NATURAL HISTORY, TORONTO REGION
mation, the Ordovician, follows after an interval long enough to destroy the Laurentian mountains, which were carved down to low hills, the region having been reduced to a peneplain.
Though the Archaean rocks are not found in place within ninety miles of Toronto, specimens of all their varieties, including marble, may be collected from the drift boulders in the neighborhood of the city.
The old Archaean surface sinks gently beneath the next sheet of rock and advancing southwards may be found by drilling at greater and greater depths below the surface. At Thornhill, fourteen miles north of Toronto, it has been found in a well at 1,200 feet below the surface and 600 feet below the level of Lake Ontario. At Toronto granite and gneiss and sometimes crystalline limestone are found at depths of 1,100 to 1,300 feet below Lake Ontario, showing a slope of about forty feet to the mile in the old land surface. It is nearly 1,000 feet below sea level at Toronto, and 2,500 on the north shore of Lake Erie.
As the ancient surface must have been leveled to a peneplain by denuding forces which can only act above the level of the sea, it is evident that vast areas of the old continental mass were warped down to form sea bottom before the Palaeozoic history of the region began.