THE CONTACT OF THE ARCH^AN AND POST- 

 ARCH^AN IN THE REGION OF THE GREAT 

 LAKES. 



The accompanying map has been compiled from one published 

 by the Geological Survey of Canada, to which was added the 

 Adirondack region from one published in Van Hise's Pre-Cam- 

 bia7i Geology. "Archaean" is used to include the Huronian and 

 Laurentian, as originally proposed by Dana. It does not here 

 include the Animikie or Keweenawan, which are considered post- 

 Archaean. With this exception, Archaean is here used as the 

 equivalent of the pre-Cambrian of many writers. 



The purpose of this paper is to bring out the peculiar step- 

 like arrangement of the contact between the Archaean and post- 

 Archaean in the region of the Great Lakes. This is shown clearly 

 on the map, where a heavy line has been drawn to emphasize 

 this feature. The contact is further marked by the almost 

 universal elevation of the Archaean several hundred feet above 

 the succeeding formations. 



Beginning at the St. Lawrence on the east, we find the line of 

 contact runs about 10° N. of W. to the southeastern corner of 

 the Georgian Bay. The Archaean fronts the Paleozoic sediments 

 nearly always as a hill rising one hundred or more feet above 

 them. It is true the Archaean underlies the sediments, and per- 

 haps as a fairly regular plain, but it would seem that there has 

 been some disturbance by which a part of the Archaean has been 

 relatively depressed. The contact of the Paleozoic follows 

 closely this line of movement covering the lower part and abut- 

 ting against the higher.' 



The east shore line of the Georgian Bay runs 30° W. of N., 

 and this may be taken as the line of contact. Only in the south- 

 east corner do post-Archaean rocks appear. The Archaean rises 

 in hills a few hundred feet above the water line. 



The contact north of Lake Huron again runs 10° N. of W. 



^Compare Wilson, Journal of Geology, Vol. XI, p. 651 ff. 



40 



