areas above the level of the lake, but a greater area was 

 added by the progressive withdrawal of the ice sheet. 

 During the first half of this stage, Lake Algonquin received 

 a large inflow of water directly from the ice sheet which 

 still formed an extensive barrier in the north. In the 

 second half, the barrier had dwindled to so small a frontage 

 that its contributions of water were relatively small and 

 unimportant. In the first half of this stage there appears 

 to have been a large discharge at Chicago as well as at 

 Port Huron, but in spite of this the stream at Port Huron 

 was apparently slightly larger than the present St. Clair 

 river. The Toleston beach of the Lake Michigan basin 

 appears to be the same as the Algonquin beach, indicating 

 that the discharge in the first part of this stage must have 

 been very large — so large that it seems hard to account 

 for its excess of volume by affluents from the ice sheet 

 alone. In the first half of this stage the volume of dis- 

 charge at Port Huron was probably slightly larger than the 

 volume of the present St. Clair river, but in the second 

 half it was a trifle less, on account of a small discharge 

 at Chicago, and the volume of Niagara river was affected 

 in a corresponding manner. 



4. Nipissing Great Lakes. — This stage of the upper 

 lakes was inaugurated by the final withdrawal of the last 

 remnant of the ice sheet from the Ottawa valley. These 

 lakes occupied the three upper lake basins and covered 

 an area only a trifle larger than that of the present lakes. 

 The ice sheet having disappeared, no additional water 

 was received from it and the volume of discharge was 

 the same as the present volume of St. Clair river. The 

 outlet during this stage was eastward from the northern 

 part of the Georgian Bay region, its head being at North 

 Bay, Ontario. The scouring effects of the outlet river 

 are well marked in the valleys of the Mattawa and Ottawa 

 rivers and indicate a river having the same volume as that 

 of the present St. Clair river. During this stage, therefore, 

 Niagara river was again left with only the discharge of 

 Lake Erie. 



5. The Present Great Lakes.— Continued uplift of 

 northern lands raised the outlet at North Bay, Ontario, 

 and sent the discharge of the three upper lakes to Port 

 Huron and thence to Lake Erie and Niagara. In the 

 transition, both outlets were active at once, but this 

 arrangement did not last long. The change of outlet 



