much more powerful agents, that is if we consider the whole area of the
land. The long lines of escarpment which stretch across several parts of
England were formerly considered to be undoubtedly ancient coast-lines;
but we now know that they stand up above the general surface merely from
resisting air, rain and frost better than the adjoining formations. It
has rarely been the good fortune of a geologist to bring conviction to
the minds of his fellow-workers on a disputed point by a single memoir;
but Mr. Whitaker, of the Geological Survey of England, was so fortunate
when, in 1867, he published his paper On sub-aerial Denudation, and on
Cliffs and Escarpments of the Chalk. {211} Before this paper appeared,
Mr. A. Tylor had adduced important evidence on sub-aerial denudation, by
showing that the amount of matter brought down by rivers must infallibly
lower the level of their drainage basins by many feet in no immense lapse
of time. This line of argument has since been followed up in the most
interesting manner by Archibald Geikie, Croll and others, in a series of
valuable memoirs. {212} For the sake of those who have never attended to
this subject, a single instance may be here given, namely, that of the
Mississippi, which is chosen because the amount of sediment brought down
by this great river has been investigated with especial care by order of
the United States Government. The result is, as Mr. Croll shows, that
the mean level of its enormous area of drainage must be lowered 1/4566 of
a foot annually, or 1 foot in 4566 years. Consequently, taking the best
estimate of the mean height of the North American continent, viz. 748
feet, and looking to the future, the whole of the great Mississippi basin
will be washed away, and brought down to the sea-level in less than
4,500,000 years, if no elevation of the land takes place. Some rivers
carry down much more sediment relatively to their size, and some much
less than the Mississippi.
Disintegrated matter is carried away by the wind as well as by running
water. During volcanic outbursts much rock is triturated and is thus
widely dispersed; and in all arid countries the wind plays an important
part in the removal of such matter. Wind-driven sand also wears down the
hardest rocks. I have shown {213} that during four months of the year a
large quantity of dust is blown from the north-western shores of Africa,
and falls on the Atlantic over a space of 1600 miles in latitude, and for
a distance of from 300 to 600 miles from the coast. But dust has been
seen to fall at a distance of 1030 miles from the shores of Africa.
During a stay of three weeks at St. Jago in the Cape Verde Archipelago,
the atmosphere was almost always hazy, and extremely fine dust coming
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