ly, they manage to bear onward the waste which they receive. Even
where the blocks of stone cling in the bed, it is only a short time
before they are again set in motion or ground to pieces. If by chance
the detritus accumulates rapidly, the slope is steepened and the work
of the torrent made more efficient. As the torrent comes toward the
base of the mountains, where it neither finds nor can create steep
slopes over which to flow, its speed necessarily diminishes. With each
reduction in this feature its carrying power very rapidly diminishes.
Thus water flowing at the rate of ten miles an hour can urge stones
four times the mass that it can move when its speed is reduced to half
that rate. The result is that on the lowlands, with their relatively
gentle slopes, the combined torrents, despite the increase in the
volume of the stream arising from their confluence, have to lay down a
large part of their load of detritus.

If we watch where a torrent enters a mountain river, we observe that
the main stream in a way sorts over the waste contributed to it,
bearing on only those portions which its rate of flow will permit it
to carry, leaving the remainder to be built into the bank in the form
of a rude terrace. This accumulation may not extend far below the
point where the torrent which imported the _debris_ joins the main
stream; a little farther down, however, we are sure to find another
such junction and a second accumulation of terrace material. As these
contributions increase, the terrace accumulations soon become
continuous, lying on one side or the other of the river, sometimes
bordering both banks of the stream. In general, it can be said that so
long as the rate of fall of the torrent exceeds one hundred feet to
the mile it does not usually exhibit these shelves of detritus. Below
that rate of descent they are apt to be formed. Much, however, depends
upon the amount of detritus which the stream bears and the coarseness
of it; moreover, where the water goes through a gorge in the manner of
a flume with steep rocky sides, it can urge a larger amount before it
than when it traverses a wide valley, through which it passes, it may
be, in a winding way.

At first sight it may seem rather a fine distinction to separate
torrents from rivers by the presence or absence of terraces. As we
follow down the stream, however, and study its action in relation to
these terraces, and the peculiar history of the detritus of which they