rated from one another, than those of any other
Order of Mammalia, it is important to fill these
vacant intervals with the fossil genera of a former
state of the earth; thus supplying links that
appeared deficient in the grand continuous chain
which connects all past and present forms of
organic life, as parts of one great system of
Creation.
As the bones of all these animals found in the
earliest series of the tertiary deposits are accompanied
by the remains of reptiles, such as now
inhabit the fresh waters of warm countries, e. g.
the Crocodile, Emys, and Trionyx (see PI. 1 ,
Figs. 80, 81, 82), and also by the leaves and
prostrate trunks of palm trees (PI. 1 , Figs. 6 6 ,
67, 6 8 , and PI. 56), we cannot but infer that the
temperature of France was much higher than it
is at present, at the time when it was occupied
by these plants and reptiles, and by Mammalia
allied to families which are natives of some of
the warmest latitudes of the present earth, e. g.
the Tapir, Rhinoceros, and Hippopotamus.
The frequent intrusion of volcanic rocks is
a remarkable accompaniment of the tertiary
strata of the Eocene period, in various parts of
Europe; and changes of level, resulting from
volcanic agency, may partially explain the fact,
that portions of the same districts became alternately
the receptacles of fresh and salt water.
The fresh-water calcareous deposits of this
period are also highly important, in relation to
the general history of the origin of limestone,
from their affording strong evidence of the
sources whence carbonate of lime has been
derived.*
* We see that thermal springs, in volcanic districts, issue
from the earth, so highly charged with carbonate of lime, as to
overspread large tracts of country with beds of calcareous tufa,
or travertino. The waters that flow from the Lago di Tartaro,
near Rome, and the hot springs of San Filippo, on the borders
of Tuscany, are well known examples of this phenomenon.
These existing operations afford a nearly certain explanation of
the origin of extensive beds of limestone in fresh-water lakes of
the tertiary period, where we know them to have been formed
during seasons of intense volcanic activity. They seem also to
indicate the probable agency of thermal waters in the formation
of still larger calcareous deposits at the bottom of the sea, during
preceding periods of the secondary and transition series.
It is a difficult problem to account for the source of the
enormous masses of carbonate of lime that compose nearly one-
eighth part of the superficial crust of the globe. Some have
referred it entirely to the secretions of marine animals; an
origin to which we must obviously assign those portions of
calcareous strata which are composed of comminuted shells and
corallines: but, until it can be shown that these animals have
the power of forming lime from other elements, we must suppose
that they derived it from the sea, either directly, or through the
medium of its plants. In either case, it remains to find the
source whence the sea obtained, not only these supplies of carbonate
of lime for its animal inhabitants, but also the still larger
quantities of the same substance, that have been precipitated
in the form of calcareous strata.
We cannot suppose it to have resulted, like sands and clays,
from the mechanical detritus of rocks of the granitic series,
because the quantity of lime these rocks contain, bears no proportion
to its large amount among the derivative rocks. The
only remaining hypothesis seems to be, that lime was continually
introduced to lakes and seas, by water that had percolated rocks
through which calcareous earth was disseminated.