PRESIDENTIAL ADDRESS. 599 



deposits of copper, nickel, and silver in Northern Canada are closely bound up 

 with the Keweenawan basic volcanic rocks or with deeper-seated diabases probably 

 of the same origin. 



Here, as in the Keewatin, we are confronted with floods of basic lava coming 

 up from unknown sources through the acid Laurentian gneiss. Do these basic 

 lavas represent heavier segregations settling to the bottom during the slow move- 

 ments of the granitic magma as it climbed into the Archaean batholiths? One 

 might imagine these heavier and more liquid parts sinking beneath the lighter, 

 more viscid, magmas of the domes, and remaining fluid until the mountain masses 

 above had become completely solid. The supposed thrust from the Atlantic basin 

 to the south-east might then bring strains to bear on the solid crust, more or less 

 shattering and shifting its masses, squeezing up the still molten diabase through 

 all the fractures and pores. 



Several remarkable basins were formed in the Archaean peneplain by the 

 ascent of these lavas, permitting the massive roof which formerly covered them 

 to collapse by block faulting or by the formation of an irregular syncline. The 

 basin of Superior seems to be of this nature. It is still rimmed by the Keweena- 

 wan lavas, sometimes accumulated to the thickness of 50,000 feet. Just to the 

 north is the smaller basin of Lake Nipigon, with its edges and islands of diabase 

 sheets ; and to the east, near Sudbury, is the extraordinary synclinal basin, with 

 which the great nickel mines are connected. These basins seem to have resulted 

 from the collapse of the solid crust because of the removal of support when basic 

 eruptives ascended from beneath. 



Palceozoic History. 



The exact relation of the Keweenawan to the Cambrian is somewhat in doubt, 

 though most geologists make it pre-Cambrian. The St. Mary's, or Lake Superior, 

 sandstone, which rests upon the Keweenawan with a slight discordance and 

 overlaps upon the Archaean, is generally called Cambrian ; it contains no fossils 

 and occurs only along the shores of Lake Superior and St. Mary's River, so 

 that its position in time is uncertain. 



Potsdam sandstone, either Upper Cambrian or Lower Ordovician, rests upon 

 the planed-down Archaean surface at the Thousand Islands and other points in 

 Eastern Canada, often with a conglomerate at its base ; and undoubted Ordovician 

 limestones feather out upon the 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 fragmental 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 ehaly 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 Tock. Drill-holes east of Lake Ontario show that there were 

 valleys hundreds of feet deep between these Archaean hills, so that in this region 

 the peneplain was far from complete. These inequalities may be considered foot- 

 hills of the Adirondack mountains further east. 



There is reason to believe that before the close of the Ordovician the sea 

 crossed from the region of Lake Winnipeg 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 discordance, and at this time 

 the sea probably submerged an even larger part of the shield, since the Silurian 

 limestone of James's Bay 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. 



