338 



NATURE 



(^September 15, 1910 



Laurentian all the way from Saskatchewan and Manitoba 

 on the north-west through Ontario to the city of Quebec 

 on the east. These limestones represent an important 

 transgression of the sea upon the Canadian Shield. 

 Apparently the old hummocky surface was often pretty 

 cleanly swept, so that limestone with very little frag- 

 niental material rests immediately upon the gneiss, but in 

 other cases there is arkose or a basal conglomerate of 

 Laurentian materials. 



Occasionally Archaean hills rise island-like through the 

 shaly limestone, which tilts away quaquaversally, as if 

 the hill had protruded through the sediments. This 

 appearance is probably due to the settling and shrinking 

 of the mud in its consolidation to rock. Drill-holes east 

 of Lake Ontario show that there were valleys hundreds of 

 feet deep between these Archa;an hills, so that in this 

 region the peneplain was far from complete. These 

 inequalities may be considered foot-hills of the Adirondack 

 mountains farther east. 



There is reason to believe that before the close of the 

 Ordovician the sea crossed from the region of Lake Winni- 

 peg to Hudson Bay, flooding all the lower parts of the 

 shield ; but probably most of Labrador and part of 

 Franklin, north-west of Hudson Bay, remained as dry 

 land. 



The Silurian follows on the Ordovician without a dis- 

 cordance, and at this time the sea probably submerged an 

 even larger part of the shield, since the Silurian limestone 

 of James's I3ay is only 250 miles from that south of the 

 Great Lakes, and there are two outliers between — on 

 Lakes Nipissing and Temiscaming. It may be added that 

 the highland of Silurian limestone crossing Southern 

 Ontario, with a bold escarpment facing north-east, rises 

 hundreds of feet higher than the watershed towards 

 Hudson Bay. The escarpment facing the Archaean " old 

 land" corresponds to the Scandinavian "glint," and has 

 a similar relation to the lakes of the Archaean border. 



The Devonian Sea also encroached south of James Bay 

 and along the south-west side of the shield from Clear 

 Lake, in Saskatchewan, to Great Bear Lake. 



What took place on the Archsean continent while the 

 coal forests flourished on the lowlands to the south and 

 to the far north is unknown, since no Carboniferous rock 

 has been found on its surface. 



Mesozoic and Cenozoic History. 



Early Mesozoic times are a blank, but a few small out- 

 crops of Cretaceous rocks resting on the Archaean toward 

 the south-west show that portions of its rim were once 

 more under water. Dr. Wilson believes that an important 

 facet of the peneplain should be dated from the Cretaceous, 

 since planation was going on in parts of the United States 

 at this time ; but no positive evidence of this is at hand. 



Nor is there any evidence as to its history in the Tertiary 

 before the oncoming of the Ice Age of the Pleistocene, 

 when its whole surface was scoured more than once by 

 great glacial sheets. The mantle of decayed rock which 

 must have accumulated during the long dry land stage 

 was almost completely swept away, leaving the rounded 

 surfaces of ancient rock fresh and clean beneath the 

 Boulder Clay. 



In an important inter-Glacial interval and in post-Glacial 

 times much of the morainic material was assorted in 

 great lakes the shore and deep-water deposits of which 

 cover large parts of the surface. With the departure of 

 the ice, the sea once more transgressed upon the lower 

 parts of the shield, but the land has been rising since, 

 leaving a belt of marine deposits up to about 500 feet 

 around the shores of Hudson Bay, the St. Lawrence, and 

 the Atlantic. 



How much of the Shield has been Covered? 

 It is generally stated that the Canadian Shield has been 

 drv land since the Archaean, and hence that erosion has 

 been taking place ever since that time. This is probably 

 true for part of the north-eastern portion of the shield, and 

 perhaps also the north-western, but much of the area, 

 especially toward the south, was buried in early days 

 under Pala;ozoic sedimentary rocks, and so protected from 

 further destruction. These sediments are still being 

 slowly stripped from the .-Xrcha^an in many places. 



NO. 2133, VOL. 84] 



This may account for the greater proportion of 

 Huronian and Keewatin rocks in the south as compared 

 with the north. It is probable that in the unprotected 

 northern parts weathering agencies have eaten the higher 

 •Archaean rocks completely away from the Laurentian 

 gneiss beneath. Before asserting this positively, however, 

 it may be well to await more thorough exploration of the 

 little known north. 



It is possible, but not very probable, that the whole 

 area was at one time covered with Ordovician or Silurian 

 shale and limestone. If so, all traces of this capping have 

 been removed from hundreds of thousands of square miles 

 of its surface. 



There is one very impressive feature of the Archaean as 

 found beneath the later rocks. The peneplain, with its 

 rounded, hummocky surface, seems exactly the same when 

 one strips from it recent Boulder Clay, early Palaeozoic 

 shale or sandstone or limestone, Keweenawan eruptives, 

 or even Lower Huronian tillite, where this has remained 

 undisturbed. It is as though all the millions of years of 

 destruction since the Middle Palaaozoic had made only 

 unimportant changes in the pre-Cambrian peneplain. 

 When it is recalled that peneplanation took place twice in 

 the pre-Cambrian, before the Lower Huronian and before 

 the Animikie, one is almost driven to think that pre- 

 Cambrian time is far longer than post-Cambrian. 



Relation 0/ the Shield to the Palaeozoic. 



Except toward the east, the Canadian Shield sinks 

 gently beneath Palaeozoic beds, in most cases retaining its 

 character as a peneplain. How far does it continue to 

 the south and west beneath the sedimentary rocks, and 

 to what depth does it extend? 



The results of drilling at Toronto, eighty miles south 

 of the contact, show gneiss and crystalline limestone at a 

 depth of 1200 feet below the surface, or 940 feet below 

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

 contact, the Archsan is reached at a depth of 3300 feet — 

 2700 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 overlying Palceozoic rocks. 



.\s the peneplain rises more than 1300 feet .above sea- 

 level at the watershed 300 miles north of Lake Erie, there 

 is a difference of 4000 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 

 6600 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 tlie 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 eleva- 

 tion 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 6 or 7 feet per mile. If we add 230 feet of deforma- 

 tion 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 4000 feet thick, 

 since it passed over the tops of the Adirondack mountains. 



