. Successive lines, indicative of
old sea-beaches, can be distinctly traced stretching inland, one behind
the other; and patches of sea-sand and water-worn stone, found at a
great distance from the coast, both in valleys and at altitudes much
greater even than 4000 feet, point to the same conclusion.[208] The
difficulty, therefore, of altitude and distance from the coast cannot be
regarded as insuperable.

_Source of Nitric Acid._

A difficulty, however, which is not so easily met, is afforded by the
presence of the nitric acid which, in combination with the soda, forms
the nitrate of soda. It is scarcely necessary to inform our readers that
nitrogen--except, of course, in small quantities in the free state--is
not a normal constituent of salt water. The question, therefore, of
greatest interest in connection with the formation of these nitrate-beds
is, Whence has the nitric acid been derived? Several theories have been
put forward to account for it.

_Guano Theory._

One is to the effect that it owes its origin to huge guano deposits,
originally covering the shores of the large salt lakes which, by the
subsequent overflowing of their shores, effected the mixture of the
guano with the salts. In this way, by a slow process of decomposition,
nitrate of soda would be ultimately formed.[209] This theory, apart from
other considerations, seems at first sight extremely plausible, more
especially when we remember that it is on this very coast that the
greatest guano deposits have been found, and that the famous Chincha
Islands, which alone have yielded over 10 million tons of this valuable
fertiliser, are comparatively near the scene of the nitrate deposits.
What seems further to support this theory, is the actual occurrence in
the nitrate-fields themselves of small quantities of guano. But however
plausible it may appear at first sight, it does not bear closer
criticism. One very serious objection is the absence in these deposits
of phosphate of lime, which is the largest constituent of guano. If they
were really due to guano, how does it happen that the insoluble
phosphate of lime should have disappeared, while the easily soluble
nitrate of soda should alone be preserved? Again, assuming this theory
to be correct, we should naturally expect to be still able to find
evidence of the chemical changes which would under such circumstances
have taken place, in the shape of portions of the guano in the
transition stage. Su