AN OUTLINE OP THE GEOLOGY OF ONTARIO. 583 



The strata of tlie district consist entii-ely of Lower Silurian forma- 

 tions, except in the extreme west, where the Medina formation of the 

 Middle Silurian series occurs. In ascending order, and succeeding each 

 other from east to west, these strata comprise the Potsdam (slightly- 

 developed near Kingston) ; Trenton (including the Black River beds 

 which cannot properly be separated from the higher Trenton strata); 

 Utica; Hudson River; and Medina formations. Of these, the Tren- 

 ton is composed of limestones and limestone shales. Some of its beds 

 yield excellent building stone; and towards its lower portion a band 

 of lithographic stone runs more or less continuously from near King- 

 ston, by Marmora, etc., to Georgian Bay. The Trenton formation 

 ranges along the lake shore from Kingston to Cobourg, and outcrops 

 on several of the interior lakes and streams, as well as on Georgian 

 Bay. The succeeding Utica formation consists of dark bituminous 

 shales, as seen at Whitby and also west of Collingwood harbour. 

 West of the Utica shales the thin-bedded sandstones, etc., of the 

 Hudson River series crop out, and range along Lake Ontario from 

 about the River Rouge to the Credit, appearing also in force on the 

 south-west shore of Georgian Bay, as at Cape Rich, Cape Crocker, 

 etc. West of the River Credit to the v^estern boundary of the dis- 

 trict in the great Niagara escarpment, the red marls and sandstones 

 of the Medina formation form the outcropping strata. The greater 

 portion of the Lake Ontario district is overlaid however, by clays, 

 sands, and gi-avels of the Glacial and Post-Glacial periods, by which 

 the underlying rocks are much concealed. Beneath these deposits, 

 the limestone strata, especially, are found very generally to be stria- 

 ted and polished by glacial action, the striae running most commonly 

 in a south-west direction. Many fresh-water shells, identical in 

 species with those now living in our lakes and streams, occur at 

 various levels in the post-Glacial accumulations ; and their presence 

 in these deposits apparently indicates the former union of our lake 

 waters into one vast freshwater sea, held up on the east by a greater 

 elevation of the gneissoid belt of rock which crosses the St. Lawrence 

 between Brockville and Kingston, and expands into the wild district 

 of the Adirondack Mountains in the State of I^ew York; or perhaps 

 by an enormous glacier descendiug from this elevated region and ex- 

 tending northwards into Canada. Bones and teeth of the beaver, 

 wapiti, and other existing mammals are also occasionally found in 

 these higher deposits, together with two extiuct types : the mammoth, 



