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Geological Observations on South America by Charles Darwin
page 103 of 461 (22%)
their original structure can be discovered. It is known that carbonate of
lime and common salt left in a mass together, and slightly moistened,
partially decompose each other (I am informed by Dr. Kane, through Mr.
Reeks, that a manufactory was established on this principle in France, but
failed from the small quantity of carbonate of soda produced. Sprengel
"Gardeners' Chronicle" 1845 page 157, states, that salt and carbonate of
lime are liable to mutual decomposition in the soil. Sir H. De la Beche
informs me, that calcareous rocks washed by the spray of the sea, are often
corroded in a peculiar manner; see also on this latter subject "Gardeners'
Chronicle" page 675 1844.): now we have at San Lorenzo and at Iquique, in
the shells and salt packed together, and occasionally moistened by the so-
called Peruvian dew, the proper elements for this action. We can thus
understand the peculiar corroded appearance of the shells on San Lorenzo,
and the great decrease of quantity in the carbonate of lime in the powder
on the upper ledge. There is, however, a great difficulty on this view, for
the resultant salts should be carbonate of soda and muriate of lime; the
latter is present, but not the carbonate of soda. Hence I am led to the
perhaps unauthorised conjecture (which I shall hereafter have to refer to)
that the carbonate of soda, by some unexplained means, becomes converted
into a sulphate.

If the above remarks be just, we are led to the very unexpected conclusion,
that a dry climate, by leaving the salt from the sea-spray undissolved, is
much less favourable to the preservation of upraised shells than a humid
climate. However this may be, it is interesting to know the manner in which
masses of shells, gradually upraised above the sea-level, decay and finally
disappear.

SUMMARY ON THE RECENT ELEVATION OF THE WEST COAST OF SOUTH AMERICA.

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