n the mountains. The deposit of each year
may acquire some degree of consistency before that of the succeeding
year is superimposed. A variety of circumstances also give rise
annually, or sometimes from day to day, to slight variations in color,
fineness of the particles, and other characters, by which alternations
of strata distinct in texture and mineral ingredients must be produced.
Thus, for example, at one period of the year, drift-wood may be carried
down, and, at another, mud, as was before stated to be the case in the
delta of the Mississippi; or at one time, when the volume and velocity
of the stream are greatest, pebbles and sand may be spread over a
certain area, over which, when the waters are low, fine matter or
chemical precipitates are formed. During inundations, the turbid current
of fresh water often repels the sea for many miles; but when the river
is low, salt water again occupies the same space. When two deltas are
converging, the intermediate space is often, for reasons before
explained, alternately the receptacle of different sediments derived
from the converging streams (see p. 272). The one is, perhaps, charged
with calcareous, the other with argillaceous matter; or one sweeps down
sand and pebbles, the other impalpable mud. These differences may be
repeated with considerable regularity, until a thickness of hundreds of
feet of alternating beds is accumulated. The multiplication, also, of
shells and corals in particular spots, and for limited periods, gives
rise occasionally to lines of separation, and divides a mass which might
otherwise be homogeneous into distinct strata.

An examination of the shell marl now forming in the Scotch lakes, or the
sediment termed "warp," which subsides from the muddy water of the
Humber and other rivers, shows that recent deposits are often composed
of a great number of extremely thin layers, either even or slightly
undulating, and preserving a general parallelism to the planes of
stratification. Sometimes, however, the laminae in modern strata are
disposed diagonally at a considerable angle, which appears to take place
where there are conflicting movements in the waters. In January, 1829, I
visited, in company with Professor L. A. Necker, of Geneva, the
confluence of the Rhone and Arve, when those rivers were very low, and
were cutting channels through the vast heaps of dabris thrown down from
the waters of the Arve in the preceding spring. One of the sandbanks