of Mr. Trenham Reeks of the Museum of Economic Geology; it consists of
carbonate of lime in abundance, of sulphate and muriate of lime, and of
muriate and sulphate of soda. The carbonate of lime is obviously derived
from the shells; and common salt is so abundant in parts of the bed, that,
as before remarked, the univalves are often filled with it. The sulphate of
lime may have been derived, as has probably the common salt, from the
evaporation of the sea-spray, during the emergence of the land; for
sulphate of lime is now copiously deposited from the spray on the shores of
Ascension. (See "Volcanic Islands" etc. by the Author.) The other saline
bodies may perhaps have been partially thus derived, but chiefly, as I
conclude from the following facts, through a different means.

On most parts of the second ledge or old sea-beach, at a height of 170
feet, there is a layer of white powder of variable thickness, as much in
some parts as two inches, lying on the angular, salt-cemented fragments of
sandstone and under about four inches of earth, which powder, from its
close resemblance in nature to the upper and most decayed parts of the
shelly mass, I can hardly doubt originally existed as a bed of shells, now
much collapsed and quite disintegrated. I could not discover with the
microscope a trace of organic structure in it; but its chemical
constituents, according to Mr. Reeks, are the same as in the powder
extracted from amongst the decaying shells on the lower ledge, with the
marked exception that the carbonate of lime is present in only very small
quantity. On the third and highest ledge, I observed some of this powder in
a similar position, and likewise occasionally in small patches at
considerably greater heights near the summit of the island. At Iquique,
where the whole face of the country is covered by a highly saliferous
alluvium, and where the climate is extremely dry, we have seen that,
according to Mr. Blake, the shells which are perfect near the beach become,
in ascending, gradually less and less perfect, until scarcely a trace of