Chapter XVII

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“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.

Fresh-water 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 formations of marl and limestone, more than 50 feet thick occur, in which the shells are of extinct species. Yet we recognise their fresh-water origin, because they are of the same genera as those now abounding in ponds, lakes, and rivers, either in our own country or in warmer latitudes.

In many parts of France—in Auvergne, for example—strata occur of limestone, marl, and sandstone hundreds of feet thick, which contain exclusively fresh-water and land shells, together with the remains of terrestrial quadrupeds. The number of land-shells scattered through some of these fresh-water 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 strewn over with innumerable dead land-shells, which have been brought down from the 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 fresh-water 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.

The absence of many fossil forms usually met with in marine strata, affords a useful negative indication of the fresh-water origin of a formation. For example, there are no sea-urchins, no corals, no chambered shells, such as the nautilus, nor microscopic Foraminifera in lacustrine or fluviatile deposits. In distinguishing the latter from formations accumulated in the sea, we are chiefly guided by the forms of the mollusca. In a fresh-water deposit, the number of individual shells is often as great as in a marine stratum, if not greater; but there is a smaller variety of species and genera. This might be anticipated from the fact that the genera and species of recent fresh-water and land shells are few when contrasted with the marine. Thus, the genera of true mollusca according to Woodward’s system, excluding those altogether extinct and those without shells, amount to 446 in number, of which the terrestrial and fresh-water genera scarcely form more than a fifth.[1]

Fig. 18: Cyrena obovata. Fig. 19: Cyrena fluminatis.

Fig. 20: Anodonta Cordierii. Fig. 21: Anodonta latimarginata. Fig. 22: Unio littoralis.

Almost all bivalve shells, or those of acephalous mollusca, are marine, about sixteen only out of 140 genera being fresh-water. 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.

Fig. 23: Gryphæa incurva.

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. 18, and Unio, Fig. 22, and the _ Monomyary,_ such as the oyster and scallop, in which there is only one of these impressions, as is seen in Fig. 23. Now, as none of these last, or the unimuscular bivalves, are fresh-water,[2] we may at once presume a deposit containing any of them to be marine.

Fig. 24: Planorbis enomphalus. Fig. 25: Limnæa longiscala. Fig. 26: Pauldina lenta. Fig. 27: Succinea amphibia. Fig. 28: Ancylus velletia. Fig. 29: Valvata piscinalis. Fig. 30: Physa hypnorum. Fig. 31: Auricula. Fig. 32: Melania inquinata. Fig. 33: Physa columnaris. Fig. 34: Melanopsis buccinoidea.

Fig. 35: Neritina globulud. Fig. 36: Nerita granulosa. The univalve shells most characteristic of fresh-water deposits are, _Planorbis, Limnæa,_ and _Paludina._ (See Figures.) But to these are occasionally added _Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, Potamides,_ and _ Neritina_ (see Figures), the four last being usually found in estuaries.

Fig. 37: Potamides cinctus.

Some naturalists include _Neritina_ (Fig. 35) and the marine _Nerita_ (Fig. 36) 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 _Neritæ,_ the inner margin of the outer lip toothed or crenulated. (See Fig. 36.)

The Potamides inhabit the mouths of rivers in warm latitudes, and are distinguishable from the marine Cerithia by their orbicular and multispiral opercula. The genus Auricula (Fig. 31) is amphibious, frequenting swamps and marshes within the influence of the tide.

The terrestrial shells are all univalves. The most important genera among these, both in a recent and fossil state, are _ Helix_ (Fig. 38), _Cyclostoma_ (Fig. 39), _Pupa_ (Fig. 40), _Clausilia_ (Fig. 41), _Bulimus_ (Fig. 42), _ Glandina_ and _Achatina._

Fig. 38: Helix Turomensis. Fig. 39: Cyclostoma elegans. Fig. 40: Pupa tridens. Fig. 41: Clausilia bidens. Fig. 42: Bulimus lubricus.

_Ampullaria_ (Fig. 43) is another genus of shells inhabiting rivers and ponds in hot countries. Many fossil species formerly referred to this genus, and which have been met with chiefly in marine formations, are now considered by conchologists to belong to _Natica_ and other marine genera.

Fig. 43: Ampullaria glauca. All univalve shells of land and fresh-water species, with the exception of _Melanopsis_ (Fig. 34), and _Achatina,_ which has a slight indentation, have entire mouths; and this circumstance may often serve as a convenient rule for distinguishing fresh-water 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 fresh-water _Ampullaria_ and the land-shells (Figs 38-42), when its outline is not interrupted by an indentation or notch, such as that seen at _b_ in _ Ancillaria_ (Fig. 45); or is not prolonged into a canal, as that seen at _a_ in _Pleurotoma_ (Fig. 44).

Fig. 44: Pleurotoma exorta. Fig. 45: Ancillaria subulata.

The mouths of a large proportion of the marine univalves have these notches or canals, and almost all species are carnivorous; whereas nearly all testacea having entire mouths are plant-eaters, whether the species be marine, fresh-water, or terrestrial.

There is, however, one genus which affords an occasional exception to one of the above rules. The _Potamides_ (Fig. 37), a subgenus of Cerithium, 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 fresh-water deposits are the shells of _Cypris,_ a minute bivalve crustaceous animal.[3] 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 fresh-water origin of a deposit, because the majority of species in another kindred genus of the same order, the _Cytherina_ of Lamarck, inhabit salt-water; and, although the animal differs slightly, the shell is scarcely distinguishable from that of the Cypris.

Fresh-water Fossil Plants.—The seed-vessels and stems of _ Chara,_ a genus of aquatic plants, are very frequent in fresh-water strata. These seed-vessels were called, before their true nature was known, gyrogonites, and were supposed to be foraminiferous shells. (See Fig. 46, _a_.)

The _Charæ_ 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. 47) 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 _Charæ,_) 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 fresh-water formations. They are generally composed of a large central tube surrounded by smaller ones; the whole stem being divided at certain intervals by transverse

## partitions or joints. (See _b,_ Fig. 46.)

Fig. 46: Chara medicaginula. Fig. 47: Chara elastica.

It is not uncommon to meet with layers of vegetable matter, impressions of leaves, and branches of trees, in strata containing fresh-water 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.”

Fresh-water and Marine Fish.—The remains of fish are occasionally useful in determining the fresh-water origin of strata. Certain genera, such as carp, perch, pike, and loach (_Cyprinus, Perca, Esox,_ and _Cobitis_), as also _Lebias,_ being peculiar to fresh-water. Other genera contain some fresh-water 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 fresh-water 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 mouth 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.

Lym-Fiord.—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 often formed and removed. The last irruption of salt water happened in 1824, when the North Sea entered, killing all the fresh-water 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, Lymnæa, Paludina,_ and _Charæ._[4]

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 fresh-water strata. When we find, as in the south-east of England (