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 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 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.

[31-A] For figures of recent species, see below, p. 183., and figs. of fossils, see p. 228.

[32-A] See Index of Principles, "Fossilization."

[33-A] See Principles, Index, "Lym-Fiord."

[33-B] See below, Chap. XVIII., on the Wealden.

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 pressure--Mineralization of organic remains--Impressions and casts how formed--Fossil wood--Goppert'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 sedimentary 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 geologists between deposits of a chemical, and those of a mechanical, origin. By the latter name are designated beds of mud, sand, or pebbles produced by the action of running water, also acc.u.mulations of stones and scoriae thrown out by a volcano, which have fallen into their present place by the force of gravitation. But the matter which forms a chemical deposit has not been mechanically suspended in water, but in a state of solution until separated by chemical action. In this manner carbonate of lime is often precipitated upon the bottom of lakes and seas in a solid form, as may be well seen in many parts of Italy, where mineral springs abound, and where the calcareous stone, called travertin, is deposited. In these springs the lime is usually held in solution by an excess of carbonic acid, or by heat if it be a hot spring, until the water, on issuing from the earth, cools or loses part of its acid. The calcareous matter then falls down in a solid state, encrusting sh.e.l.ls, fragments of wood and leaves, and binding them together.[34-A]

In coral reefs, large ma.s.ses of limestone are formed by the stony skeletons of zoophytes; and these, together with sh.e.l.ls, become cemented together by carbonate of lime, part of which is probably furnished to the sea-water by the decomposition of dead corals. Even sh.e.l.ls of which the animals are still living, on these reefs, are very commonly found to be encrusted over with a hard coating of limestone.[34-B]

If sand and pebbles are carried by a river into the sea, and these are bound together immediately by carbonate of lime, the deposit may be described as of a mixed origin, partly chemical, and partly mechanical.

Now, the remarks already made in Chapter II. on the original horizontality of strata are strictly applicable to mechanical deposits, and only partially to those of a mixed nature. Such as are purely chemical may be formed on a very steep slope, or may even encrust the vertical walls of a fissure, and be of equal thickness throughout; but such deposits are of small extent, and for the most part confined to veinstones.

_Cementing of particles._--It is chiefly in the case of calcareous rocks that solidification takes place at the time of deposition. But there are many deposits in which a cementing process comes into operation long afterwards. We may sometimes observe, where the water of ferruginous or calcareous springs has flowed through a bed of sand or gravel, that iron or carbonate of lime has been deposited in the interstices between the grains or pebbles, so that in certain places the whole has been bound together into a stone, the same set of strata remaining in other parts loose and incoherent.

Proofs of a similar cementing action are seen in a rock at Kelloway in Wiltshire. A peculiar band of sandy strata, belonging to the group called Oolite by geologists, may be traced through several counties, the sand being for the most part loose and unconsolidated, but becoming stony near Kelloway. In this district there are numerous fossil sh.e.l.ls which have decomposed, having for the most part left only their casts. The calcareous matter hence derived has evidently served, at some former period, as a cement to the siliceous grains of sand, and thus a solid sandstone has been produced. If we take fragments of many other argillaceous grits, retaining the casts of sh.e.l.ls, and plunge them into dilute muriatic or other acid, we see them immediately changed into common sand and mud; the cement of lime, derived from the sh.e.l.ls, having been dissolved by the acid.

Traces of impressions and casts are often extremely faint. In some loose sands of recent date we meet with sh.e.l.ls in so advanced a stage of decomposition as to crumble into powder when touched. It is clear that water percolating such strata may soon remove the calcareous matter of the sh.e.l.l; and, unless circ.u.mstances cause the carbonate of lime to be again deposited, the grains of sand will not be cemented together; in which case no memorial of the fossil will remain. The absence of organic remains from many aqueous rocks may be thus explained; but we may presume that in many of them no fossils were ever imbedded, as there are extensive tracts on the bottoms of existing seas even of moderate depth on which no fragment of sh.e.l.l, coral, or other living creature can be detected by dredging. On the other hand, there are depths where the zero of animal life has been approached; as, for example, in the Mediterranean, at the depth of about 230 fathoms, according to the researches of Prof. E. Forbes. In the aegean Sea a deposit of yellowish mud of a very uniform character, and closely resembling chalk, is going on in regions below 230 fathoms, and this formation must be wholly devoid of organic remains.[35-A]

In what manner silex and carbonate of lime may become widely diffused in small quant.i.ties through the waters which permeate the earth's crust will be spoken of presently, when the petrifaction of fossil bodies is considered; but I may remark here that such waters are always pa.s.sing in the case of thermal springs from hotter to colder parts of the interior of the earth; and as often as the temperature of the solvent is lowered, mineral matter has a tendency to separate from it and solidify. Thus a stony cement is often supplied to any sand, pebbles, or fragmentary mixture. In some conglomerates, like the pudding-stone of Hertfordshire, pebbles of flint and grains of sand are united by a siliceous cement so firmly, that if a block be fractured the rent pa.s.ses as readily through the pebbles as through the cement.

It is probable that many strata became solid at the time when they emerged from the waters in which they were deposited, and when they first formed a part of the dry land. A well-known fact seems to confirm this idea: by far the greater number of the stones used for building and road-making are much softer when first taken from the quarry than after they have been long exposed to the air; and these, when once dried, may afterwards be immersed for any length of time in water without becoming soft again. Hence it is found desirable to shape the stones which are to be used in architecture while they are yet soft and wet, and while they contain their "quarry-water," as it is called; also to break up stone intended for roads when soft, and then leave it to dry in the air for months that it may harden. Such induration may perhaps be accounted for by supposing the water, which penetrates the minutest pores of rocks, to deposit, on evaporation, carbonate of lime, iron, silex, and other minerals previously held in solution, and thereby to fill up the pores partially. These particles, on crystallizing, would not only be themselves deprived of freedom of motion, but would also bind together other portions of the rock which before were loosely aggregated. On the same principle wet sand and mud become as hard as stone when frozen; because one ingredient of the ma.s.s, namely, the water, has crystallized, so as to hold firmly together all the separate particles of which the loose mud and sand were composed.

Dr. MacCulloch mentions a sandstone in Skye, which may be moulded like dough when first found; and some simple minerals, which are rigid and as hard as gla.s.s in our cabinets, are often flexible and soft in their native beds; this is the case with asbestos, sahlite, tremolite, and chalcedony, and it is reported also to happen in the case of the beryl.[36-A]

The marl recently deposited at the bottom of Lake Superior, in North America, is soft, and often filled with freshwater sh.e.l.ls; but if a piece be taken up and dried, it becomes so hard that it can only be broken by a smart blow of the hammer. If the lake therefore was drained, such a deposit would be found to consist of strata of marlstone, like that observed in many ancient European formations, and like them containing freshwater sh.e.l.ls.[36-B]

It is probable that some of the heterogeneous materials which rivers transport to the sea may at once set under water, like the artificial mixture called pozzolana, which consists of fine volcanic sand charged with about 20 per cent. of oxide of iron, and the addition of a small quant.i.ty of lime. This substance hardens, and becomes a solid stone in water, and was used by the Romans in constructing the foundations of buildings in the sea.

Consolidation in these cases is brought about by the action of chemical affinity on finely comminuted matter previously suspended in water. After deposition similar particles seem to exert a mutual attraction on each other, and congregate together in particular spots, forming lumps, nodules, and concretions. Thus in many argillaceous deposits there are calcareous b.a.l.l.s, or spherical concretions, ranged in layers parallel to the general stratification; an arrangement which took place after the shale or marl had been thrown down in successive laminae; for these laminae are often traced in the concretions, remaining parallel to those of the surrounding unconsolidated rock. (See fig. 55.) Such nodules of limestone have often a sh.e.l.l or other foreign body in the centre.[37-A]

[Ill.u.s.tration: Fig. 55. Calcareous nodules in Lias.]

Among the most remarkable examples of concretionary structure are those described by Professor Sedgwick as abounding in the magnesian limestone of the north of England. The spherical b.a.l.l.s are of various sizes, from that of a pea to a diameter of several feet, and they have both a concentric and radiated structure, while at the same time the laminae of original deposition pa.s.s uninterruptedly through them. In some cliffs this limestone resembles a great irregular pile of cannon b.a.l.l.s. Some of the globular ma.s.ses have their centre in one stratum, while a portion of their exterior pa.s.ses through to the stratum above or below. Thus the larger spheroid in the annexed section (fig. 56.) pa.s.ses from the stratum _b_ upwards into _a_. In this instance we must suppose the deposition of a series of minor layers, first forming the stratum _b_, and afterwards the inc.u.mbent stratum _a_; then a movement of the particles took place, and the carbonates of lime and magnesia separated from the more impure and mixed matter forming the still unconsolidated parts of the stratum. Crystallization, beginning at the centre, must have gone on forming concentric coats, around the original nucleus without interfering with the laminated structure of the rock.

[Ill.u.s.tration: Fig. 56. Spheroidal concretions in magnesian limestone.]