transported in a
counter-trade, and that such dust is found in South America, and is taken
up there by sand-spouts, like those of the ocean in form and action. Both
Humboldt and Gibbon have graphically described them. Yet I do not think
the point well taken. South-eastward of the Cape de Verdes, where the
surface-trades--which, becoming counter-trades, pass over these islands,
and, recurving, pass over the Mediterranean and south-western
Europe--should originate, there is a vast extent of unexplored continent
in the same latitude as the portion of South America where the dust is
found; and the same dry seasons, and the same spouts, in all probability,
exist in both. Until it be shown that such forms have no "_habitat_" in
central and southern and unexplored Africa, upon the same latitudes as in
South America, it may fairly be presumed that the dust is taken up there.
Indeed, the _curve_ upon which this dust is found to fall, in the greatest
quantities, is very remarkable, and corresponds remarkably with the _law
of curvature_ of the counter-trade we have considered, and with the
progress of a storm upon that coast, and over the Mediterranean,
investigated by Colonel Reid. (See Reid, on Storms and Variable Winds, p.
276.) This _curve clearly indicates the origin of the dust in South
Africa_.

The second point is, that ashes from the volcanos of Mexico and Central
America have fallen to the north-east of the place where they were
ejected. Mr. Redfield has grouped these instances of volcanic eruption
usually cited, and I copy from him:

"We learn from Humboldt, that in the great eruption of Jorullo, a
volcano of southern Mexico, which is 2,100 feet above the sea, in
latitude 18 deg. 45', longitude 161 deg. 30', the roofs of the houses in
Queretaro, more than 150 miles north, 37 deg. east from the volcano, were
covered with the volcanic dust. In January, 1845, an eruption took
place in the volcano of Cosiguina, on the Pacific coast of Central
America, in latitude 13 deg. north, and having an elevation of 3,800
feet, the ashes from which fell on the island of Jamaica, distant 730
miles north, 60 deg. east from the volcano. The elevated currents by
which volcanic ashes are thus transported are seldom or never of a
transient or fortuitous character; and these results, therefore,
afford us one of the best indications of their general course. Thus,
the progress o