A Study of the Ontaino Basin. 357 



Toronto island is one of the most characteristic examples of 

 wave-work on the Great Lakes, its form both above and below 

 water agreeing perfectly with those of a succession of hooks 

 built into deep water. The only evidence Dr. Spencer gives 

 of its delta character is the small amount of clay occurring 

 interbedded with sand; and this is easily explained when one 

 has seen the waters opaque with clay for miles from shore dur- 

 ing an easterly storm. This drifts right past the island at 

 present and must have been deposited in earlier days over the 

 growing shoal in advance of the island. 



That the Don has deposited delta materials to a depth of 80 

 feet or more in an old deep channel beneath the present Don 

 flats is proved by wells sunk near its mouth, but its rather 

 insignificant delta ends a mile northeast of. Toronto island 

 where marshy shoals occur in Ashbridge's bay. The whole of 

 the small delta is northeast of the pier against which sand is 

 accumulating and the main structure of the island is clearly 

 due to the southwesterly drift of sand from Scarboro'. Why 

 should the little river Don build a huge hook-shaped delta 

 when the much larger Humber and Rouge rivers flowing 

 through the same type of country have done nothing of the 

 kind ? There is no hook near the mouth of the Kongo or the 

 Humber because there was no great promontory of sand and 

 clay just to the east from which the storms could obtain build- 

 ing material. This is the only difference between them and 

 the Don. 



If Lake Ontario h?s lasted at least 8,000 years it is probable 

 that Lake Iroquois, of about the same size and with shores of 

 about the same maturity, lasted in the neighborhood of 8,000 

 years, also ; though an argument from analogy must of course 

 be received with caution. 



The lapse of time between the end of Lake Iroquois and the 

 beginning of Lake Ontario is more uncertain. The thawing 

 away of the ice from the St. Lawrence region, after it had 

 sunken so far as to allow the waters to drain past the Adiron- 

 dacks, seems to have gone on rapidly, since no prominent shore 

 lines are found between those of Lake Iroquois and the marine 

 beaches formed when the St. Lawrence valley became free 

 from ice. Again, the length of time during which the Ontario 

 basin was below sea-level, forming a fresh water extension of 

 the Gulf of St. Lawrence, is not certain. The old marine 

 shores are much less mature than those of Lake Iroquois and 

 of Lake Ontario; which would seem to impl}- a much shorter 

 time in their construction, perhaps not more than half as long, 

 say 4,000 years. How long a time was required to elevate the 

 outlet of the basin and cut off Ontario from the sea, and after- 

 wards to raise the outlet until the water was ponded back as 



