It is not easy for us to
overestimate the effects which causes in every day action must produce when the
multiplying power of time is taken into account.
Attempts were made by Manfredi in 1736, and afterwards by Playfair in 1802, to
calculate the time which it would require to enable the rivers to deliver over
the whole of the land into the basin of the ocean. The data were at first too
imperfect and vague to allow them even to approximate to safe conclusions. But
in our own time similar investigations have been renewed with more prospect of
success, the amount brought down by many large rivers to the sea having been
more accurately ascertained. Mr. Alfred Tylor, in 1850, inferred that the
quantity of detritus now being distributed over the sea-bottom would, at the end
of 10,000 years, cause an elevation of the sea-level to the extent of at least
three inches. (Tylor Philosophical Magazine 4th series page 268 1850.)
Subsequently Mr. Croll, in 1867, and again, with more exactness, in 1868,
deduced from the latest measurement of the sediment transported by European and
American rivers the rate of subaerial denudation to which the surface of large
continents is exposed, taking especially the hydrographical basin of the
Mississippi as affording the best available measure of the average waste of the
land.
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