PRESIDENTIAL ADDRESS. 601 



The results of drilling at Toronto, 80 miles south of the contact, show gneiss 

 and crystalline limestone at a depth of 1,200 feet below the surface, or 940 feet 

 below sea-level. Near Lake Erie, 130 miles to the south of the contact, the 

 Archaean is reached at a depth of 3,300 feet — 2,700 feet below sea-level. Its slope 

 to Toronto is at the rate of 20 feet per mile, and from Toronto to Lake Erie 

 at the rate of 35 feet per mile. This corresponds fairly well with the dip of the 

 cverlying Palaeozoic rocks. 



As the peneplain rises more than 1,300 feet above sea-level at the watershed 

 300 miles north of Lake Erie, there is a difference of 4,000 feet in a north and 

 south direction ; and if comparison is made with the Adirondack Mountains 

 250 miles to the east the difference even amounts to 6,600 feet. It is probable, 

 however, that the Adirondacks were a residual group of mountains never reduced 

 to the general peneplain level. It is clear that the pre-Palaeozoic peneplain has 

 been greatly warped in later ages, perhaps as a result of the increasing load of 

 sediments piled on its southern edge. 



One is apt to think of these ancient crystalline rocks as an exceedingly solid 

 and resistant block of the earth's crust, likely to undergo little deformation: 

 so that this evidence of warping or doming of the surface comes as a surprise. 

 In reality shiftings of level under changes of load are normal in every region, 

 and have been going on along the southern border of the Canadian Shield all 

 through Pleistocene times, and perhaps continue now. 



The proof of this is to be found in the differential elevation of the shore-lines 

 of the great post-glacial lakes, which ascend with an increasing grade toward 

 the north (N. 20° E.). In the case of Lake Iroquois the difference in level 

 between the two ends of the earliest shore is more than 500 feet, and the grade 

 toward the north even rises to six or seven feet per mile. If we add 230 feet of 

 deformation of the marine beaches, which followed Lake Iroquois toward the 

 north-east after the final melting of the ice, there is a known change of level 

 amounting to 730 feet within late Pleistocene times. There is reason to believe 

 that similar changes of level took place during the inter-glacial period recorded 

 at Toronto and to the north. 



The Pleistocene sinkings and risings are naturally accounted for by the piling 

 up and removal of the thousands of feet of ice in the Glacial Periods, though 

 probably isostatic equilibrium was not reached in these movements. 



We know that the ice was more than 4,000 feet thick, since it passed over the 

 tops of the Adirondack mountains. This thickness of ice is equal in weight to 

 about 1,600 feet of rock, while the greatest known elevation since the removal of 

 the load is not much more than 700 feet, implying that a weight of 900 feet of 

 rock can be supported by the shield. It may be, however, that in the interior 

 of Labrador, where no beach-lines give evidence as to changes of level, the doming 

 is much greater than the amount suggested. 



It is of interest to note that these adjustments to change of load take 

 thousands of years to accomplish. The rise due to the melting of the Labrador 

 ice-sheet may be going on slowly now, thirty or forty thousand years after the 

 load was lifted. 



These sinkings and risings must be accomplished by plastic flow outwards from 

 beneath the loaded area or inward toward the area relieved of its load. 



Instead of a rigid, unyielding shield, we must conceive a stiffly flexible covering 

 over a plastic substratum, where during thousands of years adjustments of level, 

 amounting to hundreds of feet, may take place ; and during millions of years of 

 removal of load by erosion, or of piling on of load through sedimentation, changes 

 of level of thousands of feet can be accomplished. Such changes have taken 

 place on the southern and western sides of the shield without any known rupture, 

 while on the east the adjustment has been accomplished in part by great faults. 



Has the Archasan, which is supposed to underlie the stratified rocks in all 

 parts of the world, undergone the same vicissitudes ? 



Summary. 



The history of the Canadian Shield begins in pre-Keewatin times, with land 

 surfaces on which weathering took place, and seas in which mud and sand were 

 deposited. If the earth were ever molten, that stage had long been passed before 



