infall is now so great in proportion to
the evaporation that dead seas are impossible.

Turning now to the question of how lake basins are formed, we note a
great variety in the conditions which may bring about their
construction. The greatest agent, or at least that which operates in
the construction of the largest basins, are the irregular movements of
the earth, due to the mountain-building forces. Where this work goes
on on a large scale, basin-shaped depressions are inevitably formed.
If all those which have existed remained, the large part of the lands
would be covered by them. In most cases, however, the cutting action
of the streams has been sufficient to bring the drainage channels down
to the bottom of the trough, while the influx of sediments has served
to further the work by filling up the cavities. Thus at the close of
the Cretaceous period there was a chain of lakes extending along the
eastern base of the Rocky Mountains, constituting fresh-water seas
probably as large as the so-called Great Lakes of North America. But
the rivers, by cutting down and tilling up, have long since
obliterated these water areas. In other cases the tiltings of the
continent, which sometimes oppose the flow of the streams, may for a
time convert the upper part of a river basin which originally sloped
gently toward the sea into a cavity. Several cases of this description
occurred in New England in the closing stages of the Glacial period,
when the ground rose up to the northward.

We have already noted the fact that the basin of a dead sea becomes in
course of time the seat of extensive salt deposits. These may, indeed,
attain a thickness of many hundred feet. If now in the later history
of the country the tract of land with the salt beneath it were
traversed by a stream, its underground waters may dissolve out the
salt and in a way restore the basin to its original unfilled
condition, though in the second state that of a living lake. It seems
very probable that a portion at least of the areas of Lakes Ontario,
Erie, and Huron may be due to this removal of ancient salt deposits,
remains of which lie buried in the earth in the region bordering these
basins.

By far the commonest cause of lake basins is found in the
irregularities of the surface which are produced by the occupation of
the country by glaciers. When these great sheets of ice lie over a
land, they are in motion down the slopes on which they rest; they wear
the bed