Page:Popular Science Monthly Volume 31.djvu/350

336 wearing its ice-crown, can be only approximately guessed. It was, however, a long time—a time compared with which the historic period dwindles into a few days. During these long ages, the outlets of the more southern lakes were being sunken by the slow disintegration of the rock; and, doubtless, while the ice-king still reigned in the North, these now "sweet vales of Avoca" had become dry land, and possibly the homes of men.

The physical geography of the country is a strong witness to the truth of this hypothesis. To the south of the drift, where the rivers have been flowing ever since the continent was above the sea, the lake-bowls have been drained to their bottoms; in the southern part of the drift they have been only partially drained, while to the north they are still filled with water. This exactly accords with the hypothesis.

It has been argued that the southern half of the continent has been longer from under the sea, and therefore that the rivers have had more time for deepening their channels. Exactly the converse is the truth. The Laurentian Hills are not only the oldest land on the continent, but, so far as now known, the oldest on the planet.

It may be admitted, without affecting this hypothesis, except it be to re-enforce it, that the great weight of the accumulated ice must have sunk the more northern region in some parts below the sea-level, and that in its gradual melting these rose again, preserving their equilibrium.

The gradual recession of the ice northward, and therefore the first uncovering of the southern half of the drift-region, must have taken place. That this recession was slow, and during a long period of years, must be true. That during this long time the rivers must have been deepening their channels and emptying the lake-basins, is so manifest as to need only to be stated. It we should term that portion of the drift-region, south of forty-one and a half degrees, sub-glacial, we shall find the southern part of it wholly drained, the middle and northern part of it only partially so, the lakes and lakelets increasing in number and magnitude on approaching its northern boundary.

But the "dry lakes" of the Pacific slope, what of them? Their margins are still intact. True; but it is because they are dry lakes that their margins have not been cut through and no rivers connect them with the sea. These lake-basins, too, were once filled with water; the rain-supply was not equal to the evaporation, and hence their gradual drying up.

Another cause for the destruction of lakes, too important to be left out of the account, is the continuous deposit of sediment on their bottoms. While their effluents are continuously sinking their surfaces, their affluents are no less industriously raising their bottoms by deposits from the land. The bottom of Lake Superior at its deepest place is about three hundred feet below the sea-level; it can not, therefore, be wholly drained by its outlet. But its supplying streams