Part 10 (1/2)

The _Ampullaria_ (fig. 50.) is another genus of sh.e.l.ls, 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 sh.e.l.ls of land and freshwater species, with the exception of _Melanopsis_ (fig. 41.), and _Achatina_, which has a slight indentation, have entire mouths; and this circ.u.mstance may often serve as a convenient rule for distinguis.h.i.+ng 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 sh.e.l.ls as the _Ampullaria_ and the land sh.e.l.ls (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 ca.n.a.l, as that seen at _a_ in _Pleurotoma_ (fig. 51.).

[Ill.u.s.tration: Fig. 51. _Pleurotoma rotata._ Subap. hills, Italy.]

[Ill.u.s.tration: Fig. 52. _Ancillaria subulata._ London clay.]

The mouths of a large proportion of the marine univalves have these notches or ca.n.a.ls, 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 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 freshwater deposits are the sh.e.l.ls of _Cypris_, a minute crustaceous animal, having a sh.e.l.l 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 sh.e.l.ls 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 sh.e.l.l 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 sh.e.l.ls. (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 circ.u.mstance 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 sh.e.l.l 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 part.i.tions or joints. (See _b_, fig. 53.)

[Ill.u.s.tration: Fig. 53. _Chara medicaginula_; fossil. Isle of Wight.

_a._ Seed-vessel. magnified 20 diameters.

_b._ Stem, magnified.]

[Ill.u.s.tration: 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 sh.e.l.ls; 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 s.p.a.ces 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 sh.e.l.ls.

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 ill.u.s.tration of a.n.a.logous 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 sh.e.l.ls, 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.