The Student's Elements of Geology 6

The Student's Elements of Geology 6

The univalve shells 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 latitudes, 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 shells 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 shells 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 shells 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 circumstance 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 shells as the fresh-water Ampullaria

and the land-shells (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 canal, as that seen at a in Pleurotoma (Figure 44).

 

The mouths of a large proportion of the marine univalves have these notches or

canals, 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 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 fresh-water deposits are the shells 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 shells 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

shell 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

shells. (See Figure 46, a.)

 

(FIGURE 47. Chara elastica; recent, Italy.

a. Sessile seed-vessel between the divisions of the leaves of the female plant.

b. Magnified transverse section of a branch, with five seed-vessels, seen from

below upward.)

 

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 circumstance

we may attribute their abundance in a fossil state. Figure 47 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 shell-marl and in ancient

fresh-water formations. They are generally composed of a large central tube

surrounded by smaller ones; the whole stem being divided at certain intervals by

transverse partitions or joints. (See b, Figure 46.)

 

It is not uncommon to meet with layers of vegetable matter, impressions of

leaves, and branches of trees, in strata containing fresh-water 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."

 

FRESH-WATER AND MARINE FISH.

 

The remains of fish are occasionally useful in determining the fresh-water

origin of strata. Certain genera, such as carp, perch, pike, and loach

(Cyprinus, Perca, Esox, and Cobitis), as also Lebias, being peculiar to fresh-

water. Other genera contain some fresh-water 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 fresh-water 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.

 

There are also lagoons at the mouth 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.

 

LYM-FIORD.

 

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 often formed and removed. The last irruption of salt water happened in

1824, when the North Sea entered, killing all the fresh-water 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,

Lymnaea, Paludina, and Charae. (See Principles Index "Lym-Fiord.")

 

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 fresh-water strata. When we find, as in

the south-east of England (Chapter 18), a great series of fresh-water 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 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.

Sources of Lime and Silex in Solution.

 

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 mechanical, and

those of a chemical, origin. By the name mechanical are designated beds of mud,

sand, or pebbles produced by the action of running water, also accumulations 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 occasionally

precipitated upon the bottom of lakes 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, incrusting shells, fragments

of wood and leaves, and binding them together.

 

That similar travertin is formed at some points in the bed of the sea where

calcareous springs issue can not be doubted, but as a general rule the quantity

of lime, according to Bischoff, spread through the waters of the ocean is very

small, the free carbonic acid gas in the same waters being five times as much as

is necessary to keep the lime in a fluid state. Carbonate of lime, therefore,

can rarely be precipitated at the bottom of the sea by chemical action alone,

but must be produced by vital agency as in the case of coral reefs.

 

In such reefs, large masses of limestone are formed by the stony skeletons of

zoophytes; and these, together with shells, become cemented together by

carbonate of lime, part of which is probably furnished to the sea-water by the

decomposition of dead corals. Even shells, of which the animals are still living

on these reefs, are very commonly found to be incrusted over with a hard coating

of limestone.

 

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 2 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 incrust 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 vein-stones.

 

CONSOLIDATION OF STRATA.

 

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 shells 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 shells, 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 shells, 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 shells 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 shell; and unless circumstances

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.

 

In what manner silex and carbonate of lime may become widely diffused in small

quantities 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 passing 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 sand, pebbles, or

any fragmentary mixture. In some conglomerates, like the pudding-stone of

Hertfordshire (a Lower Eocene deposit), pebbles of flint and grains of sand are

united by a siliceous cement so firmly, that if a block be fractured, the rent

passes 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

crystallising, 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 mass, namely, the water, has crystallised,

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 glass

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. (Dr. MacCulloch System of Geology volume 1

page 123.)

 

The marl recently deposited at the bottom of Lake Superior, in North America, is

soft, and often filled with fresh-water shells; 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 fresh-water shells.

 

CONCRETIONARY STRUCTURE.

 

(FIGURE 48. Calcareous nodules in Lias.)

 

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 twenty per

cent of oxide of iron, and the addition of a small quantity 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

such cases is brought about by the action of chemical affinity on finely

comminuted matter previously suspended in water. After deposition similar

particles seem often 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 balls, 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 traceable through the concretions, remaining

parallel to those of the surrounding unconsolidated rock. (See Figure 48.) Such

nodules of limestone have often a shell or other foreign body in the centre.

 

(FIGURE 49. Spheroidal concretions in magnesian limestone.)

 

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 balls 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 pass

uninterruptedly through them. In some cliffs this limestone resembles a great

irregular pile of cannon-balls. Some of the globular masses have their centre in

one stratum, while a portion of their exterior passes through to the stratum

above or below. Thus the larger spheroid in the section (Figure 49) passes from

the stratum b upward into a. In this instance we must suppose the deposition of

a series of minor layers, first forming the stratum b, and afterwards the

incumbent 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. Crystallisation,

beginning at the centre, must have gone on forming concentric coats around the

original nucleus without interfering with the laminated structure of the rock.

 

(FIGURE 50. Section through strata of grit.)

 

When the particles of rocks have been thus rearranged by chemical forces, it is

sometimes difficult or impossible to ascertain whether certain lines of division

are due to original deposition or to the subsequent aggregation of several

particles. Thus suppose three strata of grit, A, B, C, are charged unequally

with calcareous matter, and that B is the most calcareous. If consolidation

takes place in B, the concretionary action may spread upward into a part of A,

where the carbonate of lime is more abundant than in the rest; so that a mass, d

e f, forming a portion of the superior stratum, becomes united with B into one

solid mass of stone. The original line of division, d e, being thus effaced, the

line d f would generally be considered as the surface of the bed B, though not

strictly a true plane of stratification. (Figure 50.)

 

PRESSURE AND HEAT.

 

When sand and mud sink to the bottom of a deep sea, the particles are not

pressed down by the enormous weight of the incumbent ocean; for the water, which

becomes mingled with the sand and mud, resists pressure with a force equal to

that of the column of fluid above. The same happens in regard to organic remains

which are filled with water under great pressure as they sink, otherwise they

would be immediately crushed to pieces and flattened. Nevertheless, if the

materials of a stratum remain in a yielding state, and do not set or solidify,

they will be gradually squeezed down by the weight of other materials

successively heaped upon them, just as soft clay or loose sand on which a house

is built may give way. By such downward pressure particles of clay, sand, and

marl may become packed into a smaller space, and be made to cohere together

permanently.

 

Analogous effects of condensation may arise when the solid parts of the earth's

crust are forced in various directions by those mechanical movements hereafter

to be described, by which strata have been bent, broken, and raised above the

level of the sea. Rocks of more yielding materials must often have been forced

against others previously consolidated, and may thus by compression have

acquired a new structure. A recent discovery may help us to comprehend how fine

sediment derived from the detritus of rocks may be solidified by mere pressure.

The graphite or "black lead" of commerce having become very scarce, Mr.

Brockedon contrived a method by which the dust of the purer portions of the

mineral found in Borrowdale might be recomposed into a mass as dense and compact

as native graphite. The powder of graphite is first carefully prepared and freed

from air, and placed under a powerful press on a strong steel die, with air-

tight fittings. It is then struck several blows, each of a power of 1000 tons;

after which operation the powder is so perfectly solidified that it can be cut

for pencils, and exhibits when broken the same texture as native graphite.

 

But the action of heat at various depths in the earth is probably the most

powerful of all causes in hardening sedimentary strata. To this subject I shall

refer again when treating of the metamorphic rocks, and of the slaty and jointed

structure.

 MINERALISATION OF ORGANIC REMAINS.