TYPICAL LAURENTIAN AREA OF CANADA. 33/ 



The high elevations attained by these rocks in portions of the 

 Protaxis in the north may, of course, be due to differential ele- 

 vation, but immediately along the southern edge of the area 

 there can have been but little differential change of level as 

 compared with the flat-lying Potsdam strata which border it 

 and lie but little above the present sea leveU Further evi- 

 dence of the original height or continued uprising of the area 

 is afforded by the fact that all the material of which the North 

 American continent was built up (with the possible exception of 

 some of the limestones) was derived originally from the Archean 

 Protaxis of the continent, a considerable proportion of this at 

 least coming from the main Protaxis of which this typical 

 Laurentian area forms a part. We must conclude therefore 

 that in early Cambrian or pre-Cambrian times, in portions of 

 the Protaxis at least, the Laurentian mountains rose several 

 hundred and possibly in places several thousand feet above the 

 sea level. 



The intrusion of the granites and anorthosites as well as the 

 folding of the whole system of rocks took place before Upper 

 Cambrian times. The whole series was moreover without doubt 

 at that time in the "metamorphic" condition in which we now 

 find it, for along the margin of the area the Potsdam sandstone 

 rests in flat undisturbed beds on the deeply eroded remnants of 

 these old mountains, its basal beds often consisting of a con- 

 glomerate with pebbles of the underlying gneissic rocks. These 

 Cambrian strata cover up the gneisses, granites and anorthosites 

 alike and are evidently of much more recent age, being separated 

 from the Laurentian by the long interval occupied in the upheaval 

 and erosion of the Laurentian area. 



How long before Upper Cambrian times this folding and 

 erosion took place cannot be determined from a study of this 

 area, but further west along the edge of the Protaxis in the Lake 

 Superior district we find that the Keweenawan, Nipigon and 

 Animikie Series also repose in flat undisturbed beds on the 

 eroded remnants of a series of crystalline rocks which have the 

 petrographical character of the Fundamental Gneiss. This 



