ch evidence, however, the most careful
investigations have failed to detect. Apart, however, from the above
objections, there seems to be little doubt, from evidence afforded by
traces of birds' nests, &c., that the guano found in the nitrate-beds
was deposited subsequent to the formation of the nitrate of soda.

_Nitric Acid derived from Sea-weed._

The most probable theory seems to be that put forward by Noellner. The
origin of the nitric acid is, according to him, to be ascribed to the
decay of great masses of sea-weed, which, by means of hurricanes such as
are still prevalent in these districts, were driven into the lagoons.
The chief difficulty in the way of accepting this theory is the enormous
quantity of sea-weed required to produce the millions of tons of nitric
acid these deposits contain. It must be remembered, however, as bearing
upon this point, that the occurrence of gigantic masses of sea-weed in
the Pacific Ocean[210] is by no means uncommon even at the present time.
If, to understand the formation of coal, we must suppose the
Carboniferous period to be one during which exceptionally luxuriant
growth of vegetation took place, we may be permitted to suppose a
similar luxuriant growth of sea-weed during the formation of the nitrate
deposits. Very strong confirmation of the truth of this theory is
further afforded by the presence in large quantities, in the raw nitrate
of soda, of iodine, a substance characteristic of sea-weed; while pieces
of sea-weed still undecomposed are met with here and there. On the
whole, therefore, this theory, while not free from difficulties, seems
to be the most worthy of acceptance as regards the origin of the nitrate
deposits.[211]

_Appearance of Nitrate-fields._

Having thus discussed the origin of the nitrate-fields, we may now give
a more detailed description of their appearance. The chief deposits at
present being worked are those lying in the Pampa de Tamarugal, in the
province of Tarapaca. They stretch to a distance of thirty or forty
miles inland, from Pisagua southwards to somewhat beyond the town of
Iquique. This huge desert, as has been already indicated, seems to be
entirely destitute of all vegetation and animal life. Even in the
immediately adjoining country the only kind of vegetation that seems to
grow is a species of _acacia_. The few streams that are found in this
neighbourhood are entirely fed by the melting snow from the Cordilleras.
Darwin describe