eeds the primary in this respect, so a third series may hereafter
arise from the depths of the sea, which may exceed the last in the
proportion of its calcareous strata."

If these propositions went no farther than to suggest that every
particle of lime that now enters into the crust of the globe, may
possibly in its turn have been subservient to the purposes of life, by
entering into the composition of organized bodies, I should not deem the
speculation improbable; but, when it is hinted that lime may be an
animal product combined by the powers of vitality from some simple
elements, I can discover no sufficient grounds for such an hypothesis,
and many facts militate against it.

If a large pond be made in almost any soil, and filled with rain water,
it may usually become tenanted by testacea; for carbonate of lime is
almost universally diffused in small quantities. But if no calcareous
matter be supplied by waters flowing from the surrounding high grounds,
or by springs, no tufa or shell-marl are formed. The thin shells of one
generation of mollusks decompose, so that their elements afford
nutriment to the succeeding races; and it is only where a stream enters
a lake, which may introduce a fresh supply of calcareous matter, or
where the lake is fed by springs, that shells accumulate and form marl.

All the lakes in Forfarshire which have produced deposits of shell-marl
have been the sites of springs, which still evolve much carbonic acid,
and a small quantity of carbonate of lime. But there is no marl in Loch
Fithie, near Forfar, where there are _no springs_, although that lake is
surrounded by these calcareous deposits, and although, in every other
respect, the site is favorable to the accumulation of aquatic testacea.

We find those Charae which secrete the largest quantity of calcareous
matter in their stems to abound near springs impregnated with carbonate
of lime. We know that, if the common hen be deprived altogether of
calcareous nutriment, the shells of her eggs will become of too slight a
consistency to protect the contents; and some birds eat chalk greedily
during the breeding season.

If, on the other hand, we turn to the phenomena of inorganic nature, we
observe that, in volcanic countries, there is an enormous evolution of
carbonic acid, either free, in a gaseous form, or mixed with water; and
the springs of such districts are usually impregnated with carbonate of
lime in great abundance. No one who ha