CHAPTER III.

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.

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.

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]

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.

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.

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.)

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]

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.)

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.

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.

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.).

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.)

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.