﻿254 Canadian Record of Science. 



F. D. Adams, rather as preliminary to a more detailed 

 report to be issued later. It is a somewhat general 

 description of a small part of the Laurentian area whose 

 north-western extension beyond the limits of this map 

 has been more minutely studied. 



About half of the Laurentian area shown on this map 

 consists of Anorthosite. This rock was formerly regarded 

 by Logan, Hunt and other early investigators as a sedi- 

 mentary deposit, and was called the Upper Laurentian or 

 Norian formation. 



An elaborate study of this and other occurrences was- 

 made by Dr. Adams, the result of which was to establish 

 the fact that Anorthosite is an igneous rock which has 

 been intruded through the Laurentian prior to the depo- 

 sition of the overlying Palaeozoic strata. 



The Laurentian system here, as in many other places^ 

 has been found to consist of two parts, an upper series 

 which is sedimentary in part at least, and is highly 

 metamorphosed, and an underlying mass of altered igneous 

 rock which constitutes the lowest known member of the 

 earth's crust. 



The former of these, which is known as the Grenville 

 series, has been nmch altered by igneous intrusions. 

 It contains almost all the mineral wealth of the Laurentian. 



The underlying mass is commonly designated as the 

 Fundamental Gneiss. From recent researches it seems 

 probable that it represents the downward extension of its 

 original crust of the Earth, into which when subsequently 

 softened by heating, the Grenville series has sagged down. 

 If so, it has probably been the chief agency in the 

 metamorphism of the Grenville series. The prominently 

 petrographical character of this chapter and the absence 

 of structural details may somewhat lessen its popular, 

 though not its technical, interest. This is, however, 

 amply compensated for by the admirable description of 

 the area which introduces the chapter. 



