148 Canadian Record of Science. 



extent. Its eastern shores form here the western limits so 

 far as observable, of the great eozoic rocks, and were also, 

 no doubt, the eastern shore of the great inland sea. 



It has been proposed to lower the level of Lake Manitoba 

 by removing the obstructions in the channel through which 

 its waters are conducted by way of St. Martin's Lake to Lake 

 Winnipeg, and there is no doubt that ij this could be effected 

 to the extent of only a few feet, large tracts of country 

 would be reclaimed which around its margin are presently 

 more or less under water. The southern end of the lake is 

 now bounded by a narrow sand bank elevated a few feet 

 above the water. Inside of this are very considerable tracts 

 once forming a part of the lake and now more or less sub- 

 merged, but in which the process of growth and decay of 

 the grasses and aquatic plants and the resulting annual de- 

 posit of soil will eventually end in their reclamation from the 

 water. This same process is going on in a large tract cov- 

 ering four or five townships about ten miles to the westward 

 of Lake Manitoba, known as the Big Grass Marsh, as well 

 as in many other places in the province, and will, in coming 

 years, result in the formation of prairie land with a rich 

 covering of black vegetable loam. 



The County of Essex in Ontario has a considerable ex- 

 tent of prairie land which was no doubt largely formed 

 under similar conditions of annual growth and decay, and 

 which in its origin points to a time when Lakes Erie and 

 St. Clair, were more intimately connected than they now 

 are. Long Point, Point Pekle and Sandusky Harbour, all 

 on Lake Erie, are illustrations of prairies now in process of 

 formation. These prairies all have a fresh water origin. 

 Those south of Montreal, and extending beyond St. Johns 

 and St. Hyacinthe, are rather of marine origin, dating back 

 to the Leda clay period, when the drift clays were re-assorted 

 under water and added to, and the land then elevated to its 

 present level. 



Probably contemporaneous with the formation of the 

 prairies was the creation of the deep valleys of the Assini- 

 boine and the Qu'Appelle Rivers. The valley of the Assini- 

 boine above Brandon has an average depth of towards 200 



