NATURAL HISTORY, TORONTO REGION
Spirifera duodenarius. Stropheodonta demissa. Conocardium cuneus. Paracyclas elliptica. Platyceras carinatum. Proetus rowi.
As previously mentioned, the greater part of the region under consideration is covered with drift deposits of the Pleistocene, sometimes to the thickness of 600 or 700 feet ; and a very complex history has been worked out from them. Between the Devonian and the end of the Pliocene no record has been preserved, but it is certain that superficial erosion went on to a great extent in the long period of dry land conditions after the middle Palaeozoic. There was time to strip much of the Palaeozoic beds from the Archaean floor and to cut back for many miles toward the west and south the Silurian shales under their protective capping of Niagara limestone, thus producing the striking escarpment which crosses the province. Great river valleys were carved below the present level of the sea, showing that the land stood higher than now, the most important being the " Laurentian River," as it has been named by Dr. Spencer, which drained the Upper Lakes region through what is now Georgian Bay to the Ontario