degree of atmospheric dryness was accompanied by continual flashes of
lightning. Is it not an uncommon case, thus to find a remarkable
degree of aerial transparency with such a state of weather?
Generally the atmosphere is hazy; and this is caused by the falling of
impalpably fine dust, which was found to have slightly injured the
astronomical instruments. The morning before we anchored at Porto
Praya, I collected a little packet of this brown-coloured fine dust,
which appeared to have been filtered from the wind by the gauze of the
vane at the mast-head. Mr. Lyell has also given me four packets of
dust which fell on a vessel a few hundred miles northward of these
islands. Professor Ehrenberg [3] finds that this dust consists in
great part of infusoria with siliceous shields, and of the siliceous
tissue of plants. In five little packets which I sent him, he has
ascertained no less than sixty-seven different organic forms! The
infusoria, with the exception of two marine species, are all
inhabitants of fresh-water. I have found no less than fifteen
different accounts of dust having fallen on vessels when far out in the
Atlantic. From the direction of the wind whenever it has fallen, and
from its having always fallen during those months when the harmattan is
known to raise clouds of dust high into the atmosphere, we may feel
sure that it all comes from Africa. It is, however, a very singular
fact, that, although Professor Ehrenberg knows many species of
infusoria peculiar to Africa, he finds none of these in the dust which
I sent him. On the other hand, he finds in it two species which
hitherto he knows as living only in South America. The dust falls in
such quantities as to dirty everything on board, and to hurt people's
eyes; vessels even have run on shore owing to the obscurity of the
atmosphere. It has often fallen on ships when several hundred, and
even more than a thousand miles from the coast of Africa, and at points
sixteen hundred miles distant in a north and south direction. In some
dust which was collected on a vessel three hundred miles from the land,
I was much surprised to find particles of stone above the thousandth of
an inch square, mixed with finer matter. After this fact one need not
be surprised at the diffusion of the far lighter and smaller sporules
of cryptogamic plants.
The geology of this island is the most interesting part of its natural
history. On entering the harbour, a perfectly horizontal white band,
in the face of the sea cliff, may be seen running for some miles along
7