I have no doubt that this
upper layer originally existed as a bed of shells, like that on the
eighty-five-feet ledge; but it does not now contain even a trace of
organic structure. The powder has been analysed for me by Mr. T.
Reeks; it consists of sulphates and muriates both of lime and soda,
with very little carbonate of lime. It is known that common salt
and carbonate of lime left in a mass for some time together partly
decompose each other; though this does not happen with small
quantities in solution. As the half-decomposed shells in the lower
parts are associated with much common salt, together with some of
the saline substances composing the upper saline layer, and as
these shells are corroded and decayed in a remarkable manner, I
strongly suspect that this double decomposition has here taken
place. The resultant salts, however, ought to be carbonate of soda
and muriate of lime, the latter is present, but not the carbonate
of soda. Hence I am led to imagine that by some unexplained means
the carbonate of soda becomes changed into the sulphate. It is
obvious that the saline layer could not have been preserved in any
country in which abundant rain occasionally fell: on the other hand
this very circumstance, which at first sight appears so highly
favourable to the long preservation of exposed shells, has probably
been the indirect means, through the common salt not having been
washed away, of their decomposition and early decay.
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