e
obtained by the evaporation of sea-water, or by the mutual reactions of
its salts with the constituents of the adjacent rocks. As there is
"indisputable evidence of the recent elevation of the whole of this
coast," volcanic upheaval might be reasonably held to explain their
altitude. Their comparative proximity to the coast would seem further to
favour this theory. On these grounds, therefore, Forbes is inclined to
think that they owe their origin to the evaporation, under the influence
of a tropical sun, of lagoons of salt water, the communication of which
with the sea had been cut off by the rising of the land.

_Forbes and Darwin on the Theory of their Origin._

The obvious difficulty of accounting for the formation of the larger
deposits by such a theory he meets by saying that it is only necessary
to suppose that, even after the partial isolation of the lagoons by the
elevations of the coast, they might still have maintained tidal or
occasional communication with the sea by means of lateral openings in
the chain of hills separating them from the ocean. In such cases there
would be a gradual accumulation of salts, very much greater in amount
than that due simply to the evaporation of the water originally
contained in the lagoons. The above theory of the origin of the lower
saline deposits may go to explain the mode of formation of the
nitrate-fields; but in this case several difficulties present
themselves. One is the much greater altitude of the latter, as well as
their greater distance inland. This difficulty, however, may be met by
assuming that they are of older origin than the lower deposits, and have
been subjected to a correspondingly greater amount of volcanic upheaval.
There is abundance of proof that this part of the continent has been the
scene in the past of such volcanic upheaval. Forbes is of opinion that
there is the fullest evidence to prove that, even since the arrival of
the Spaniards, a very considerable elevation of the land has taken place
over the greater part, if not the whole extent, of the line of coast;
while Darwin states that he has convincing proof that this part of the
continent has been elevated from 400 to 1200 feet since the epoch of
existing shells. Furthermore, elevations of the coast-line, amounting in
many cases to several feet, are known to have happened within recent
times, while earthquakes and volcanic disturbances of a less striking
nature are still of common occurrence