Rainy Lake Archman Geology. 557 



metamorpliic rocks which were proved to be okler than the 

 ' Laurentiau ' : the Coutchiching, a series of mica-schists and para- 

 gneisses, and above it the Keewatin, a series made up largely of 

 volcanic rocks. The conclusions then arrived at as to the strati- 

 graphical position of the Coutchiching have been called in question 

 by the United States Geological Survey and by the International 

 Committee on Geoloi;ical Nomenclature. They denied the existence 

 of such a series beloio the Keewatin, and decided that on at least 

 one line of section the rocks referred to the (youtchiching were 

 certainly stratigraphically higher than parts of the Keewatin. This 

 controversy naturally led to a re-examination of the district; by 

 Dr. Lawson, and his revised conclusions are now before us. In all 

 its essentials he maintains his original position, and having forcibly 

 presented his case with a mass of detailed and convincing field 

 evidence, he challenges his critics to deny the superposition of the 

 Keewatin upon the Coutchiching as he mapped them a quarter of 

 a century ago. 



The International Committee of 1905 decided that the term 

 ' Laurentian' should be restricted to the granites and gneissose 

 granites which antedate or protrude through the Keewatin and which 

 are pre-Huronian. Since then it has become increasingly evident 

 that a great pi'oportion of the igneous rocks mapped as ' Laurentian' 

 are of post-Huronian age. In the Rainy Lake region at least two 

 periods of granite intrusion, widely separated in age, have been 

 recognized. For the older of these Dr. Lawson proposes to retain the 

 terra Laurentian in accordance with the now accepted definition, and 

 for the later (post-Huronian) he suggests the term Algoman, from the 

 old district of Algoma, in which such intrusions extensively occur. 



Dr. Lawson discusses the mechanics of batholithic intrusion as 

 illustrated in liis area, and comes to the conclusion that the facts are 

 somewhat difficult to harmonize with Daly's conception of stoping on 

 a large scale. The folded and faulted structures of the roofs are 

 suggestive of a mode of intrusion analogous to that of a laccolith. 



Only two divisions of the Huronian formations are recognized, the 

 Animikie, as shown in the classification given below, being separated 

 from the Upper Huronian (Middle Huronian of Van Hise and Leith) 

 by the intrusion of the Algoman granites and by the immensely h)ng 

 period of erosion which followed. The pre-Cambrian sequence 

 suggested by Dr. Lawson is as follows: — 



{Keweenawan. 

 Unconformity. 

 Animikie. 



Eparchsean interval. 

 /'Algoman granites, gneisses, etc. 



Irruptive contact. 

 Upper Huronian. 



Unconformity. 

 Lower Huronian. 



Unconformity. 

 Laurentian granites, gneisses, etc. 

 Irruptive contact. 



Keewatin. 1 r» ,t ,.• „ 

 r^ i 1 • 1 • /-(Jntarian. 

 i.Coutchicmng.J 



Arch.'EAN ■[ 



