entering
the shell of a pearl oyster is at once enveloped or washed in the
creature's exudations ; that the organic matter of the secretions
forms a filmy envelope in which the mineral contained in them is
precipitated or crystallizes in wave-like layers of crystals of great
tenuity, and that as these layers harden the process is repeated, and
that during the process the creature either revolves the object, or
about it, as it is free, or fastened to the shell. It is also possible
that changes in the organic matter interwoven with the calcium
carbonate may produce some chemical action resulting in the
crystallization of the lime, and the crystallization in turn be
provocative of another deposit, each process in turn being almost
simultaneous and that the process is continued until a paucity of
mineral in the exudations induces a rest for recuperation, after which
the process is repeated, the result being a succession of composite
skins as we find them. Whatever the cause, it is evident in all parts
of the shell and in the pearl that continuity of construction is
periodically arrested to be resumed upon exactly the same plan, except
that the material used in the suc-
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