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 Ill.u.s.trations: _Cytheridae_ and _Foraminifera_ from the chalk.

Fig. 21. _Cythere_, Mull.

_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 acc.u.mulation 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 sh.e.l.ls, 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 const.i.tuted 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 sh.e.l.ls are princ.i.p.ally, 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 lat.i.tudes.

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 sh.e.l.ls, together with the remains of terrestrial quadrupeds. The number of land sh.e.l.ls 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-sh.e.l.ls (_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 sh.e.l.ls, 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 sh.e.l.ls 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 sh.e.l.ls, 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 sh.e.l.ls 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 antic.i.p.ated from the fact that the genera and species of recent freshwater and land sh.e.l.ls 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 sh.e.l.ls, amount to about 200 in number, of which the terrestrial and freshwater genera scarcely form more than a sixth.[28-A]

[Ill.u.s.tration: Fig. 25. _Cyclas obovata_; fossil. Hants.]

[Ill.u.s.tration: Fig. 26. _Cyrena consobrina_; fossil. Grays, Ess.e.x.]

[Ill.u.s.tration: Fig. 27. _Anodonta Cordierii_; fossil. Paris.]

[Ill.u.s.tration: Fig. 28. _Anodonta latimarginatus_; recent. Bahia.]

[Ill.u.s.tration: Fig. 29. _Unio littoralis_; recent. Auvergne.]

Almost all bivalve sh.e.l.ls, 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 pa.s.s into each other.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Fig. 31. _Planorbis euomphalus_; fossil. Isle of Wight.]

[Ill.u.s.tration: Fig. 32. _Lymnea longiscata_; fossil. Hants.]

[Ill.u.s.tration: Fig. 33. _Paludina lenta_; fossil. Hants.]

The univalve sh.e.l.ls 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.)

[Ill.u.s.tration: Fig. 34. _Succinea amphibia_; fossil. Loess, Rhine.]

[Ill.u.s.tration: Fig. 35. _Ancylus elegans_; fossil. Hants.]

[Ill.u.s.tration: Fig. 36. _Valvata_; fossil. Grays, Ess.e.x.]

[Ill.u.s.tration: Fig. 37. _Physa hypnorum_; recent.]

[Ill.u.s.tration: Fig. 38. _Auricula_; recent. Ava.]

[Ill.u.s.tration: Fig. 39. _Melania inquinata._ Paris Basin.]

[Ill.u.s.tration: Fig. 40. _Physa columnaris._ Paris Basin.]

[Ill.u.s.tration: 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 sh.e.l.l, however, of the _Ancylus_ is usually thinner.[29-A]

[Ill.u.s.tration: Fig. 42. _Neritina globulus._ Paris basin.]

[Ill.u.s.tration: 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.)

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Fig. 45. _Helix Turonensis._ Faluns, Touraine.]

[Ill.u.s.tration: Fig. 46. _Cyclostoma elegans._ Loess.]

[Ill.u.s.tration: Fig. 47. _Pupa tridens._ Loess.]

[Ill.u.s.tration: Fig. 48. _Clausilia bidens._ Loess.]

[Ill.u.s.tration: Fig. 49. _Bulimus lubricus._ Loess, Rhine.]

The terrestrial sh.e.l.ls 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 pa.s.s into each other.

[Ill.u.s.tration: Fig. 50. _Ampullaria glauca_, from the Jumna.]