Page:Popular Science Monthly Volume 31.djvu/351

Rh are constantly sifting sediment on its bottom. The St. Louis River has a large delta, making access to Superior City so difficult as to require 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 Rhône 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 continuously 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. Allowing 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 greatest 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 temperature has not yet been reached, and that the line of greatest snowfall 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