among these facts which have already been discussed, are the 

 following : The chilled border f acies which developed as an upper 

 as well as an "outer margin resting directly upon and against the 

 Marcy anorthosite; failure to find masses of Grenville farther 

 down in the body of the anorthosite than just below the level 'of the 

 inner margin of the chilled border, thus indicating the power of the 



