ultimately reach the
sea, unless it is first deposited in some lake, or by the overflowing of a
river goes to form an alluvial plain. The universality of this subaerial
denudation, both as regards space and time, renders it certain that its
cumulative effects must be very great; but no attempt seems to have been
made to determine the magnitude of these effects till Mr. Alfred Tylor, in
1853,[33] pointed out that by measuring the quantity of solid matter
brought down by rivers (which can be done with considerable accuracy), we
may obtain the amount of lowering of the land-area, and also the rise of
the ocean level, owing to the quantity of matter deposited on its floor. A
few years later Dr. Croll applied the same method in more detail to an
estimate of the amount by which the land is lowered in a given period; and
the validity of this method has been upheld by Sir A. Geikie, Sir Charles
Lyell, and all our best geologists, as affording a means of actually
determining with some approach to accuracy, the time occupied by one
important phase of geological change.

The quantity of matter carried away from the land by a river is greater
than at first sight appears, and is more likely to be under- than
over-estimated. By taking samples of water near the mouth of a river (but
above the influence of the tide) at a sufficient number of points in its
channel and at different depths, and repeating this daily or at other short
intervals throughout the year, it is easy to determine the quantity of
solid matter held in suspension and solution; and if corresponding
observations determine the quantity of water that is discharged, the total
amount of solid matter brought down annually may be calculated. But besides
this, a considerable quantity of sand or even gravel is carried along the
bottom or bed of the river, and this has rarely been estimated, so that the
figures hitherto obtained are usually under the real quantities. There is
also another source of error caused by the quantity of matter the river may
deposit in lakes or in flooded lands during its course, for this adds to
the amount of denudation performed by the river, although {104} the matter
so deposited does not come down to the sea. After a careful examination of
all the best records, Sir A. Geikie arrives at the following results, as to
the quantity of matter removed by seven rivers from their basins, estimated
by the number of years required to lower the whole surface an