62 



THE ICE AGE IN CANADA. 



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same recession of the sea which 

 produced the Saxicava sand. At 

 Montreal, where the isolated mass 

 of trap flanked with lower Silurian 

 beds, constituting Mount Eoyal, 

 forms a great tide-gauge for the 

 recession of the Post-pliocene sea, 

 there are four principal sea margins, 

 with several others less distinctly 

 marked. The lowest of these, at a 

 level of about 120 feet above the 

 level of the sea at lake St. Peter, 

 may be considered to correspond 

 with the general level of the great 

 plain of Leda clay in this part of 

 Canada. On this terrace in many 

 places the Saxicava sand forms the 

 surface, and the Leda and boulder- 

 clay may be seen beneath it. This 

 may be called at Montreal the Sher- 

 brooke Street terrace. Another, the 

 Water-work terrace, is about 220 

 feet high, and is marked by an 

 indentation on the lower Silurian 

 limestone. At this level some 

 boulder-clay appears, and in places 

 the calcareous shales are decom- 

 posed to a great depth, evidencing 

 long sub-aerial action. Three other 

 terraces occur at heights of 386, 

 440, and 470 feet, and the latter 

 has, at one place above the village 

 of Cote des Neiges, a beach of sand 



