effect, to which neither of them unaided by the other could give
rise,--as when repeated earthquakes unite with running water to widen a
valley; or when a thermal spring rises up from a great depth, and
conveys the mineral ingredients with which it is impregnated from the
interior of the earth to the surface. Sometimes the organic combine with
the inorganic causes; as when a reef, composed of shells and corals,
protects one line of coast from the destroying power of tides or
currents, and turns them against some other point; or when drift timber,
floated into a lake, fills a hollow to which the stream would not have
had sufficient velocity to convey earthy sediment.

It is necessary, however, to divide our observations on these various
causes, and to classify them systematically, endeavoring as much as
possible to keep in view that the effects in nature are mixed and not
simple, as they may appear in an artificial arrangement.

In treating, in the first place, of the aqueous causes, we may consider
them under two divisions; first, those which are connected with the
circulation of water from the land to the sea, under which are included
all the phenomena of rain, rivers, glaciers, and springs; secondly,
those which arise from the movements of water in lakes, seas, and the
ocean, wherein are comprised the phenomena of waves, tides, and
currents. In turning our attention to the former division, we find that
the effects of rivers may be subdivided into, first, those of a
destroying and transporting, and, secondly, those of a renovating
nature; in the former are included the erosion of rocks and the
transportation of matter to lower levels; in the renovating class, the
formation of deltas by the influx of sediment, and the shallowing of
seas; but these processes are so intimately related to each other, that
it will not always be possible to consider them under their separate
heads.

_Fall of Rain._--It is well known that the capacity of the atmosphere to
absorb aqueous vapor, and hold it in suspension, increases with every
increment of temperature. This capacity is also found to augment in a
higher ratio than the augmentation of the heat. Hence, as was first
suggested by the geologist, Dr. Hutton, when two volumes of air, of
different temperatures, both saturated with moisture, mingle together,
clouds and rain are produced, for a mean degree of heat having resulted
from the union of the two moist airs, the excess of vapor