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animals of all the kinds which have been enumerated； occur in the
chalk； in greater or less abundance； while not one of those forms
of shell…fish which are characteristic of fresh water has yet been
observed in it。
When we consider that the remains of more than three thousand
distinct species of aquatic animals have been discovered among the
fossils of the chalk； that the 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 waterthe
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。＇67＇
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 ＇68＇ became enveloped
in chalky mud。〃
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 Globigerinae。 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
corraline； since corallines do not live imbedded in mud。
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 Crania 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 which the sea…urchin rests。 Then； as the
coralline could not have fixed itself； if the Crania had been
covered up with chalk mud； and could not have lived had itself been
so covered it follows； that an inch of chalk mud could not have
accumulated within the time between the death and decay of the soft
parts of the sea…urchin and the growth of the coralline to the full
size which it has attained。 If the decay of the soft parts of the
sea…urchin； the attachment； growth to maturity； and decay of the
Crania； and the subsequent attachment and growth of the coralline；
took a year （which is a low estimate enough）； the accumulation of
the inch of chalk must have taken more than a year： and the deposit
of a thousand feet of chalk must； consequently； have taken more
than twelve thousand years。
The foundation of all this calculation is； of course； a knowledge
of the length of time the Crania and the coralline needed to attain
their full size； and； on this head； precise knowledge is at present
wanting。 But there are circumstances which tend to show； that
nothing like an inch of chalk has accumulated during the life of a
Crania； and； on any probable estimate of the length of that life；
the chalk period must have had a much longer duration than that
thus roughly assigned to it。
Thus； not only is it certain that the chalk is the mud of an
ancient sea…bottom； but it is no less certain； that the chalk sea
existed during an extremely long period； though we may not be
prepared to give a precise estimate of the length of that period in
years。 The relative duration is clear； though the absolute
duration may not be definable。 The attempt to affix any precise
date to the period at which the chalk sea began； or ended； its
existence； is baffled by difficulties of the same kind。 But the
relative age of the cretaceous epoch may be determined with as
great ease and certainty as the long duration of that epoch。
You will have heard of the interesting discoveries recently made；
in various parts of Western Europe； of flint implements； obviously
worked into shape by human