ARRANGEMENT OF FOSSILS IN STRATA--FRESHWATER AND MARINE.
Successive deposition indicated by fossils--Limestones formed of
corals and shells Proofs of gradual increase of strata derived from
fossils--Serpula attached to spatangus--Wood bored by
Teredina--Tripoli and semi-opal formed of infusoria--Chalk derived
principally from organic bodies--Distinction of freshwater from marine
formations--Genera of freshwater and land shells--Rules for
recognizing marine testacea--Gyrogonite and chara--Freshwater
fishes--Alternation of marine and freshwater deposits--Lym-Fiord.
Having in the last chapter considered the forms of stratification so far as
they are determined by the arrangement of inorganic matter, we may now turn
our attention to the manner in which organic remains are distributed
through stratified deposits. We should often be unable to detect any signs
of stratification or of successive deposition, if particular kinds of
fossils did not occur here and there at certain depths in the mass. At one
level, for example, univalve shells of some one or more species
predominate; at another, bivalve shells; and at a third, corals; while in
some formations we find layers of vegetable matter, commonly derived from
land plants, separating strata.
It may appear inconceivable to a beginner how mountains, several thousand
feet thick, can have become filled with fossils from top to bottom; but the
difficulty is removed, when he reflects on the origin of stratification, as
explained in the last chapter, and allows sufficient time for the
accumulation of sediment. He must never lose sight of the fact that, during
the process of deposition, each separate layer was once the uppermost, and
covered immediately by the water in which aquatic animals lived. Each
stratum in fact, however far it may now lie beneath the surface, was once
in the state of shingle, or loose sand or soft mud at the bottom of the
sea, in which shells and other bodies easily became enveloped.
By attending to the nature of these remains, we are often enabled to
determine whether the deposition was slow or rapid, whether it took
place in a deep or shallow sea, near the shore or far from land, and
whether the water was salt, brackish, or fresh. Some limestones consist
almost exclusively of corals, and in many cases it is evident that the
present position of each fossil zoophyte has been determined by the
manner in which it grew originally. The axis of the coral, for example,
if its natural growth is erect, still remains at right angles to the
plane of stratification. If the stratum be now horizontal, the round
spherical heads of certain species continue uppermost, and their points
of attachment are directed downwards. This arrangement is sometimes
repeated throughout a great succession of strata. From what we know of
the growth of similar zoophytes in modern reefs, we infer that the rate
of increase was extremely slow, and some of the fossils must have
flourished for ages like forest trees, before they attained so large a
size. During these ages, the water remained clear and transparent, for
such corals cannot live in turbid water.
[Illustration: Fig. 10. Fossil _Gryphaea_, covered both on the outside and
inside with fossil serpulae.]
In like manner, when we see thousands of full-grown shells dispersed every
where throughout a long series of strata, we cannot doubt that time was
required for the multiplication of successive generations; and the evidence
of slow accumulation is rendered more striking from the proofs, so often
discovered, of fossil bodies having lain for a time on the floor of the
ocean after death before they were imbedded in sediment. Nothing, for
example, is more common than to see fossil oysters in clay, with serpulae,
or barnacles (acorn-shells), or corals, and other creatures, attached to
the inside of the valves, so that the mollusk was certainly not buried in
argillaceous mud the moment it died. There must have been an interval
during which it was still surrounded with clear water, when the testacea,
now adhering to it, grew from an embryo state to full maturity. Attached
shells which are merely external, like some of the serpulae (_a_) in the
annexed figure (fig. 10.), may often have grown upon an oyster or other
shell while the animal within was still living; but if they are found on
the inside, it could only happen after the death of the inhabitant of the
shell which affords the support. Thus, in fig. 10., it will be seen that
two serpulae have grown on the interior, one of them exactly on the place
where the adductor muscle of the _Gryphaea_ (a kind of oyster) was fixed.
Some fossil shells, even if simply attached to the _outside_ of others,
bear full testimony to the conclusion above alluded to, namely, that an
interval elapsed between the death of the creature to whose shell they
adhere, and the burial of the same in mud or sand. The sea-urchins or
_Echini_, so abundant in white chalk, afford a good illustration. It is
well known that these animals, when living, are invariably covered with
numerous spines, which serve as organs of motion, and are supported by rows
of tubercles, which last are only seen after the death of the sea-urchin,
when the spines have dropped off. In fig. 12. a living species of
_Spatangus_, common on our coast, is represented with one half of its shell
stripped of the spines. In fig. 11. a fossil of the same genus from the
white chalk of England shows the naked surface which the individuals of
this family exhibit when denuded of their bristles. The full-grown
_Serpula_, therefore, which now adheres externally, could not have begun to
grow till the _Spatangus_ had died, and the spines were detached.
[Illustration: Fig. 11. _Serpula_ attached to a fossil _Spatangus_
from the chalk.]
[Illustration: Fig. 12. Recent _Spatangus_ with the spines removed
from one side.
_b._ Spine and tubercles, nat. size.
_a._ The same magnified.]
[Illustration: Fig. 13.
_a._ _Echinus_ from the chalk, with lower valve of the _Crania_ attached.
_b._ Upper valve of the _Crania_ detached.]
Now the series of events here attested by a single fossil may be carried a
step farther. Thus, for example, we often meet with a sea-urchin in the
chalk (see fig. 13.), which has fixed to it the lower valve of a
_Crania_, a genus of bivalve mollusca. The upper valve (_b_, fig. 13.)
is almost invariably wanting, though occasionally found in a perfect
state of preservation in 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.
It may be well to mention one more illustration of the manner in which
single fossils may sometimes throw light on a former state of things, both
in the bed of the ocean and on some adjoining land. We meet with many
fragments of wood bored by ship-worms at various depths in the clay on
which London is built. Entire branches and stems of trees, several feet in
length, are sometimes dug out, drilled all over by the holes of these
borers, the tubes and shells of the mollusk still remaining in the
cylindrical hollows. In fig. 15. _e_, a representation is given of a piece
of recent wood pierced by the _Teredo navalis_, or common ship-worm, which
destroys wooden piles and ships. When the cylindrical tube _d_ has been
extracted from the wood, a shell is seen at the larger extremity, composed
of two pieces, as shown at _c_. In like manner, a piece of fossil wood
(_a_, fig. 14.) has been perforated by an animal of a kindred but extinct
genus, called _Teredina_ by Lamarck. The calcareous tube of this mollusk
was united and as it were soldered on to the valves of the shell (_b_),
which therefore cannot be detached from the tube, like the valves of the
recent _Teredo_. The wood in this fossil specimen is now converted into a
stony mass, a mixture of clay and lime; but it must once have been buoyant
and floating in the sea, when the _Teredinae_ lived upon it, perforating it
in all directions. Again, before the infant colony settled upon the drift
wood, the branch of a tree must have been floated down to the sea by a
river, uprooted, perhaps, by a flood, or torn off and cast into the waves
by the wind: and thus our thoughts are carried back to a prior period, when
the tree grew for years on dry land, enjoying a fit soil and climate.
[2 Illustrations: Fossil and recent wood drilled by perforating Mollusca.
Fig. 14. _a_. Fossil wood from London clay, bored by _Teredina_.
_b_. Shell and tube of _Teredina personata_, the right-hand
figure the ventral, the left the dorsal view.
Fig. 15. _e_. Recent wood bored by _Teredo_.
_d_. Shell and tube of _Teredo navalis_, from the same.
_c_. Anterior and posterior view of the valves of same detached
from the tube.]
It has been already remarked that there are rocks in the interior of
continents, at various depths in the earth, and at great heights above
the sea, almost entirely made up of the remains of zoophytes and
testacea. Such masses may be compared to modern oyster-beds and coral
reefs; and, like them, the rate of increase must have been extremely
gradual. But there are a variety of stony deposits in the earth's crust,
now proved to have been derived from plants and animals, of which the
organic origin was not suspected until of late years, even by
naturalists. Great surprise was therefore created by the recent
discovery of Professor Ehrenberg of Berlin, that a certain kind of
siliceous stone, called tripoli, was entirely composed of millions of
the remains of organic beings, which the Prussian naturalist refers to
microscopic Infusoria, but which most others now believe to be plants.
They abound in freshwater lakes and ponds in England and other
countries, and are termed Diatomaceae by those naturalists who believe in
their vegetable origin. The substance alluded to has long been well
known in the arts, being used in the form of powder for polishing stones
and metals. It has been procured, among other places, from Bilin, in
Bohemia, where a single stratum, extending over a wide area, is no less
than 14 feet thick. This stone, when examined with a powerful
microscope, is found to consist of the siliceous plates or frustules of
the above-mentioned Diatomaceae, united together without any visible
cement. It is difficult to convey an idea of their extreme minuteness;
but Ehrenberg estimates that in the Bilin tripoli there are 41,000
millions of individuals of the _Gaillonella distans_ (see fig. 17.) in
every cubic inch, which weighs about 220 grains, or about 187 millions
in a single grain. At every stroke, therefore, that we make with this
polishing powder, several millions, perhaps tens of millions, of perfect
fossils are crushed to atoms.
[3 Illustrations: These figures are magnified nearly 300 times, except
the lower figure of _G. ferruginea_ (fig. 18. _a_), which is magnified
2000 times.
Fig. 16. _Bacillaria vulgaris?_
Fig. 17. _Gaillonella distans._
Fig. 18. _Gaillonella ferruginea._]
[2 Illustrations: Fragment of semi-opal from the great bed of
Tripoli, Bilin.
Fig. 19. Natural size.
Fig. 20. The same magnified, showing circular articulations of a species of
_Gaillonella_, and spiculae of _Spongilla_.]
The remains of these Diatomaceae are of pure silex, and their forms are
various, but very marked and constant in particular genera and species.
Thus, in the family _Bacillaria_ (see fig. 16.), the fossils preserved in
tripoli are seen to exhibit the same divisions and transverse lines which
characterize the living species of kindred form. With these, also, the
siliceous spiculae or internal supports of the freshwater sponge, or
_Spongilla_ of Lamarck, are sometimes intermingled (see the needle-shaped
bodies in fig. 20.). These flinty cases and spiculae, although hard, are
very fragile, breaking like glass, and are therefore admirably adapted,
when rubbed, for wearing down into a fine powder fit for polishing the
surface of metals.
Besides the tripoli, formed exclusively of the fossils above described,
there occurs in the upper part of the great stratum at Bilin another
heavier and more compact stone, a kind of semi-opal, in which innumerable
parts of Diatomaceae and spiculae of the _Spongilla_ are filled with, and
cemented together by, siliceous matter. It is supposed that the siliceous
remains of the most delicate Diatomaceae have been dissolved by water, and
have thus given rise to this opal in which the more durable fossils are
preserved like insects in amber. This opinion is confirmed by the fact that
the organic bodies decrease in number and sharpness of outline in
proportion as the opaline cement increases in quantity.
In the Bohemian tripoli above described, as in that of Planitz in Saxony,
the species of Diatomaceae (or Infusoria, as termed by Ehrenberg) are
freshwater; but in other countries, as in the tripoli of the Isle of
France, they are of marine species, and they all belong to formations of
the _tertiary_ period, which will be spoken of hereafter.
A well-known substance, called bog-iron ore, often met with in peat-mosses,
has also been shown by Ehrenberg to consist of innumerable articulated
threads, of a yellow ochre colour, composed partly of flint and partly of
oxide of iron. These threads are the cases of a minute microscopic body,
called _Gaillonella ferruginea_ (fig. 18.).
[4 Illustrations: _Cytheridae_ and _Foraminifera_ from the chalk.
Fig. 21. _Cythere_, Muell.
_Cytherina_, Lam.
Fig. 22. Portion of _Nodosaria_.
Fig. 23. _Cristellaria rotulata._
Fig. 24. _Rosalina._]
It is clear that much time must have been required for the accumulation of
strata to which countless generations of Diatomaceae have contributed their
remains; and these discoveries lead us naturally to suspect that other
deposits, of which the materials have usually been supposed to be
inorganic, may in reality have been derived from microscopic organic
bodies. That this is the case with the white chalk, has often been
imagined, this rock having been observed to abound in a variety of marine
fossils, such as shells, echini, corals, sponges, crustacea, and fishes.
Mr. Lonsdale, on examining, in Oct. 1835, in the museum of the Geological
Society of London, portions of white chalk from different parts of England,
found, on carefully pulverizing them in water, that what appear to the eye
simply as white grains were, in fact, well preserved fossils. He obtained
above a thousand of these from each pound weight of chalk, some being
fragments of minute corallines, others entire Foraminifera and Cytheridae.
The annexed drawings will give an idea of the beautiful forms of many of
these bodies. The figures _a_ _a_ represent their natural size, but, minute
as they seem, the smallest of them, such as _a_, fig. 24., are gigantic in
comparison with the cases of Diatomaceae before mentioned. It has, moreover,
been lately discovered that the chambers into which these Foraminifera are
divided are actually often filled with thousands of well-preserved organic
bodies, which abound in every minute grain of chalk, and are especially
apparent in the white coating of flints, often accompanied by innumerable
needle-shaped spiculae of sponges. After reflecting on these discoveries, we
are naturally led on to conjecture that, as the formless cement in the
semi-opal of Bilin has been derived from the decomposition of animal and
vegetable remains, so also even those parts of chalk flints in which no
organic structure can be recognized may nevertheless have constituted a
part of microscopic animalcules.
"The dust we tread upon was once alive!"--BYRON.
How faint an idea does this exclamation of the poet convey of the real
wonders of nature! for here we discover proofs that the calcareous and
siliceous dust of which hills are composed has not only been once alive,
but almost every particle, albeit invisible to the naked eye, still retains
the organic structure which, at periods of time incalculably remote, was
impressed upon it by the powers of life.
_Freshwater and marine fossils._--Strata, whether deposited in salt or
fresh water, have the same forms; but the imbedded fossils are very
different in the two cases, because the aquatic animals which frequent
lakes and rivers are distinct from those inhabiting the sea. In the
northern part of the Isle of Wight a formation of marl and limestone, more
than 50 feet thick, occurs, in which the shells are principally, if not
all, of extinct species. Yet we recognize their freshwater origin, because
they are of the same genera as those now abounding in ponds and lakes,
either in our own country or in warmer latitudes.
In many parts of France, as in Auvergne, for example, strata of
limestone, marl, and sandstone are found, hundreds of feet thick, which
contain exclusively freshwater and land shells, together with the
remains of terrestrial quadrupeds. The number of land shells scattered
through some of these freshwater deposits is exceedingly great; and
there are districts in Germany where the rocks scarcely contain any
other fossils except snail-shells (_helices_); as, for instance, the
limestone on the left bank of the Rhine, between Mayence and Worms, at
Oppenheim, Findheim, Budenheim, and other places. In order to account
for this phenomenon, the geologist has only to examine the small deltas
of torrents which enter the Swiss lakes when the waters are low, such as
the newly-formed plain where the Kander enters the Lake of Thun. He
there sees sand and mud strewed over with innumerable dead land shells,
which have been brought down from valleys in the Alps in the preceding
spring, during the melting of the snows. Again, if we search the sands
on the borders of the Rhine, in the lower part of its course, we find
countless land shells mixed with others of species belonging to lakes,
stagnant pools, and marshes. These individuals have been washed away
from the alluvial plains of the great river and its tributaries, some
from mountainous regions, others from the low country.
Although freshwater formations are often of great thickness, yet they are
usually very limited in area when compared to marine deposits, just as
lakes and estuaries are of small dimensions in comparison with seas.
We may distinguish a freshwater formation, first, by the absence of many
fossils almost invariably met with in marine strata. For example, there
are no sea-urchins, no corals, and scarcely any zoophytes; no chambered
shells, such as the nautilus, nor microscopic Foraminifera. But it is
chiefly by attending to the forms of the mollusca that we are guided in
determining the point in question. In a freshwater deposit, the number
of individual shells is often as great, if not greater, than in a marine
stratum; but there is a smaller variety of species and genera. This
might be anticipated from the fact that the genera and species of recent
freshwater and land shells are few when contrasted with the marine.
Thus, the genera of true mollusca according to Blainville's system,
excluding those of extinct species and those without shells, amount to
about 200 in number, of which the terrestrial and freshwater genera
scarcely form more than a sixth.[28-A]
[Illustration: Fig. 25. _Cyclas obovata_; fossil. Hants.]
[Illustration: Fig. 26. _Cyrena consobrina_; fossil. Grays, Essex.]
[Illustration: Fig. 27. _Anodonta Cordierii_; fossil. Paris.]
[Illustration: Fig. 28. _Anodonta latimarginatus_; recent. Bahia.]
[Illustration: Fig. 29. _Unio littoralis_; recent. Auvergne.]
Almost all bivalve shells, or those of acephalous mollusca, are marine,
about ten only out of ninety genera being freshwater. Among these last, the
four most common forms, both recent and fossil, are _Cyclas_, _Cyrena_,
_Unio_, and _Anodonta_ (see figures); the two first and two last of which
are so nearly allied as to pass into each other.
[Illustration: Fig. 30. _Gryphaea incurva_, Sow. (_G. arcuata_, Lam.) upper
valve. Lias.]
Lamarck divided the bivalve mollusca into the _Dimyary_, or those having
two large muscular impressions in each valve, as _a b_ in the Cyclas, fig.
25., and the _Monomyary_, such as the oyster and scallop, in which there is
only one of these impressions, as is seen in fig. 30. Now, as none of these
last, or the unimuscular bivalves, are freshwater, we may at once presume a
deposit in which we find any of them to be marine.
[Illustration: Fig. 31. _Planorbis euomphalus_; fossil. Isle of Wight.]
[Illustration: Fig. 32. _Lymnea longiscata_; fossil. Hants.]
[Illustration: Fig. 33. _Paludina lenta_; fossil. Hants.]
The univalve shells most characteristic of freshwater deposits are,
_Planorbis_, _Lymnea_, and _Paludina_. (See figures.) But to these are
occasionally added _Physa_, _Succinea_, _Ancylus_, _Valvata_, _Melanopsis_,
_Melania_, and _Neritina_. (See figures.)
[Illustration: Fig. 34. _Succinea amphibia_; fossil. Loess, Rhine.]
[Illustration: Fig. 35. _Ancylus elegans_; fossil. Hants.]
[Illustration: Fig. 36. _Valvata_; fossil. Grays, Essex.]
[Illustration: Fig. 37. _Physa hypnorum_; recent.]
[Illustration: Fig. 38. _Auricula_; recent. Ava.]
[Illustration: Fig. 39. _Melania inquinata._ Paris Basin.]
[Illustration: Fig. 40. _Physa columnaris._ Paris Basin.]
[Illustration: Fig. 41. _Melanopsis buccinoidea_; recent. Asia.]
In regard to one of these, the _Ancylus_ (fig. 35.), Mr. Gray observes that
it sometimes differs in no respect from the marine _Siphonaria_, except in
the animal. The shell, however, of the _Ancylus_ is usually thinner.[29-A]
[Illustration: Fig. 42. _Neritina globulus._ Paris basin.]
[Illustration: Fig. 43. _Nerita granulosa._ Paris basin.]
Some naturalists include _Neritina_ (fig. 42.) and the marine _Nerita_
(fig. 43.) in the same genus, it being scarcely possible to distinguish the
two by good generic characters. But, as a general rule, the fluviatile
species are smaller, smoother, and more globular than the marine; and they
have never, like the _Neritae_, the inner margin of the outer lip toothed or
crenulated. (See fig. 43.)
[Illustration: Fig. 44. _Cerithium cinctum._ Paris basin.]
A few genera, among which _Cerithium_ (fig. 44.) is the most abundant, are
common both to rivers and the sea, having species peculiar to each. Other
genera, like _Auricula_ (fig. 38.), are amphibious, frequenting marshes,
especially near the sea.
[Illustration: Fig. 45. _Helix Turonensis._ Faluns, Touraine.]
[Illustration: Fig. 46. _Cyclostoma elegans._ Loess.]
[Illustration: Fig. 47. _Pupa tridens._ Loess.]
[Illustration: Fig. 48. _Clausilia bidens._ Loess.]
[Illustration: Fig. 49. _Bulimus lubricus._ Loess, Rhine.]
The terrestrial shells are all univalves. The most abundant genera among
these, both in a recent and fossil state, are _Helix_ (fig. 45.),
_Cyclostoma_ (fig. 46.), _Pupa_ (fig. 47.), _Clausilia_ (fig. 48.),
_Bulimus_ (fig. 49.), and _Achatina_; which two last are nearly allied and
pass into each other.
[Illustration: Fig. 50. _Ampullaria glauca_, from the Jumna.]
The _Ampullaria_ (fig. 50.) is another genus of shells, inhabiting
rivers and ponds in hot countries. Many fossil species have been
referred to this genus, but they have been found chiefly in marine
formations, and are suspected by some conchologists to belong to
_Natica_ and other marine genera.
All univalve shells of land and freshwater species, with the exception of
_Melanopsis_ (fig. 41.), and _Achatina_, which has a slight indentation,
have entire mouths; and this circumstance may often serve as a convenient
rule for distinguishing freshwater from marine strata; since, if any
univalves occur of which the mouths are not entire, we may presume that the
formation is marine. The aperture is said to be entire in such shells as
the _Ampullaria_ and the land shells (figs. 45-49.), when its outline is
not interrupted by an indentation or notch, such as that seen at _b_ in
_Ancillaria_ (fig. 52.); or is not prolonged into a canal, as that seen at
_a_ in _Pleurotoma_ (fig. 51.).
[Illustration: Fig. 51. _Pleurotoma rotata._ Subap. hills, Italy.]
[Illustration: Fig. 52. _Ancillaria subulata._ London clay.]
The mouths of a large proportion of the marine univalves have these notches
or canals, and almost all such species are carnivorous; whereas nearly all
testacea having entire mouths, are plant-eaters; whether the species be
marine, freshwater, or terrestrial.
There is, however, one genus which affords an occasional exception to one
of the above rules. The _Cerithium_ (fig. 44.), although provided with a
short canal, comprises some species which inhabit salt, others brackish,
and others fresh water, and they are said to be all plant-eaters.
Among the fossils very common in freshwater deposits are the shells of
_Cypris_, a minute crustaceous animal, having a shell much resembling that
of the bivalve mollusca.[31-A] Many minute living species of this genus
swarm in lakes and stagnant pools in Great Britain; but their shells are
not, if considered separately, conclusive as to the freshwater origin of a
deposit, because the majority of species in another kindred genus of the
same order, the _Cytherina_ of Lamarck (see above, fig. 21. p. 26.),
inhabit salt water; and, although the animal differs slightly, the shell is
scarcely distinguishable from that of the _Cypris_.
The seed-vessels and stems of _Chara_, a genus of aquatic plants, are very
frequent in freshwater strata. These seed-vessels were called, before their
true nature was known, gyrogonites, and were supposed to be foraminiferous
shells. (See fig. 53. _a._)
The _Charae_ inhabit the bottom of lakes and ponds, and flourish mostly
where the water is charged with carbonate of lime. Their seed-vessels are
covered with a very tough integument, capable of resisting decomposition;
to which circumstance we may attribute their abundance in a fossil state.
The annexed figure (fig. 54.) represents a branch of one of many new
species found by Professor Amici in the lakes of northern Italy. The
seed-vessel in this plant is more globular than in the British _Charae_, and
therefore more nearly resembles in form the extinct fossil species found in
England, France, and other countries. The stems, as well as the
seed-vessels, of these plants occur both in modern shell marl and in
ancient freshwater formations. They are generally composed of a large tube
surrounded by smaller tubes; the whole stem being divided at certain
intervals by transverse partitions or joints. (See _b_, fig. 53.)
[Illustration: Fig. 53. _Chara medicaginula_; fossil. Isle of Wight.
_a._ Seed-vessel. magnified 20 diameters.
_b._ Stem, magnified.]
[Illustration: Fig. 54. _Chara elastica_; recent. Italy.
_a._ Sessile seed vessel between the division of the leaves of the
female plant.
_b._ Transverse section of a branch, with five seed-vessels magnified,
seen from below upwards.]
It is not uncommon to meet with layers of vegetable matter, impressions of
leaves, and branches of trees, in strata containing freshwater shells; and
we also find occasionally the teeth and bones of land quadrupeds, of
species now unknown. The manner in which such remains are occasionally
carried by rivers into lakes, especially during floods, has been fully
treated of in the "Principles of Geology."[32-A]
The remains of fish are occasionally useful in determining the freshwater
origin of strata. Certain genera, such as carp, perch, pike, and loach
(_Cyprinus_, _Perca_, _Esox_, and _Cobitis_), as also _Lebias_, being
peculiar to freshwater. Other genera contain some freshwater and some
marine species, as _Cottus_, _Mugil_, and _Anguilla_, or eel. The rest are
either common to rivers and the sea, as the salmon; or are exclusively
characteristic of salt water. The above observations respecting fossil
fishes are applicable only to the more modern or tertiary deposits; for in
the more ancient rocks the forms depart so widely from those of existing
fishes, that it is very difficult, at least in the present state of
science, to derive any positive information from ichthyolites respecting
the element in which strata were deposited.
The alternation of marine and freshwater formations, both on a small and
large scale, are facts well ascertained in geology. When it occurs on a
small scale, it may have arisen from the alternate occupation of certain
spaces by river water and the sea; for in the flood season the river forces
back the ocean and freshens it over a large area, depositing at the same
time its sediment; after which the salt water again returns, and, on
resuming its former place, brings with it sand, mud, and marine shells.
There are also lagoons at the mouths of many rivers, as the Nile and
Mississippi, which are divided off by bars of sand from the sea, and
which are filled with salt and fresh water by turns. They often
communicate exclusively with the river for months, years, or even
centuries; and then a breach being made in the bar of sand, they are for
long periods filled with salt water.
The Lym-Fiord in Jutland offers an excellent illustration of analogous
changes; for, in the course of the last thousand years, the western
extremity of this long frith, which is 120 miles in length, including
its windings, has been four times fresh and four times salt, a bar of
sand between it and the ocean having been as often formed and removed.
The last irruption of salt water happened in 1824, when the North Sea
entered, killing all the freshwater shells, fish, and plants; and from
that time to the present, the sea-weed _Fucus vesiculosus_, together
with oysters and other marine mollusca, have succeeded the _Cyclas_,
_Lymnea_, _Paludina_, and _Charae_.[33-A]
But changes like these in the Lym-Fiord, and those before mentioned as
occurring at the mouths of great rivers, will only account for some cases
of marine deposits of partial extent resting on freshwater strata. When we
find, as in the south-east of England, a great series of freshwater beds,
1000 feet in thickness, resting upon marine formations and again covered by
other rocks, such as the cretaceous, more than 1000 feet thick, and of
deep-sea origin, we shall find it necessary to seek for a different
explanation of the phenomena.[33-B]
FOOTNOTES:
[28-A] See Synoptic Table in Blainville's Malacologie.
[29-A] Gray, Phil. Trans., 1835, p. 302.
[31-A] For figures of recent species, see below, p. 183., and figs. of
fossils, see p. 228.
[32-A] See Index of Principles, "Fossilization."
[33-A] See Principles, Index, "Lym-Fiord."
[33-B] See below, Chap. XVIII., on the Wealden.