teepness of which would depend on
the volume of water. The erosive action would then cease, but the
weathering of the sides and consequent widening would continue, and the
river would wander from one part of its valley to another, spreading the
materials and forming a river plain. At length, as the rapidity still
further diminished, it would no longer have sufficient power even to
carry off the materials brought down. It would form, therefore, a cone
or delta, and instead of meandering, would tend to divide into different
branches. These three stages, we may call those of--

1. Deepening and widening;

2. Widening and levelling;

3. Filling up;

and every place in the second stage has passed through the first; every
one in the third has passed through the second.

A velocity of 6 inches per second will lift fine sand, 8 inches will
move sand as coarse as linseed, 12 inches will sweep along fine gravel,
24 inches will roll along rounded pebbles an inch diameter, and it
requires 3 feet per second at the bottom to sweep along angular stones
of the size of an egg.

When a river has so adjusted its slope that it neither deepens its bed
in the upper portion of its course, nor deposits materials, it is said
to have acquired its "regimen," and in such a case if the character of
the soil remains the same, the velocity must also be uniform. The
enlargement of the bed of a river is not, however, in proportion to the
increase of its waters as it approaches the sea. If, therefore, the
slope did not diminish, the regimen would be destroyed, and the river
would again commence to eat out its bed. Hence as rivers enlarge, the
slope diminishes, and consequently every river tends to assume some such
"regimen" as that shown in Fig. 46.

Now, suppose that the fall of the river is again increased, either by a
fresh elevation, or locally by the removal of a barrier. Then once more
the river regains its energy. Again it cuts into its old bed, deepening
the valley, and leaving the old plain as a terrace high above its new
course. In many valleys several such terraces may be seen, one above the
other. In the case of a river running in a transverse valley, that is to
say of a valley lying at right angles to the "strike" or direction of
the strata (such, for instance, as the Reuss), the water acts more
effectively than in longitudinal valleys running along the strike. Hence
the lateral valleys have been less deeply excavated th