The name _Cretaceous_ (from _creta_, chalk) is given to this epoch in
the history of our globe because the rocks deposited by the sea, towards
its close, are almost entirely composed of chalk (carbonate of lime).
Carbonate of lime, however, does not now appear for the first time as a
part of the earth’s crust; we have already seen limestone occurring,
among the terrestrial materials, from the Silurian period; the Jurassic
formation is largely composed of carbonate of lime in many of its beds,
which are enormous in number as well as extent; it appears, therefore,
that in the period called _Cretaceous_ by geologists, carbonate of lime
was no new substance in the constitution of the globe. If geologists
have been led to give this name to the period, it is because it accords
better than any other with the characteristics of the period; with the
vast accumulations of chalky or earthy limestone in the Paris basin, and
the beds of so-called Greensand, and Chalk of the same age, so largely
developed in England.
We have already endeavoured to establish the origin of lime, in speaking
of the Silurian and Devonian periods, but it may be useful to
recapitulate the explanation here, even at the risk of repeating
ourselves.
We have said that lime was, in all probability, introduced to the globe
by thermal waters flowing abundantly through the fissures, dislocations,
and fractures in the ground, which were themselves caused by the gradual
cooling of the globe; the central nucleus being the grand reservoir and
source of the materials which form the solid crust. In the same manner,
therefore, as the several eruptive substances--such as granites,
porphyries, trachytes, basalts, and lava--have been ejected, so have
thermal waters charged with carbonate of lime, and often accompanied by
silica, found their way to the surface in great abundance, through the
fissures, fractures, and dislocations in the crust of the earth. We need
only mention here the Iceland geysers, the springs of Plombières, and
the well-known thermal springs of Bath and elsewhere in this country.
But how comes lime in a state of bicarbonate, dissolved in these
thermal waters, to form rocks? That is what we propose to explain.
During the primary geological periods, thermal waters, as they reached
the surface, were discharged into the sea and united themselves with the
waves of the vast primordial ocean, and the waters of the sea became
sensibly calcareous--they contained, it is believed, from one to two per
cent. of lime. The innumerable animals, especially Zoophytes, and
Mollusca with solid shells, with which the ancient seas swarmed,
secreted this lime, out of which they built up their mineral
dwelling--or shell. In this liquid and chemically calcareous medium, the
Foraminifera and Polyps of all forms swarmed, forming an innumerable
population. Now what became of the bodies of these creatures after
death? They were of all sizes, but chiefly microscopic; that is, so
small as to be individually all but invisible to the naked eye. The
perishable animal matter disappeared in the bosom of the waters by
decomposition, but there still remained behind the indestructible
inorganic matter, that is to say, the carbonate of lime forming their
testaceous covering; these calcareous deposits accumulating in thick
beds at the bottom of the sea, became compacted into a solid mass, and
formed a series of continuous beds superimposed on each other. These,
increasing imperceptibly in the course of ages, ultimately formed the
rocks of the _Cretaceous_ period, which we have now under consideration.
These statements are not, as the reader might conceive from their
nature, a romantic conception invented to please the imagination of
those in search of a system--the time is past when geology should be
regarded as the romance of Nature--nor has what we advance at all the
character of an arbitrary conception. One is no doubt struck with
surprise on learning, for the first time, that all the limestone rocks,
all the calcareous stones employed in the construction of our dwellings,
our cities, our castles and cathedrals, were deposited in the seas of an
earlier world, and are only composed of an aggregation of shells of
Mollusca, or fragments of the testaceous coverings of Foraminifera and
other Zoophytes--nay, that they were secreted from the water itself, and
then assimilated by these minute creatures, and that this would appear
to have been the great object of their creation in such myriads. Whoever
will take the trouble to observe, and reflect on what he observes, will
find all his doubts vanish. If chalk be examined with a microscope, it
will be found to be composed of the remains of numerous Zoophytes, of
minute and divers kinds of shells, and, above all, of Foraminifera, so
small that their very minuteness seems to have rendered them
indestructible. A hundred and fifty of these small beings placed end to
end, in a line, will only occupy the space of about one-twelfth part of
an inch.
[Illustration: Chalk under the Microscope.