But while its shores are being enriched, the soil-beds creep out with incessant growth, contracting its area, while the lighter mud-particles deposited on the bottom cause it to grow constantly shallower, until at length the last remnant of the lake vanishes,--closed forever in ripe and natural old age.

And now its feeding-stream goes winding on without halting through the new gardens and groves that have taken its place.
The length of the life of any lake depends ordinarily upon the capacity of its basin, as compared with the carrying power of the streams that flow into it, the character of the rocks over which these streams flow, and the relative position of the lake toward other lakes.

In a series whose basins lie in the same canon, and are fed by one and the same main stream, the uppermost will, of course, vanish first unless some other lake-filling agent comes in to modify the result; because at first it receives nearly all of the sediments that the stream brings down, only the finest of the mud-particles being carried through the highest of the series to the next below.

Then the next higher, and the next would be successively filled, and the lowest would be the last to vanish.

But this simplicity as to duration is broken in upon in various ways, chiefly through the action of side-streams that enter the lower lakes direct.