The Student's Elements of Geology 5

The Student's Elements of Geology 5

It is well known that the torrents and streams which now descend from the Alpine

declivities to the shore, bring down annually, when the snow melts, vast

quantities of shingle and sand, and then, as they subside, fine mud, while in

summer they are nearly or entirely dry; so that it may be safely assumed that

deposits like those of the valley of the Magnan, consisting of coarse gravel

alternating with fine sediment, are still in progress at many points, as, for

instance, at the mouth of the Var. They must advance upon the Mediterranean in

the form of great shoals terminating in a steep talus; such being the original

mode of accumulation of all coarse materials conveyed into deep water,

especially where they are composed in great part of pebbles, which can not be

transported to indefinite distances by currents of moderate velocity. By

inattention to facts and inferences of this kind, a very exaggerated estimate

has sometimes been made of the supposed depth of the ancient ocean. There can be

no doubt, for example, that the strata a, Figure 7, or those nearest to Monte

Calvo, are older than those indicated by b, and these again were formed before

c; but the vertical depth of gravel and sand in any one place can not be proved

to amount even to 1000 feet, although it may perhaps be much greater, yet

probably never exceeding at any point 3000 or 4000 feet. But were we to assume

that all the strata were once horizontal, and that their present dip or

inclination was due to subsequent movements, we should then be forced to

conclude that a sea several miles deep had been filled up with alternate layers

of mud and pebbles thrown down one upon another.

 

In the locality now under consideration, situated a few miles to the west of

Nice, there are many geological data, the details of which can not be given in

this place, all leading to the opinion that, when the deposit of the Magnan was

formed, the shape and outline of the Alpine declivities and the shore greatly

resembled what we now behold at many points in the neighbourhood. That the beds

a, b, c, d are of comparatively modern date is proved by this fact, that in

seams of loamy marl intervening between the pebbly beds are fossil shells, half

of which belong to species now living in the Mediterranean.

 

RIPPLE-MARK.

 

The ripple-mark, so common on the surface of sandstones of all ages (see Figure

8), and which is so often seen on the sea-shore at low tide, seems to originate

in the drifting of materials along the bottom of the water, in a manner very

similar to that which may explain the inclined layers above described. This

ripple is not entirely confined to the beach between high and low water mark,

but is also produced on sands which are constantly covered by water. Similar

undulating ridges and furrows may also be sometimes seen on the surface of drift

snow and blown sand.

 

The ripple-mark is usually an indication of a sea-beach, or of water from six to

ten feet deep, for the agitation caused by waves even during storms extends to a

very slight depth. To this rule, however, there are some exceptions, and recent

ripple-marks have been observed at the depth of 60 or 70 feet. It has also been

ascertained that currents or large bodies of water in motion may disturb mud and

sand at the depth of 300 or even 450 feet. (Darwin Volcanic Islands page 134.)

Beach ripple, however, may usually be distinguished from current ripple by

frequent changes in its direction. In a slab of sandstone, not more than an inch

thick, the furrows or ridges of an ancient ripple may often be seen in several

successive laminae to run towards different points of the compass.

 

 

CHAPTER III.

 

ARRANGEMENT OF FOSSILS IN STRATA.-- FRESH-WATER AND MARINE FOSSILS.

 

Successive Deposition indicated by Fossils.

Limestones formed of Corals and Shells.

Proofs of gradual Increase of Strata derived from Fossils.

Serpula attached to Spatangus.

Wood bored by Teredina.

Tripoli formed of Infusoria.

Chalk derived principally from Organic Bodies.

Distinction of Fresh-water from Marine Formations.

Genera of Fresh-water and Land Shells.

Rules for recognising Marine Testacea.

Gyrogonite and Chara.

Fresh-water Fishes.

Alternation of Marine and Fresh-water Deposits.

Lym-Fiord.

 

Having in the last chapter considered the forms of stratification so far as they

are determined by the arrangement of inorganic matter, we may now turn our

attention to the manner in which organic remains are distributed through

stratified deposits. We should often be unable to detect any signs of

stratification or of successive deposition, if particular kinds of fossils did

not occur here and there at certain depths in the mass. At one level, for

example, univalve shells of some one or more species predominate; at another,

bivalve shells; and at a third, corals; while in some formations we find layers

of vegetable matter, commonly derived from land plants, separating strata.

 

It may appear inconceivable to a beginner how mountains, several thousand feet

thick, can have become full of fossils from top to bottom; but the difficulty is

removed, when he reflects on the origin of stratification, as explained in the

last chapter, and allows sufficient time for the accumulation of sediment. He

must never lose sight of the fact that, during the process of deposition, each

separate layer was once the uppermost, and immediately in contact with the water

in which aquatic animals lived. Each stratum, in fact, however far it may now

lie beneath the surface, was once in the state of shingle, or loose sand or soft

mud at the bottom of the sea, in which shells and other bodies easily became

enveloped.

 

RATE OF DEPOSITION INDICATED BY FOSSILS.

 

By attending to the nature of these remains, we are often enabled to determine

whether the deposition was slow or rapid, whether it took place in a deep or

shallow sea, near the shore or far from land, and whether the water was salt,

brackish, or fresh. Some limestones consist almost exclusively of corals, and in

many cases it is evident that the present position of each fossil zoophyte has

been determined by the manner in which it grew originally. The axis of the

coral, for example, if its natural growth is erect, still remains at right

angles to the plane of stratification. If the stratum be now horizontal, the

round spherical heads of certain species continue uppermost, and their points of

attachment are directed downward. This arrangement is sometimes repeated

throughout a great succession of strata. From what we know of the growth of

similar zoophytes in modern reefs, we infer that the rate of increase was

extremely slow, and some of the fossils must have flourished for ages like

forest-trees, before they attained so large a size. During these ages, the water

must have been clear and transparent, for such corals can not live in turbid

water.

 

(FIGURE 9. Fossil Gryphaea, covered both on the outside and inside with fossil

Serpulae.)

 

In like manner, when we see thousands of full-grown shells dispersed everywhere

throughout a long series of strata, we can not doubt that time was required for

the multiplication of successive generations; and the evidence of slow

accumulation is rendered more striking from the proofs, so often discovered, of

fossil bodies having lain for a time on the floor of the ocean after death

before they were imbedded in sediment. Nothing, for example, is more common than

to see fossil oysters in clay, with Serpulae, or barnacles (acorn-shells), or

corals, and other creatures, attached to the inside of the valves, so that the

mollusk was certainly not buried in argillaceous mud the moment it died. There

must have been an interval during which it was still surrounded with clear

water, when the creatures whose remains now adhere to it grew from an embryonic

to a mature state. Attached shells which are merely external, like some of the

Serpulae (a) in Figure 9, may often have grown upon an oyster or other shell

while the animal within was still living; but if they are found on the inside,

it could only happen after the death of the inhabitant of the shell which

affords the support. Thus, in Figure 9, it will be seen that two Serpulae have

grown on the interior, one of them exactly on the place where the adductor

muscle of the Gryphaea (a kind of oyster) was fixed.

 

(FIGURE 10. Serpula attached to a fossil Micraster from the Chalk.)

 

(FIGURE 11. Recent Spatangus with the spines removed from one side.

b. Spine and tubercles, natural size.

a. The same magnified.)

 

Some fossil shells, even if simply attached to the OUTSIDE of others, bear full

testimony to the conclusion above alluded to, namely, that an interval elapsed

between the death of the creature to whose shell they adhere, and the burial of

the same in mud or sand. The sea-urchins, or Echini, so abundant in white chalk,

afford a good illustration. It is well known that these animals, when living,

are invariably covered with spines supported by rows of tubercles. These last

are only seen after the death of the sea-urchin, when the spines have dropped

off. In Figure 11 a living species of Spatangus, common on our coast, is

represented with one half of its shell stripped of the spines. In Figure 10 a

fossil of a similar and allied genus from the white chalk of England shows the

naked surface which the individuals of this family exhibit when denuded of their

bristles. The full-grown Serpula, therefore, which now adheres externally, could

not have begun to grow till the Micraster had died, and the spines became

detached.

 

(FIGURE 12.

a. Ananchytes from the chalk with lower valve of Crania attached.

b. Upper valve of Crania detached.)

 

Now the series of events here attested by a single fossil may be carried a step

farther. Thus, for example, we often meet with a sea-urchin (Ananchytes) in the

chalk (see Figure 12) which has fixed to it the lower valve of a Crania, a genus

of bivalve mollusca. The upper valve (b, Figure 12) is almost invariably

wanting, though occasionally found in a perfect state of preservation in white

chalk at some distance. In this case, we see clearly that the sea-urchin first

lived from youth to age, then died and lost its spines, which were carried away.

Then the young Crania adhered to the bared shell, grew and perished in its turn;

after which the upper valve was separated from the lower before the Ananchytes

became enveloped in chalky mud.

 

(FIGURES 13 AND 14. Fossil and recent wood drilled by perforating Mollusca.

 

(FIGURE 13.

a. Fossil wood from London Clay, bored by Teredina.

b. Shell and tube of Teredina personata, the right-hand figure the ventral, the

left the dorsal view.)

 

(FIGURE 14.

e. Recent wood bored by Toredo.

d. Shell and tube of Teredo navalis, from the same.

c. Anterior and posterior view of the valves of same detached from the tube.))

 

It may be well to mention one more illustration of the manner in which single

fossils may sometimes throw light on a former state of things, both in the bed

of the ocean and on some adjoining land. We meet with many fragments of wood

bored by ship-worms at various depths in the clay on which London is built.

Entire branches and stems of trees, several feet in length, are sometimes found

drilled all over by the holes of these borers, the tubes and shells of the

mollusk still remaining in the cylindrical hollows. In Figure 14, e, a

representation is given of a piece of recent wood pierced by the Teredo navalis,

or common ship-worm, which destroys wooden piles and ships. When the cylindrical

tube d has been extracted from the wood, the valves are seen at the larger or

anterior extremity, as shown at c. In like manner, a piece of fossil wood (a,

Figure 13) has been perforated by a kindred but extinct genus, the Teredina of

Lamarck. The calcareous tube of this mollusk was united and, as it were,

soldered on to the valves of the shell (b), which therefore can not be detached

from the tube, like the valves of the recent Teredo. The wood in this fossil

specimen is now converted into a stony mass, a mixture of clay and lime; but it

must once have been buoyant and floating in the sea, when the Teredinae lived

upon, and perforated it. Again, before the infant colony settled upon the drift

wood, part of a tree must have been floated down to the sea by a river,

uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind:

and thus our thoughts are carried back to a prior period, when the tree grew for

years on dry land, enjoying a fit soil and climate.

 

STRATA OF ORGANIC ORIGIN.

 

(FIGURE 15. Gaillonella ferruginea, Ehb.)

 

(FIGURE 16. Gaillonella distans, Ehb.)

 

(FIGURE 17. Bacillaria paradoxa.

a. Front view.

b. Side view.)

 

It has been already remarked that there are rocks in the interior of continents,

at various depths in the earth, and at great heights above the sea, almost

entirely made up of the remains of zoophytes and testacea. Such masses may be

compared to modern oyster-beds and coral-reefs; and, like them, the rate of

increase must have been extremely gradual. But there are a variety of stone

deposits in the earth's crust, now proved to have been derived from plants and

animals of which the organic origin was not suspected until of late years, even

by naturalists. Great surprise was therefore created some years since by the

discovery of Professor Ehrenberg, of Berlin, that a certain kind of siliceous

stone, called tripoli, was entirely composed of millions of the remains of

organic beings, which were formerly referred to microscopic Infusoria, but which

are now admitted to be plants. They abound in rivulets, lakes, and ponds in

England and other countries, and are termed Diatomaceae by those naturalists who

believe in their vegetable origin. The subject alluded to has long been well-

known in the arts, under the name of infusorial earth or mountain meal, and is

used in the form of powder for polishing stones and metals. It has been

procured, among other places, from the mud of a lake at Dolgelly, in North

Wales, and from Bilin, in Bohemia, in which latter place a single stratum,

extending over a wide area, is no less than fourteen feet thick. This stone,

when examined with a powerful microscope, is found to consist of the siliceous

plates or frustules of the above-figured Diatomaceae, united together without

any visible cement. It is difficult to convey an idea of their extreme

minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000

millions of individuals of the Gaillonella distans (see Figure 16) in every

cubic inch (which weighs about 220 grains), or about 187 millions in a single

grain. At every stroke, therefore, that we make with this polishing powder,

several millions, perhaps tens of millions, of perfect fossils are crushed to

atoms.

 

A well-known substance, called bog-iron ore, often met with in peat-mosses, has

often been shown by Ehrenberg to consist of innumerable articulated threads, of

a yellow ochre colour, composed of silica, argillaceous matter, and peroxide of

iron. These threads are the cases of a minute microscopic body, called

Gaillonella ferruginea (Figure 15), associated with the siliceous frustules of

other fresh-water algae. Layers of this iron ore occurring in Scotch peat bogs

are often called "the pan," and are sometimes of economical value.

 

It is clear much time must have been required for the accumulation of strata to

which countless generations of Diatomaceae have contributed their remains; and

these discoveries lead us naturally to suspect that other deposits, of which the

materials have been supposed to be inorganic, may in reality be composed chiefly

of microscopic organic bodies. That this is the case with the white chalk, has

often been imagined, and is now proved to be the fact. It has, moreover, been

lately discovered that the chambers into which these Foraminifera are divided

are actually often filled with thousands of well-preserved organic bodies, which

abound in every minute grain of chalk, and are especially apparent in the white

coating of flints, often accompanied by innumerable needle-shaped spiculae of

sponges (see Chapter 17.).

 

"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 shells 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 latitudes.

 

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 shells, together with the remains of terrestrial quadrupeds. The

number of land-shells 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-shells (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-shells, 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-shells 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 shells, such as the

nautilus, nor microscopic Foraminifera in lacustrine or fluviatile deposits. In

distinguishing the latter from formations accumulated in the sea, we are chiefly

guided by the forms of the mollusca. In a fresh-water deposit, the number of

individual shells 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 anticipated from

the fact that the genera and species of recent fresh-water and land shells 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

shells, 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, Essex.)

 

(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 shells, 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 pass 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.)