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. For, notwithstanding many of these side tributaries are quite
short, and, during late summer, feeble, they all become powerful
torrents in springtime when the snow is melting, and carry not only sand
and pine-needles, but large trunks and boulders tons in weight, sweeping
them down their steeply inclined channels and into the lake basins with
astounding energy. Many of these side affluents also have the advantage
of access to the main lateral moraines of the vanished glacier that
occupied the canon, and upon these they draw for lake-filling material,
while the main trunk stream flows mostly over clean glacier pavements,
where but little moraine matter is ever left for them to carry. Thus a
small rapid stream with abundance of loose transportable material within
its reach may fill up an extensive basin in a few centuries, while a
large perennial trunk stream, flowing over clean, enduring pavements,
though ordinarily a hundred times larger, may not fill a smaller basin
in thousands of years.

The comparative influence of great and small streams as lake-fillers is
strikingly illustrated in Yosemite Valley, through which the Merced
flows. The bottom of the valley is now composed of level meadow-lands
and dry, sloping soil-beds planted with oak and pine, but it was once a
lake stretching from wall to wall and nearly from one end of the valley
to the other, forming one of the most beautiful cliff-bound sheets of
water that ever ex