Cushing — Igneous Rocks of the Adirondack Region. 293 



the younger sediments are lacking in New York so that precise 

 comparisons cannot be made, it nevertheless appeared to me 

 that the mutual relationships of the two sets of eruptives were 

 precisely those which prevailed in Canada, and that the ortho- 

 gneisses of the Thousand Island vicinity were precisely like 

 the Laurentian and Ottawa gneisses which I had seen in a host 

 of Ontarian localities, under the guidance of Canadian 

 geologists, chiefly Adams and Barlow. I therefore made the 

 tentative correlation of the orthogneiss with the Laurentian. 

 In eastern Canada Adams has described the anorthosite as 

 holding inclusions of orthogneiss, correlated as Ottawa gneiss, 

 and in a personal letter just received reiterates the statement 

 that the Morin anorthosite has a number of large inclusions of 

 the regular orthoclase gneiss through which it cuts. The 

 Ottawa gneiss is regarded as of Laurentian age. In the 

 Adirondacks the anorthosite is the oldest of the set of younger 

 eruptives, so far as the evidence goes. Both in New York and 

 in Canada, the anorthosite cuts out older granite gneiss along 

 the foliation strike, and holds inclusions of it. 



This old orthogneiss, which is everywhere associated with 

 the Grenville in eastern Canada and in New York, and which 

 further west has equally close association with the Keewatin, 

 seems to me to represent a widespread granitic invasion whose 

 separate masses cannot differ greatly in age. It also seems to 

 me that, in Precambrian rocks, great periods of igneous activ- 

 ity, such as this, afford a safer basis of correlation than the 

 sediments do. It is because the Grenville is older than this 

 orthogneiss, which I correlate with the Laurentian, that I have 

 for some time felt the impossibility of correlating it with any 

 part of the Huronian, and believed it more akin to the 

 Keewatin, as several of the Canadian geologists are now 

 classifying it. I think the classification of the orthogneisses of 

 the northwestern Adirondacks as Laurentian is as safe a cor- 

 relation as can be made in Precambrian rocks. And even if 

 this correlation is unjustifiable and erroneous, it would not 

 alter the fact that, in this region, there are two granites which 

 differ widely in age. 



Quantity of the Orthogneiss in New York. — Dr. Miller 

 states in his paper that " If any such older granite does exist, 

 it must be small in amount, and the writer believes it could 

 rarely be successfully mapped as a formation distinct from the 

 granites, which are certainly only phases of the syenite."* 



Since I radically differ from both these statements, I am led to 

 believe that a partial reason for the view that Dr. Miller holds 

 may be that his detailed work has been chiefly confined to the 



*Op. cit.,p. 252. 



