oad has been several times turned to avoid
this cavity, the enlargement of which is still proceeding, and the old
line of road may be seen to have held its course directly over what is
now the wildest part of the ravine. In the perpendicular walls of this
great chasm appear beds of clay and sand, red, white, yellow, and
green, produced by the decomposition in situ of hornblendic gneiss,
with layers and veins of quartz, which remain entire, to prove that the
whole mass was once solid and crystalline.

[Illustration: Fig. 14.

Ravine on the farm of Pomona, near Milledgeville, Georgia, as it
appeared January, 1846.

Excavated in twenty years, 55 feet deep, and 180 feet broad.]

I infer, from the rapidity of the denudation which only began here after
the removal of the native wood, that this spot, elevated about 600 feet
above the sea, has been always covered with a dense forest, from the
remote time when it first emerged from the sea. The termination of the
cavity on the right hand in the foreground is the head or upper end of
the ravine, and in almost every case, such gulleys are lengthened by the
streams cutting their way backwards. The depth at the upper end is
often, as in this case, considerable, and there is usually at this
point, during floods, a small cascade.

_Sinuosities of rivers._--In proportion as such valleys are widened,
sinuosities are caused by the deflection of the stream first to one
side and then to the other. The unequal hardness of the materials
through which the channel is eroded tends partly to give new directions
to the lateral force of excavation. When by these, or by accidental
shiftings of the alluvial matter in the channel, the current is made to
cross its general line of descent, it eats out a curve in the opposite
bank, or in the side of the hills bounding the valley, from which curve
it is turned back again at an equal angle, so that it recrosses the line
of descent, and gradually hollows out another curve lower down in the
opposite bank, till the whole sides of the valley, or river bed, present
a succession of salient and retiring angles. Among the causes of
deviation from a straight course, by which torrents and rivers tend in
mountainous regions to widen the valleys through which they flow, may be
mentioned the confluence of lateral torrents, swollen irregularly at
different seasons by partial storms, and discharging at different times
unequal quantities of sand, mud, and pebbles, i