Some naturalists include Neritina (fig. 42) and the marine Nerita (fig. 43) in the same genus, it being scarcely possible to distinguish Fig. 42. Fig. 48. Fig. 44. Neritina globulus. Paris basin. Nerita granulosa. Paris basin. 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 Nerita, the inner margin of the outer lip toothed or crenulated. (See fig. 43.) The Potamides inhabit the mouths of rivers in warm latitudes, and are distinguished from the marine cerithia by their orbicular and multispiral opercula. The genus Potamides auricula (fig. 38) is amphibious, frequenting swamps and cinctus,Sow, marshes within the influence of the tide. Paris basin. 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 AchaAmpullaria glauca, tina, which has a slight indentation, have entire from the Jumna. 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. 4549), 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.* 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 Chara inhabit the bottom of lakes and ponds, and flourish mostly where the water is charged with carbonate of lime. Their seedvessels 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 Chara, and therefore more nearly resembles in form the extinct fossil species found in England, France, and other countries. The For figures of fossil species of Purbeck, see below, ch. xx. stems, as well as the seed-vessels, of these plants occur both in modern shell marl and in ancient freshwater formations. They are generally Fig. 53. Fig. 54. a Chara medicaginula; b. Stem, magnified. Chara elastica; recent. Italy. a. Sessile seed-vessel between the divisions of b. Magnified transverse section of a branch, 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."* 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 icthyolites 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. * See Index of Principles, “Fossilization." 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 Chara.* 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 southeast 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.† CHAPTER IV. CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS. Chemical and mechanical deposits-Cementing together of particles-Hardening by exposure to air-Concretionary nodules-Consolidating effects of pressureMineralization of organic remains-Impressions and casts how formed-Fossil wood-Göppert's experiments-Precipitation of stony matter most rapid where putrefaction is going on-Source of lime in solution-Silex derived from decomposition of felspar-Proofs of the lapidification of some fossils soon after burial, of others when much decayed. HAVING spoken in the preceding chapters of the characters of sedi mentary formations, both as dependent on the deposition of inorganic matter and the distribution of fossils, I may next treat of the consolidation of stratified rocks, and the petrifaction of imbedded organic remains. Chemical and mechanical deposits.-A distinction has been made by *See Principles, Index, "Lym-Fiord." See below, Chap. XVIII., on the Wealden. stems, as well as the seed-vessels, of these plants occur both in modern shell marl and in ancient freshwater formations. They are generally Fig. 53. Fig. 54. a Chara medicaginula; a. Seed-vessel, b. Stem, magnified. Chara elastica; recent. Italy. a. Sessile seed-vessel between the divisions of b. Magnified transverse section of a branch, 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."* 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 icthyolites 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. * See Index of Principles, “Fossilization." |