158 THE PHYSICAL GEOGRAPHY OF THE SEA. 



empty them into the sea. It is also admitted that the great lakes 

 would, from this cause, be salt, if they had no sea drainage. The 

 Niagara River passes these river salts from the upper lakes into 

 Ontario, and the St. Lawrence conveys them thence to the sea. 

 Now the basins or bottoms of all these upper lakes are far below 

 the top of the rock over which the Niagara pitches its flood. And, 

 were the position assumed by this writer correct, viz., that if the 

 water in any of these lakes should, in consequence of its specific 

 gravity, once sink below the level of the shoals in the rivers and 

 straits which connect them, it never could flow out again, and 

 consequently must remain there forever* — were this principle 

 physically correct, would not the water at the bottom of the lakes 

 gradually have received salt sufiicient, during the countless ages 

 that they have been sending it off to the sea, to make this ever- 

 lastingly pent-up water briny, or at least quite different in its con- 

 stituents from that of the surface ? We may presume that the 

 water at the bottom of every extensive and quiet sheet of water, 

 whetlicr salt or fresh, is at the bottom by reason of specific grav- 

 ity ; but tliat it does not remain there forever we have abundant 

 proof. If so, the Niagara River would be fed by Lake Erie only 

 from that layer of water which is above the level of the top of the 

 rock at the Falls. Consequently, wherever the breadth of that 

 river is no greater than it is at the Falls, we should have a cur- 

 rent as rapid as it is at the moment of passing the top of the rock 

 to make the leap. To see that such is not the way of Nature, we 

 have but to look at any common mill-pond when the water is run- 

 ning over the dam. The current in the pond that feeds the over- 

 flow is scarcely perceptible, for " still water runs deep." More- 

 over, we know it is not such a skimming current as the geologist 

 would make, which runs from one lake to another ; for wherever 

 above the Niagara Falls the water is deep, there we are sure to 

 find tlie current sluggish, in comparison with the rate it assumes 

 as it approaches the Falls ; and it is sluggish in deep places, rapid 

 in shallow ones, because it is fed from below. The common 

 "wastes" in our canals teach us this fact. 



433. The reasoning of this celebrated geologist appears to be 



* See paragraph quoted (^ 430) from " Lyell's Principles of Geology." 



