which had formed, in the spring of 1828, where the opposing currents of
the two rivers neutralized each other, and caused a retardation in the
motion, had been undermined; and the following is an exact
representation of the arrangement of laminae exposed in a vertical
section. The length of the portion here seen is about twelve feet, and
the height five. The strata A A consist of irregular alternations of
pebbles and sand in undulating beds: below these are seams of very fine
sand B B, some as thin as paper, others about a quarter of an inch
thick. The strata C C are composed of layers of fine greenish-gray sand
as thin as paper. Some of the inclined beds will be seen to be thicker
at their upper, others at their lower extremity, the inclination of some
being very considerable. These layers must have accumulated one on the
other by lateral apposition, probably when one of the rivers was very
gradually increasing or diminishing in velocity, so that the point of
greatest retardation caused by their conflicting currents shifted
slowly, allowing the sediment to be thrown down in successive layers on
a sloping bank. The same phenomenon is exhibited in older strata of all
ages.[376]

[Illustration: Fig. 26.

Section of a sand-bank in the bed of the Arve at its confluence with the
Rhone, showing the stratification of deposits where currents meet.]

If the bed of a lake or of the sea be sinking, whether at a uniform or
an unequal rate, or oscillating in level during the deposition of
sediment, these movements will give rise to a different class of
phenomena, as, for example, to repeated alternations of shallow-water
and deep-water deposits, each with peculiar organic remains, or to
frequent repetitions of similar beds, formed at a uniform depth, and
inclosing the same organic remains, and to other results too complicated
and varied to admit of enumeration here.

_Formation of conglomerates._--Along the base of the Maritime Alps,
between Toulon and Genoa, the rivers, with few exceptions, are now
forming strata of conglomerate and sand. Their channels are often
several miles in breadth, some of them being dry, and the rest easily
forded for nearly eight months in the year, whereas during the melting
of the snow they are swollen, and a great transportation of mud and
pebbles takes place. In order to keep open the main road from France to
Italy, now carried along the sea-coast, it is necessary to remove
annually great masses of