Mr. Stanislas Meunier communicates to _Le Nature_ an account of some
interesting specimens of globular calcareous matter, resembling
pisolites or peastones both in appearance and structure, which were
accidentally formed as follows: The Northern Railway Company, France,
desiring to purify some calciferous water designed for use in steam
boilers, hit upon the ingenious expedient of treating it with lime water
whose concentration was calculated exactly from the amount of lime
held in the liquid to be purified. The liquids were mixed in a vast
reservoir, to which they were led by parallel pipes, and by which they
were given a rapid eddying motion. The transformation of the
bicarbonate into neutral carbonate of lime being thus effected with
the accompaniment of a circling motion, the insoluble salt which
precipitated, instead of being deposited in an amorphous state, hardened
into globules, the sizes of which were strictly regulated by the
velocity of the currents. Those that have been formed at one and the
same operation are uniform, but those formed at different times vary
greatly--their diameters varying by at least one millimeter to one and
a half centimeters. The surface of the smaller globules is smooth, but
that of the larger ones is rough. Even by the naked eye, it may be
seen that both the large and small globules are formed of regularly
superposed concentric layers. If an extremely thin section be made
through one of them it is found that the number of layers is very great
and that they are remarkably regular (A). By the microscope, it has been
ascertained that each layer is about 0.007 of a millimeter in thickness.
On observing it under polarized light the calcareous substance is
discovered to be everywhere crystallized, and this suggests the question
whether the carbonate has here taken the form of aragonite or of
calcite. Examination has shown it to be the latter. The density of the
globules (2.58) is similar to that of ordinary varieties of calcite. It
is probable that if the operation were to take place under the influence
of heat, under the conditions above mentioned, aragonite would be
formed. It is hardly necessary to dwell upon the possible geological
applications of this mode of forming calcareous oolites and pisolites.