326 



Glacial Phenomena of Canada, 



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Mr. Moffat's 

 Garden. 



Dorchester St. 



glaciers. We are indebted to Dr 

 Dawson of Montreal for the three 

 important subdivisions of the su- 

 perficial deposits, — namely, 1st, at 

 the base, lower boulder-clay and 

 gravel ; 2ndly, an unctuous clay, 

 with many marine shells, called by 

 him the " Leda-clay" (Led-Port- 

 landica), on which lie, 3rdly, beds 

 of gravel and sand, with shells, 

 one of the most common of which 

 is Saxicava Rugosa. These sub- 

 formations occasionnally pass into 

 each other where they join. The 



Saxicava sand he considers to have 

 been a shallow and sublittoral de- 

 posit ; the Leda-clay to have been 

 accumulated at depths of from 100 

 to 300 feet or more ; and the true 

 boulder-clay to have been formed 

 at an earlier period of subsidence, 

 during which an ocean spread over 

 the greater part of North America. 

 I shall have occasion to show that 

 at one time this sea was, in places, 

 probably over 3000 feet in depth. 

 The section (fig. 1)* across the 

 drift, which I drew at Montreal, 

 nearly agrees with Dr. Dawson's, 

 with the exception that I show five 

 terraces in the drift, while he gives 

 two. Their number may vary in 

 different localities; but they have 

 certainly been formed during the 

 last emergence of the country, each 

 terrace indicating a pause in eleva- 

 tion ; and in a great degree the 

 shells of the upper strata lie in a 

 debris of remodelled drift. The 

 two upper terraces, to the left of 



* For the Silurian geology of this diagram, I am indebted to the des- 

 cription of Sir Wm, Logan. 



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