THICKNESS AND FIELD RELATIONS. 229 
rocks recognized as Keewatin, and lies conformably, so far as observed, 
beneath the latter, there seems no reason why the two together should 
not be classed as Huronian. 
_ The Huronian of western Ontario presents, then, an immense series 
of shallow-water sediments, in the upper part mixed with eruptives, per- 
haps largely later injections, but partly pyroclastic; the whole equal in 
magnitude to the thick series of clastics found in the preparatory troughs 
of great mountain ranges, such as the Appalachians and the Rockies. 
Friritp RerLATIONS oF LAURENTIAN AND HURONIAN 
The field relations of the Laurentian, Couchiching, and Keewatin have 
been so admirably shown by Lawson that only a brief summary will be 
required here. He was struck by the fact that the lower Archean “ oc- 
curs in large, isolated, central areas more or less completely surrounded 
by the schists of the upper Archean, the encircling belts anastomosing 
and forming a continuous meshwork.”’* 
An examination of the Rainy Lake region, where the innumerable bare 
shores of islands and promontories give an unsurpassed exposure of the 
Archean, shows that the Laurentian consists chiefly of coarse reddish, 
often porphyritic rock, usually granite in the central part of the area, but 
showing foliation toward the margin where it comes in contact with the 
Huronian. The foliation of the gneiss is generally parallel to that of the 
schist beside it. Before the actual contact is reached one generally sees 
strips and fragments of the Huronian embedded in the gneiss, sometimes 
sharply angular, at others with softened outlines. Often the actual line 
of contact is hard to define. so mixed are the gneiss and the schist. Dikes 
of granite, pegmatite, or felsite generally run from the gneiss into the 
Huronian. Where the latter is Keewatin one usually finds it hardened 
into hornblende-schist instead of the more common chlorite-schist. The 
Couchiching rarely shows much contact metamorphism, though occa- 
sionally garnets and staurolites are developed. 
The Huronian schists have almost always a steep dip away from the 
gneiss, seldom less than 45 degrees and often vertical, and are folded 
into sharp synclines. 
The areas of Laurentian are of all dimensions from a mile or less in 
diameter to a width of 50 miles or perhaps even more, and are usually 
rounded in form. Good examples of the larger Laurentian areas are 
found in the one including the north arm of Rainy lake, nearly 50 miles 
long from east to west and 25 wide; and in the Grande Presqu’ile of the 
lake of the Woods, 30 miles in length from east to west and 18 in width. 
* Geol, Survey of Canada, 1887, p. 142, F. 
