GEOLOGY OF THE SCHROON LAKE QUADRANGLE 33 



Some of the more important objections to the view just expressed 

 are: (i) the anorthosite represents a separate and distinctly older 

 intrusion than the syenite-granite, and so the sheetlike arrange- 

 ment advocated by Bbwen is out of the question; (2) the Adi- 

 rondack anorthosite area is by no means practically free from 

 masses of syenite-granite, this being particularly true of the whole 

 northeastern half of the area where there are many large and 

 small bodies of syenite and granite in the form of real intrusives in 

 the anorthosite; and (3) it is not at all necessary to assume that 

 both syenite-granite and anorthosite were batholithic intrusions. 



An explanation offered by the writer to account for the absence 

 of Grenville and syenite-granite from so much of the anorthosite 

 area may be briefly stated as follows. The anorthosite is con- 

 sidered to be a laccolith not much greater across than the present 

 area of outcrop. Its intrusion was soon followed by a very irregu- 

 lar intrusion of the great body of generally rather highly fluid 

 syenite-granite magma. That the syenite-granite magma was 

 mostly rather highly fluid is proved by its great power to cross- 

 cut, intimately penetrate, break up and tilt the Grenville strata. 

 Only exceptionally did local portions of this magma invade the 

 Grenville strata in true laccolithic fashion. Both the anorthosite 

 and the syenite-granite are believed to have intruded a very thick 

 mass of essentially undisturbed Grenville strata, largely or alto- 

 gether free from orthogneiss. The southwestern half of the 

 anorthosite body, which is so free from masses of Grenville and 

 syenite-granite, is believed to represent the greatest bulk of the 

 anorthosite where the laccolithic magma was thickest and reached 

 its highest level. The northeastern half of the anorthosite as now 

 exposed is regarded as the portion where the anorthosite magma 

 spread out as a relatively much thinner layer whose surface was 

 at a notably lower level than that of the thicker portion to the 

 southwest (see figure 2). Because of the greater uplift of the 

 southwestern portion, the Grenville cover has there been almost, if 

 not completely, removed by erosion. But many areas of the Gren- 

 ville roof remain over the thinner northeastern part of the anortho- 

 site where the uplift was much less. Thus we have a smiple 

 explanation of the absence of the Grenville from so much of the 

 anorthosite area. After the solidification of the great body of 

 anorthosite, the syenite-granite magma was batholithically intruded 

 in a rather highly fluid state, and it tended to avoid penetration of 

 the anorthosite which was much more massive, homogeneous and 



