GEOLOGY OF THE SCHROON LAKE QUADRANGLE 



35 



The writer believes the Adirondack anortliosite (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. 



Probably aabbro or pendatiie 

 '^^^^^^enife.-qraniie, series 

 '^Chilled barclerof anorfhosiife Line ABz present erosion surface. 



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 facies which developed as an Upper 

 as well as an outer margin resting directly upon and against tljie 

 Marcy anorthosite; failure to find masses of Grenville farther 

 down in the body of the anorthosite than just below the level of tl^e 

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



