Ch. VI.] 



OF AQUEOUS CAUSES CONTEOVERTED 



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of mountain 



In the latter case, 



mountains themselves could do no more 



certain quantity of atmospheric air, whereas, the instanta- 



neous formation of the shoal would displace a vast body ot 

 water, which*being heaved up to a great height might roll 

 over and permanently submerge a large portion of a continent. 



If we restrict ourselves to combinations of causes at present 

 known, it would seem that the two principal sources of extra- 

 ordinary inundations are, first, the escape of the waters of a 

 large lake raised far above the sea ; and, secondly, the pouring 

 down of a marine current into lands depressed below the 

 mean level of the ocean. 



As an example of the first of these cases, we may take 



I 



Lake Superior, which is more than 400 geographical miles in 

 length and about 150 in breadth, having an average depth of 

 from 500 to 900 feet. The surface of this vast body of fresh 

 water is no less than 600 feet above the level of the ocean ; 

 the lowest part of the barrier which separates the lake on its 

 south-west side from those streams which flow into the head 

 waters of the Mississippi being about 600 feet high. If, 

 therefore, a series of subsidences should lower any part of this 

 barrier, even a few yards at a time, or if earthquakes should 

 rend it open, the breaches thus made might allow the sudden 

 escape of vast floods of water into a hydrographical basin of 

 enormous extent. If the event happened in the dry season, 

 when the ordinary channels of the Mississippi and its tribu- 

 taries are in a great degree empty, the inundation might not 

 be considerable ; but if in the flood-season, a region capable 

 of supporting a population of many millions might be 

 suddenly submerged. But even this event would be insuf- 

 ficient to cause a violent rush of water, and to produce those 

 effects usually called diluvial ; for the difference of level of 

 600 feet between Lake Superior and the Gulf of Mexico, when 

 distributed over a distance of 1,800 miles, would give an 

 average fall of only four inches per mile. 



The second case before adverted to is where there are large 

 tracts of dry land beneath the mean level of the ocean. It 

 seems, after much controversy, to be at length a settled point, 

 that the Caspian is really 83 feet 6 inches lower than the 





