THE GREAT LAKES OF NORTH AMERICA. 471 



to the sea by the river St. Lawrence. These great 

 lakes form admirable examples of overflow reservoirs, 

 such as we have spoken of. We here see a mighty 

 river channel still in process of formation, and in ages 

 yet to come, should the Niagara falls cut their way 

 back through the rocky obstruction into softer strata, 

 the world might yet see these great lakes much re- 

 duced in size, and portions of their beds converted into 

 alluvial plains, such as we see many examples of 

 throughout the world. Great level plains of this sort 

 often excite the astonishment of beholders, and give 

 rise to many surmises respecting their probable origin. 

 In many cases we believe that they have been thus 

 formed by the deposits of water, beneath which they 

 have been submerged when they were the beds of 

 immense lakes, at an earlier epoch of the world's 

 existence. In some cases it may be that these waters 

 may have been those of the ocean; but if so, the fact 

 is generally made apparent by the discovery of marine 

 shells and other remains of the ancient sea ; but there 

 are other notable cases, where for great depths below 

 the present surface of the land the remains are all of 

 fresh water origin; and that being so, is strong pre- 

 sumptive evidence of the former existence of some 

 mighty lake, from whose waters these alluvial deposits 

 were formed, throughout an epoch of geological time 

 of whose magnitude the human mind can form no 

 adequate idea, and in comparison with which the whole 

 period of historical time is dwarfed into absolute insig- 

 nificance. The time occupied in the formation of 

 lacustrine deposits is immense. Mr. G. F. Richardson, 

 formerly one of the staff of the British Museum, in 

 regard to this matter says: "Lakes are ascertained to 



