THE NORTH AMERICAN LAKES. 337 



are constantly sifting sediment on its bottom. The St. Louis River 

 has a large delta, making access to Superior City so difficult as to re- 

 quire annual dredging. It has been estimated that the sediment yearly 

 carried to the Gulf by the Mississippi is sufficient to raise a square 

 mile two hundred and forty-one feet, or equaling a cubic mile in a 

 little less than twenty-two years. Where the Rhone enters Lake 

 Geneva, its water is loaded to its capacity with sediment ; where it 

 leaves the lake, it is crystal clear. This solid matter is being continu- 

 ously deposited, raising the bottom, while the deepening channel is 

 sinking the surface. In Indiana and Ohio are numerous shallow lake- 

 basins, now dry land, their bottoms, level as a floor, with often several 

 feet of rich alluvium, still bearing testimony to the agencies that have 

 despoiled them of their waters. The celebrated Walnut Level of the 

 former State is but an ancient lake-basin, and it is to this deposit of 

 sediment that it owes its far-famed fertility. It is not improbable 

 that by the time the Falls of Niagara shall have broken through the 

 rim of Lake Superior, the sinking surface of the water may reach the 

 rising bottom only a little above the ocean-level. 



But why should the lakes begin to increase in size and frequency 

 at about the forty-first parallel ? The answer is to be found in the 

 relative amount of snowfall during the glacial epoch. More snow 

 falls at the south end of Hudson Bay than at Boothia Felix ; more 

 at Cape Farewell than at Cape Hatherton ; more at twenty degrees 

 south of the Arctic Circle than at any parallel north of it. The line 

 of greatest snowfall, like the isothermal line, is irregularly extended, 

 depending greatly on the wind-currents. The water of the southern 

 winds condenses and falls as they reach the colder latitudes. Allow- 

 ing the line of greatest snowfall to pass through Hudson Bay, it 

 must have been far to the south of it during the ice period. At or 

 near the forty-second parallel the glacier probably attained its great- 

 est thickness. Here it intrenched itself to stay ; and for a very long 

 time the winter snows must have compensated for the summer thaws. 

 While, therefore, that part of the drift-region lying farther to the 

 south was uncovered, and the water-courses actively at work digging 

 out their beds and draining the land, the whole country to the north 

 was a field of ice. Simultaneously the ice and the line of greatest 

 snowfall receded northward. As the day's greatest heat is not when 

 the sun is on the meridian, but an hour or two later ; as the summer's 

 greatest heat is not when the sun is at its greatest altitude, but a 

 month or two later ; so it is probable that the highest average tem- 

 perature has not yet been reached, and that the line of greatest snow- 

 fall is still receding toward the poles. This fact, if it be one, must 

 presage for Arctic explorers wider and more open fields of work a 

 thousand years hence than to-day. 



The cause of the saltness of some American lakes is too patent to 

 require many words of explanation. It is probable that, when the 



TOL. XXXI. 22 



