THE SUCCESSION OF DEPOSITS. 55 



crosses the St. Lawrence, and where the same ancient 

 rocks cross the Ottawa; and in general the Leda clay 

 may be said to be limited to the lower Silurian plain, and 

 not to mount up the Laurentian and metamorphic hills 

 bounding it. Since, however, the level of the water, as 

 indicated by the terraces in Lower Canada, and by the 

 probable depth at which the Leda clay was deposited, 

 would carry the sea level far beyond the limits above 

 indicated, and even to the base of the Magara escarp- 

 ment, we must suppose, either (1) that the supply of 

 this sediment failed toward the west; or (2) that the 

 mud has been removed by denudation or worked over 

 again by the fresh waters so as to lose its marine fossils ; 

 or (3) that the relative levels of the western or eastern 

 parts of Canada were different from those at present ; or 

 (4) that the water may have been freshened and rendered 

 cold by the influx of melting snow and ice into a landlocked 

 water area or one with a narrow opening. As already 

 stated, there are indications that the first may be an 

 element in the cause. The second is no doubt true of the 

 clays which lie in the immediate vicinity of the lake 

 basins. Dr. Spencer has detailed many observations in 

 favour of the third, more especially in the later glacial 

 and Post-glacial periods. 



I believe, however, that much more rigorous investiga- 

 tions of the clays of western Canada are required before 

 we can certainly affirm that none of them hold marine 

 fossils.* 



Whittlesey has described the western drift deposits in 

 the Smithsonion Contributions, Vol. XV., and according 



* It is to be observed that even near the coast the greater part of 

 the thickness of the Leda clay is often unfossiliferous. 



