The Student's Elements of Geology Part 3

"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 sh.e.l.ls 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 lat.i.tudes.

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 sh.e.l.ls, together with the remains of terrestrial quadrupeds. The number of land-sh.e.l.ls 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-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 strewn over with innumerable dead land-sh.e.l.ls, 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-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 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 sh.e.l.ls, such as the nautilus, nor microscopic Foraminifera in lacustrine or fluviatile deposits. In distinguishing the latter from formations acc.u.mulated in the sea, we are chiefly guided by the forms of the mollusca. In a fresh-water deposit, the number of individual sh.e.l.ls 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 antic.i.p.ated from the fact that the genera and species of recent fresh-water and land sh.e.l.ls 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 sh.e.l.ls, amount to 446 in number, of which the terrestrial and fresh-water genera scarcely form more than a fifth. (See Woodward's Manual of Mollusca 1856.)

(FIGURE 18. Cyrena obovata, Sowerby; fossil. Hants.)

(FIGURE 19. Cyrena (Corbicella) fluminalis, Moll.; fossil. Grays, Ess.e.x.)

(FIGURE 20. Anodonta Cordierii; D'Orbigny; fossil. Paris.)

(FIGURE 21. Anodonta latimarginata; recent. Bahia.)

(FIGURE 22. Unio littoralis. Lamarck; recent. Auvergne.)

(FIGURE 23. Gryphaea incurva, Sowerby; (G. arcuata, Lamarck) upper valve. Lias.)

Almost all bivalve sh.e.l.ls, 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 18-22); the two first and two last of which are so nearly allied as to pa.s.s 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, Figure 18, and Unio, Figure 22, and the Monomyary, such as the oyster and scallop, in which there is only one of these impressions, as is seen in Figure 23. Now, as none of these last, or the unimuscular bivalves, are fresh-water, we may at once presume a deposit containing any of them to be marine. (The fresh-water Mulleria, when young, forms a single exception to the rule, as it then has two muscular impressions, but it has only one in the adult state.)

(FIGURE 24. Planorbis euomphalus, Sowerby; fossil. Isle of Wight.)

(FIGURE 25. Limnaea longiscala, Brongniart; fossil. Isle of Wight.)

(FIGURE 26. Paludina lenta, Brand.; fossil. Isle of Wight.)

(FIGURE 27. Succinea amphibia, Drap. (S. putris, L.); fossil. Loess, Rhine.)

(FIGURE 28. Ancylus velletia (A. elegans), Sowerby; fossil. Isle of Wight.)

(FIGURE 29. Valvata piscinalis, Mull.; fossil. Grays, Ess.e.x.)

(FIGURE 30. Physa hypnorum, Linne; recent. Isle of Wight.)

(FIGURE 31. Auricula; recent. Ava.)

(FIGURE 32. Melania inquinata, Def. Paris basin.)

(FIGURE 33. Physa columnaris, Desh. Paris basin.)

(FIGURE 34. Melanopsis buccinoidea, Ferr.; recent. Asia.)

The univalve sh.e.l.ls most characteristic of fresh-water deposits are, Planorbis, Limnaea, and Paludina. (See Figures 24-26.) But to these are occasionally added Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, Potamides, and Neritina (see Figures 27-34), the four last being usually found in estuaries.

(FIGURE 35. Neritina globulus, Def. Paris basin.)

(FIGURE 36. Nerita granulosa, Desh. Paris basin.)

Some naturalists include Neritina (Figure 35) and the marine Nerita (Figure 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 Neritae, the inner margin of the outer lip toothed or crenulated. (See Figure 36.)

(FIGURE 37. Potamides cinctus, Sowerby. Paris basin.)

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

(FIGURE 38. Helix Turonensis, Desh.; faluns, Touraine.)

(FIGURE 39. Cyclostoma elegans, Mull.; Loess.)

(FIGURE 40. Pupa tridens, Drap.; Loess.)

(FIGURE 41. Clausilia bidens, Drap.; Loess.)

(FIGURE 42. Bulimus lubricus, Mull.; Loess, Rhine.)

The terrestrial sh.e.l.ls are all univalves. The most important genera among these, both in a recent and fossil state, are Helix (Figure 38), Cyclostoma (Figure 39), Pupa (Figure 40), Clausilia (Figure 41) Bulimus (Figure 42), Glandina and Achatina.

(FIGURE 43. Ampullaria glauca, from the Jumna.)

Ampullaria (Figure 43) is another genus of sh.e.l.ls 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.

(FIGURE 44. Pleurotoma exorta, Brand. Upper and Middle Eocene. Barton and Bracklesham.)

(FIGURE 45. Ancillaria subulata, Sowerby. Barton clay. Eocene.)

All univalve sh.e.l.ls of land and fresh-water species, with the exception of Melanopsis (Figure 34), and Achatina, which has a slight indentation, have entire mouths; and this circ.u.mstance 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 sh.e.l.ls as the fresh-water Ampullaria and the land-sh.e.l.ls (Figures 38-42), when its outline is not interrupted by an indentation or notch, such as that seen at b in Ancillaria (Figure 45); or is not prolonged into a ca.n.a.l, as that seen at a in Pleurotoma (Figure 44).

The mouths of a large proportion of the marine univalves have these notches or ca.n.a.ls, 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 (Figure 37), a subgenus of Cerithium, although provided with a short ca.n.a.l, 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 sh.e.l.ls of Cypris, a minute bivalve crustaceous animal. (For figures of fossil species of Purbeck see below, Chapter 19.) Many minute living species of this genus swarm in lakes and stagnant pools in Great Britain; but their sh.e.l.ls 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 sh.e.l.l is scarcely distinguishable from that of the Cypris.

FRESH-WATER FOSSIL PLANTS.

(FIGURE 46. Chara medicaginula; fossil. Upper Eocene, Isle of Wight.)

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 sh.e.l.ls. (See Figure 46, a.)

(FIGURE 47. Chara elastica; recent, Italy.