Discourses: Biological and Geological Essays | Page 9

great majority of them are of such forms as are now
met with only in the sea, and that there is no reason to believe that any
one of them inhabited fresh water--the collateral evidence that the chalk
represents an ancient sea-bottom acquires as great force as the proof
derived from the nature of the chalk itself. I think you will now allow
that I did not overstate my case when I asserted that we have as strong
grounds for believing that all the vast area of dry land, at present
occupied by the chalk, was once at the bottom of the sea, as we have
for any matter of history whatever; while there is no justification for
any other belief.
No less certain it is that the time during which the countries we now
call south-east England, France, Germany, Poland, Russia, Egypt,
Arabia, Syria, were more or less completely covered by a deep sea, was
of considerable duration. We have already seen that the chalk is, in

places, more than a thousand feet thick. I think you will agree with me,
that it must have taken some time for the skeletons of animalcules of a
hundredth of an inch in diameter to heap up such a mass as that. I have
said that throughout the thickness of the chalk the remains of other
animals are scattered. These remains are often in the most exquisite
state of preservation. The valves of the shell-fishes are commonly
adherent; the long spines of some of the sea-urchins, which would be
detached by the smallest jar, often remain in their places. In a word, it
is certain that these animals have lived and died when the place which
they now occupy was the surface of as much of the chalk as had then
been deposited; and that each has been covered up by the layer of
Globigerina mud, upon which the creatures imbedded a little higher up
have, in like manner, lived and died. But some of these remains prove
the existence of reptiles of vast size in the chalk sea. These lived their
time, and had their ancestors and descendants, which assuredly implies
time, reptiles being of slow growth.
There is more curious evidence, again, that the process of covering up,
or, in other words, the deposit of Globigerina skeletons, did not go on
very fast. It is demonstrable that an animal of the cretaceous sea might
die, that its skeleton might lie uncovered upon the sea-bottom long
enough to lose all its outward coverings and appendages by
putrefaction; and that, after this had happened, another animal might
attach itself to the dead and naked skeleton, might grow to maturity,
and might itself die before the calcareous mud had buried the whole.
Cases of this kind are admirably described by Sir Charles Lyell. He
speaks of the frequency with which geologists find in the chalk a
fossilized sea-urchin, to which is attached the lower valve of a Crania.
This is a kind of shell-fish, with a shell composed of two pieces, of
which, as in the oyster, one is fixed and the other free.
"The upper valve is almost invariably wanting, though occasionally
found in a perfect state of preservation in the white chalk at some
distance. In this case, we see clearly that the sea-urchin first lived from
youth to age, then died and lost its spines, which were carried away.
Then the young Crania adhered to the bared shell, grew and perished in

its turn; after which, the upper valve was separated from the lower,
before the Echinus became enveloped in chalky mud."[4]
A specimen in the Museum of Practical Geology, in London, still
further prolongs the period which must have elapsed between the death
of the sea- urchin, and its burial by the Globigerinoe. For the outward
face of the valve of a Crania, which is attached to a sea-urchin,
(Micraster), is itself overrun by an incrusting coralline, which spreads
thence over more or less of the surface of the sea-urchin. It follows that,
after the upper valve of the Crania fell off, the surface of the attached
valve must have remained exposed long enough to allow of the growth
of the whole coralline, since corallines do not live embedded in
mud.[4]
[Footnote 4: Elements of Geology, by Sir Charles Lyell, Bart. F.B.S., p.
23.]
The progress of knowledge may, one day, enable us to deduce from
such facts as these the maximum rate at which the chalk can have
accumulated, and thus to arrive at the minimum duration of the chalk
period. Suppose that the valve of the Cronia upon which a coralline has
fixed itself in the way just described, is so attached to the sea-urchin
that no part of it is more than an inch above the face upon