THE PROBLEM OF THE ANORTHOSITES 229 



ently a counterpart of the anorthosite-syenite association in the 

 Adirondacks. 



While the writer has nothing very definite to offer concerning a 

 similar association in the Morin area, certain indications were 

 found tending to show that detailed study might definitely bring 

 out its existence there. In the vicinity of Piedmont and extending 

 southeastward beyond Shawbridge the gneiss which here forms the 

 southern boundary of the anorthosite mass is a rather fine-grained 

 greenish rock looking very similar to the syenite of the Adirondacks. 

 Specimens of this taken at various distances from the borders of 

 the anorthosite show that it varies considerably. In all cases the 

 rock is composed principally of plagioclase but, on receding from 

 the border of the anorthosite, orthoclase continually increases in 

 importance. There is apparently a perfect transition from anortho- 

 .site toward syenite, though in none of the specimens collected had 

 the change gone to completion, that is, none of the specimens could 

 be called typical syenite. In one specimen, however, orthoclase 

 made up about 30 per cent of the rock, and was accompanied by 

 some quartz, so that probably the change would not have to be 

 followed much farther to afford typical syenite. 1 On account of 

 this transitional relation it is very difficult, at least where seen by 

 the writer, to fix a boundary between anorthosite and gneiss. 

 Specimens that are apparently typical anorthosite and taken well 

 within the boundary of the mass as mapped by Adams, are found 

 to be like those types of anorthosite of the Adirondacks which show 

 the beginning of transition to syenite in that the plagioclase con- 

 tains orthoclase inclusions. Specimens from the cliffs north of 

 Piedmont station show the orthoclase in streaks forming an anti- 

 perthite much richer in orthoclase than any seen in Adirondack 

 specimens. 2 Even specimens taken four miles within the border of 

 the anorthosite, in the village of Ste. Adele, show abundant ortho- 

 clase inclusions in the plagioclase. 



1 Professor Adams informed the writer in conversation that intermediate mon- 

 zonite types analogous to the Norwegian types of Kolderup occur in the region, so 

 that, while they are not described at length in his report, he undoubtedly recognized 

 such types. 



2 In none of the Canadian specimens was there seen any peculiar zone of plagioclase 

 surrounding the orthoclase inclusions as described from the Adirondack localities. 



