THE SUCCESSION OF DEPOSITS. 57 



deposit occurring on higher ground and farther from the 

 .sea. 



Where the Leda clay rests on marine boulder-clay, the 

 change of the deposits implies a diminution of ice- 

 transport relatively to deposition of fine sediment from 

 water ; and with this, more favourable circumstances for 

 marine animals. This may have arisen from geographical 

 changes diminishing the supply of ice from local glaciers, 

 or obstructing the access of heavy icebergs from the 

 arctic regions. At the present time, for example, the 

 action of the heaviest bergs is limited to the outer coasts 

 of Labrador and Newfoundland, and a deposit resembling 

 the Leda clay is forming in the gulf of St. Lawrence; 

 but a subsidence which would determine the arctic 

 current and tlie trains of heavy bergs into the gulf, 

 would bring with it the conditions for the formation of a 

 boulder-clay, more especially if there were glaciers on the 

 Laurentide hills to the north. Where the Leda clay rests 

 on boulder-clay which may be supposed to be of terres- 

 trial origin, subsidence is of course implied ; and it is 

 interesting to observe that the conditions thus required 

 are the reverse of each other. In other words, elevation 

 of land or sea bottom might be required to enable Leda 

 clay to take the place of marine boulder-clay, but depres- 

 sion of the land would be necessary to enable Leda clay 

 to replace the moraine of a glacier. I cannot say, how- 

 ever, that I know any case in Canada where I can 

 certainly affirm that this last* change has occurred ; 

 though on the north shore of the St. Lawrence there are 

 cases in which the Leda clay rests directly on striated 

 surfaces which might be attributed to glaciers ; just as in 

 the west the Erie clay occupies this position. 



Deposits referable to the sliores of the Leda clay sea, 



