from Africa was continually falling. In some of this dust which fell in
the open ocean at a distance of between 330 and 380 miles from the
African coast, there were many particles of stone, about 1/1000 of an
inch square. Nearer to the coast the water has been seen to be so much
discoloured by the falling dust, that a sailing vessel left a track
behind her. In countries, like the Cape Verde Archipelago, where it
seldom rains and there are no frosts, the solid rock nevertheless
disintegrates; and in conformity with the views lately advanced by a
distinguished Belgian geologist, De Koninck, such disintegration may be
attributed in chief part to the action of the carbonic and nitric acids,
together with the nitrates and nitrites of ammonia, dissolved in the dew.
In all humid, even moderately humid, countries, worms aid in the work of
denudation in several ways. The vegetable mould which covers, as with a
mantle, the surface of the land, has all passed many times through their
bodies. Mould differs in appearance from the subsoil only in its dark
colour, and in the absence of fragments or particles of stone (when such
are present in the subsoil), larger than those which can pass through the
alimentary canal of a worm. This sifting of the soil is aided, as has
already been remarked, by burrowing animals of many kinds, especially by
ants. In countries where the summer is long and dry, the mould in
protected places must be largely increased by dust blown from other and
more exposed places. For instance, the quantity of dust sometimes blown
over the plains of La Plata, where there are no solid rocks, is so great,
that during the gran seco, 1827 to 1830, the appearance of the land,
which is here unenclosed, was so completely changed that the inhabitants
could not recognise the limits of their own estates, and endless lawsuits
arose. Immense quantities of dust are likewise blown about in Egypt and
in the south of France. In China, as Richthofen maintains, beds
appearing like fine sediment, several hundred feet in thickness and
extending over an enormous area, owe their origin to dust blown from the
high lands of central Asia. {215} In humid countries like Great Britain,
as long as the land remains in its natural state clothed with vegetation,
the mould in any one place can hardly be much increased by dust; but in
its present condition, the fields near high roads, where there is much
traffic, must receive a considerable amount of dust, and when fields are
harrowed during dry and windy weather, clouds of dust may be seen to be
blown away. But in all these cases the surface-soil is merely
transported from one place to another. The dust which falls so thickly
within our houses consists largely of organic matter, and if spread over
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