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PROl!'. r. DAWSON ADAMS ON THE 



[May 1908, 



IX. Relation of the Geenville Seeies to othee 

 Pee-Cambeian Seeies. 



In the southern portion of the Laurentian Highlands to the west 

 of the area occupied by the Grenville Series — that is, north of 

 Lake Huron and in the district about Lake Superior, Rainy Lake, 

 and Lake of the Woods — other pre-Cambrian series, differing 

 essentially in petrographical character from the Grrenville Series, 

 are found. These are, enumerated in ascending order, the Keewatin, 

 Huronian, and Keeweenawan Series. Up to the present time the 

 Grenville Series has nowhere been found in contact with these ; but 

 it is hoped that the relation of these eastern and western pre- 

 Cambrian developments may eventually be determined, in order that 

 a correlation may be made between them. Until this has been done, 

 however, their relations must remain a matter of mere conjecture. 

 The two successions, then, are as follows : — 



Western District. 

 Upper Cambrian— Potsdam Sandstone. 



Unconformity. 



''Keeweenawan. 



Unconformity. 



f Upper(Animikie). 



I Unconformity. 

 ^av.-vxc«^^,xx«^ . Huronian. •( Middle. 

 (Proterozoic). ] ] Unconformity. 



1^ Lower. 



Pre-Cambrian 



Eastern District. 

 Upper Cambrian — Potsdam Sand- 

 [stone. 

 Unconformity. 



f 



Unconformity. 

 Keewatin. 



Intrusive Contact. 



Iiaurentian. 



Pre-Cambrian 

 (Proterozoic). 



Grenville Series. 

 Intrusive Contact. 



Laurentian. 



It will be noticed that here the term Laurentian is used in a 

 somewhat different sense from that in which it was employed by 

 Logan. In Logan's original classification of the Laurentian, this 

 term — apart from the Upper Laurentian, which was proved to be 

 composed essentially of anorthosite-intrusions — included two series 

 differing in character, namely, the Lower Orthoclase-C Funda- 

 mental ') Gneiss and the Grenville Series. Now that investigations 

 have shown that these two series differ in origin (one being essen- 

 tially a great development of very ancient sediments, and the other 

 consisting of great bodies of igneous rock underlying and intruded 

 through them), it becomes necessary to separate these two develop- 

 ments in drawing up a scheme of classification. As the lower 

 gneisses forming what has been termed the ' Fundamental Gneiss ' 

 have an enormously-greater areal development than the overlying 

 sedimentary series, constituting as they do a very large part of 

 the whole northern protaxis, and forming the basis upon which the 



