THE SUCCKSSION OF DEPOSITS. 57 



dep(j.sit oecurrino' on hij^'licr ground uiid farlher from llio 

 sen. 



Where the Leda elay rests on nuirine l)oulder-cliiy, tlie 

 change of the deposits implies a dinunution of ice- 

 transport relatively to deposition of tine sediment from 

 water; and with this, more favourable circumstances for 

 marine animals. This may have arisen from geographical 

 changes dimini.shing the "Supply of ice from local glaciers, 

 or ol)structing the access of lieavy icebergs from the 

 arctic region.s. At tlie })resent time, for e.\ami)le, the 

 action of the heaviest bergs is limited to the outer coasts 

 of Labrador and Newfoundland, and a deposit rc.-;end)ling 

 the Leda clay is fornnng in the gulf of 8t. Lawrence: 

 but a subsidence which would deternune the arctic 

 current and the trains of heavy bergs into the gulf, 

 would l)ring with it the conditions for the formation of a 

 ])0ulder-clay, nioi'c especially if there were glaciers on the 

 Laurentide hills to the nortli. Where the Leda elay rests 

 on l)oulder-clay which may be sui)})osed to be of terres- 

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

 interesting to observe that the conditions thus recpiired 

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

 of land or sea bottom ndght l)e reijuired to enable Leda 

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

 sion of the land would l)e 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 atlirni that this last change has occurred ; 

 though on the nortli shore of the St. Lawrence the)-e are 

 cases in which the Leda clay rests directly on striated 

 surfaces which might l)e attril)uted to glaciers ; just as in 

 the west the Erie clay occupies this position. 



Deposits referable to the shores of the Leda clay sea, 



