It would carry us too far from our present purpose to draw out this

theory in all its details. But we cannot for-bear briefly to touch upon

some of the bold and startling conclusions to which it has led. The

Geologist having, by patient and varied exercise, in the regions of

existing Glaciers, trained his eye and his judgment in the observation

of those phenomena that mark the action of moving ice, soon begins to

discover that they are not wanting in other countries. They are not

to be found, indeed, beneath the burning sun of Africa, nor on the

borders of the Mediterranean Sea. But as he travels northward they

begin by degrees to appear; and when at length he reaches the shores

of the Baltic, they are spread out profusely before him as they were

in the bosom of the Alps. All this had puzzled Geologists for years;

but the clue has been found at last. What is going on to-day in

Switzerland, and in Greenland, and on the shores of Labrador, must have

been going on, ages ago, in Germany, and in Denmark, and on the shores

of the Baltic. We may argue from the effect to the cause. Here are the

moraines, the erratics, the perched blocks, and the surfaces of rock

furrowed and scratched with ice: at some past time there must have been

the moving Glaciers and the floating Icebergs.

Following out this line of argument, and applying it to countries

nearer home, Geologists have come to the conclusion that the Grampian

Hills in Scotland, the mountains of Kerry in Ireland, the Snowdonian

heights in Wales, and many other ranges of hills in these islands,

were in former times subjected to the action of moving ice. Nay, it

is contended, with much show of reason, that these islands must have

been, for a considerable time, in great part submerged beneath the

sea, and traversed by floating Icebergs. When large erratic blocks are

found in the immediate neighborhood of the formation from which they

have been derived, then it is easy to explain their origin and to trace

their course. But it often happens that the nearest rock of the same

mineral composition, and therefore, the nearest rock from which they

can possibly have been derived, is separated from the site which they

now occupy by a lofty chain of mountains. By what means, then, have

they been transported hither? Not by moving water, for their sharp

edges and projecting angles are still preserved. Not by Glaciers; for

a Glacier cannot climb a steep mountain ridge. It would seem, indeed,

that in the present geographical distribution of land and water, there

is no natural cause which could carry them from the parent rocks to

their present position. But if we suppose that in some long past age of

the world, Great Britain and Ireland were submerged beneath the sea,

and that Icebergs floated in the waters above, the problem is solved

at once. The fragments of far distant rocks frozen into the Icebergs

might then have been carried over the summits of what are now lofty

mountains, and as the ice melted away, might have been deposited all

along their slopes and even on their highest crests.

The presence of marine shells, belonging chiefly to species which now

exist only in the arctic seas, affords a strong confirmation of this

hypothesis. For they are found intimately associated with the erratic

blocks, not merely in valleys, to which the sea might be supposed

to have had access in times of extraordinary flood, but upon lofty

mountains at a height of five hundred, six hundred, and even thirteen

hundred feet above the level of the sea. There is no difficulty in

accounting for this phenomenon if we suppose the country to have been

at one time submerged, and the glacial drift in which the shells are

found embedded to have been deposited by Icebergs on the floor of the

ocean. If we refuse to make this supposition the difficulty is simply

insurmountable.[34]

* * * * *

But it is somewhat beside our purpose to wander so far into the region

of theory and speculation. Our main object in these chapters has been

to establish the fact that Denudation is actually taking place to an

almost incredible extent, in the present age of the world. For this

purpose we have enumerated the principal agents by which this process

is carried on; and we have endeavored to show from the authenticated

researches of travellers and scientific men that they have been at work

within the period of history, and are still at work around us. Our

summary is, indeed, brief; but it is still sufficient to demonstrate

that, even during the present age, the whole surface of the Globe has

been ever in a constant state of change; that mountain heights have

been worn away, and valleys have been scooped out, and lofty cliffs

have disappeared, and bold headlands have been rent in twain, and rocks

and earths have day by day been broken up and dissolved and decomposed,

by the never ceasing operation of natural causes; and that the broken

fragments are at every moment moving along over the surface of the land

or through the depths of the sea.

Now Geologists tell us that these are the raw materials of a new

building which is going on in these latter times under the guiding hand

of Nature. Indeed, they say it is not so much a new building as the

uppermost story of an old building. If we descend into the Crust of

the Earth we may trace this building even from the foundations, which

are laid upon the solid granite, up through each successive stage of

limestone, and sandstone, slate, conglomerate, and clay, until we come

to the surface, where new strata, composed of the same elements, and

exhibiting the same general characteristics, are slowly growing up

before our eyes. Thus will the idea gradually steal upon the mind, that

the works of ages long gone by are reproduced once again in our own

days, and that we may study the history of the past in the mirror of

the present which nature holds up to our view.

This is the branch of Geological argument upon which we are now about

to enter. We have visited Nature, as it were, in her quarry, and we

have seen how she collects her materials, how she fashions them to

her purpose, how she transports them to the place for which they are

designed. If it be true, as alleged, that with these materials she is

actually engaged, at the present moment, in building upon the existing

surface of our Globe a new series of stratified rocks, which are the

exact counterpart of those beneath, this fact affords at least a very

strong presumption in favor of one very important principle in the

theory of Geologists. Let us, then, follow the course of her operations

and judge for ourselves.

[Illustration]

[Illustration]

_CHAPTER V._

STRATIFIED ROCKS OF MECHANICAL ORIGIN--THEORY DEVELOPED AND ILLUSTRATED.

_Formation of stratified rocks ascribed to the agency of

natural causes--This theory supported by facts--The argument

stated--Examples of mechanical rocks--Materials of which they

are composed--Origin and history of these materials traced

out--Process of deposition--Process of consolidation--Instances

of consolidation by pressure--Consolidation perfected by

natural cements--Curious illustrations--Consolidation of

sandstone in Cornwall--Arrangement of strata explained by

intermittent action of the agents of Denudation._

The Stratification of Rocks is one of the most remarkable features

which the Crust of the Earth presents to our notice; and the principles

by which this phenomenon is explained belong to the very foundation of

Geological theory. It is now universally agreed that the successive

layers or strata, which constitute such a very large proportion of the

Earth’s Crust, and which cannot fail to attract the notice even of the

most careless observer, have been slowly built up during a long series

of ages by the action of natural causes. In support of this bold and

comprehensive theory, geologists appeal to the operations which are

going on in nature at the present day, or which have been observed and

recorded within historic times. There is a vast machinery, they say,

even now at work all over the world, breaking up the rocks that appear

at the surface of the Earth, transporting the materials to different

sites, and there constructing new strata, just the counterpart of those

which we see piled up one above the other, wherever a section of the

Earth’s Crust is exposed to view. It is given to us, therefore, on the

one hand to contemplate the finished work as it exists in the Crust

of the Earth, and on the other, to examine the work still in progress

upon its surface; and if both are found to agree in all their most

remarkable characteristics, it is not unreasonable to infer that the

one was produced in bygone ages by the very same causes that are now

busy in the production of the other.

In the examination of this argument we first turned our attention to

the numerous and powerful agents that are now employed in the breaking

up and transporting of existing rocks. It was impossible within our

narrow limits to enumerate them all. But we selected those which are

at the same time the most familiar in their operations, and the most

striking in their results:--mighty rivers discharging daily and hourly

into the sea the accumulated spoils of vast continents; the breakers

of the ocean dashing with unceasing energy against all the cliffs and

coasts of the world; the tides and currents of the sea taking up the

ruins which the breakers have made, and carrying them far away to the

lonely depths of the ocean; the frozen rain bursting massive rocks

asunder with its expansive force, and sending the fragments over lofty

cliffs and steep precipices to become the prey of roaring mountain

torrents, or perhaps, more fortunate, to find a place of tranquil rest

on the bosom of the glittering Glacier; then this wondrous Glacier

itself, a moving sea of ice, bearing along its ponderous burden from

the summits of lofty mountains far down into the smiling plains, and

meanwhile, with tremendous power, grinding, and furrowing, and wearing

away the floor of the valley, and leaving behind it an impress which

even time cannot efface; and lastly, the massive Icebergs which stud

the northern and southern seas, drifting along like floating islands

above the fathomless abysses of the ocean, and scattering their huge

boulders over the surface of submarine mountains and valleys.

All these phenomena have been learned from actual and repeated

observation. They are not philosophical speculations, but ascertained

facts. We cannot doubt, therefore, that the work of demolition is going

on; it remains for us now to inquire about the work of reconstruction.

The reader will remember that Geologists divide the stratified rocks

into three distinct classes, Mechanical, Chemical, and Organic. This

distinction, they say, is founded on the actual operations of Nature.

From a close examination of the natural agents now at work in the

world, it appears that some strata are being formed chiefly by the

action of mechanical force; others chiefly by the influence of chemical

laws; and others again chiefly by the intervention of organic life.

Thus we have three distinct classes of rock at present coming into

existence, each exhibiting its own peculiar characteristics, and each,

moreover, having its counterpart among the strata that compose the

Crust of the Earth. We shall now proceed to set forth some of the

evidence that may be advanced in favor of these important conclusions,

beginning with those rocks that are called Mechanical.

And first it is important to have, at least, a general idea of the

appearance which Mechanical Rocks present to the eye. We shall

take three familiar examples, Conglomerate, Sandstone, and Clay.

Conglomerate, or Pudding-stone as it is sometimes called, is composed

of pebbles, gravel, and sand, more or less compacted together, and

generally forming a hard and solid mass. The various materials of

which it is composed, though united in the one rock, nevertheless

remain their own external forms, and may be distinctly recognized even

by the unpractised eye. Sandstone, as the name implies, is made up of

grains of sand closely compressed and cemented together. The quality

and appearance of this rock vary very much according to the size and

character of its constituent particles. Often the grains of sand are

as large as peas, or even larger; sometimes they are so minute that

they cannot be distinguished without the aid of a lens. For the most

part they consist of quartz, with grains of limestone intermixed; and

they are usually rounded, as if by the action of running water. Clay

is a rather vague and general term, now commonly employed to denote

any finely-divided mineral matter which contains from ten to thirty

per cent. of Alumina, and is thereby rendered plastic, and capable,

when softened with water, of being moulded like paste with the hand.

It occurs in many different forms among the strata of the Earth,

according to the different minerals that enter into its composition

and the different influences to which it has been subjected. Marl and

Loam may be taken as well-known illustrations: the former is a clay in

which there is a large proportion of calcareous matter; the latter is

a mixture of clay and sand. Sometimes by pressure clay is condensed

into a kind of slaty rock called Shale, which has the property of being

easily split up into an immense number of thin plates or laminæ.

It should be remembered that there is not always a perfect uniformity

in the structure of these rocks. In Conglomerate, for example, the

pebbles may be as large as cannon balls, or they may be only the size

of walnuts. So, too, we have every variety of fineness and coarseness

in the quality of Sandstone. Again, both Conglomerate and Sandstone

are often largely adulterated with clay, and on the other hand, clay

will sometimes contain more than its usual proportion of sand or lime.

Lastly, these materials are in one place compacted into hard and solid

rock, in another they are found in a loose and incoherent condition.

But amidst all these varieties of form and texture, the rocks we have

been describing generally preserve their peculiar characteristics,

and with a little experience can be easily recognized. They are found

to constitute a very large part, perhaps we might say the larger

part, of the stratified rocks in every country that has hitherto been

explored by Geologists. Wherever we go we are met by the same familiar

appearances;--beds of Conglomerate, Sandstone, Clay, Marl, Shale,

recurring again and again through a series of many hundred strata,

sometimes in one order, and sometimes in another; sometimes without any

formation of a different kind intervening, and sometimes alternating

with limestone or other rocks of which we shall speak hereafter.

Such is the general character and appearance of those strata which are

known among Geologists as Aqueous Rocks of Mechanical origin. Now,

it must at once strike the reader, that these rocks are made up of

just those very materials--the same both in kind and in form--that we

have already shown to be daily prepared and fashioned by a vast and

complex machinery in the great workshop of Nature. He will remember

how enormous blocks are detached from the mountain side, or from the

cliffs on the seashore, and broken up into fragments; how the fragments

in time become pebbles, sand, and mud; and how these are caught up by

rivers, tides, and currents, and carried far away to sea. Here we have

certainly all the materials that are necessary for the building up of

Conglomerate, Sandstone, and Shale. We have seen how they are prepared

by the hand of Nature, how they are moulded into shape, how they are

transported from place to place. Let us now pursue the sequel of their

history, and follow them on to the end.

It is plain they cannot remain forever suspended in water; sooner

or later they must fall to the bottom. Yet they will not all fall

together. For though all are carried downward by the one force of

gravity, those materials that are smaller and lighter will be more

impeded by the resistance of the water. The pebbles and coarse gravel

will be the first to reach the bottom, then the sand, and last of all

the fine, impalpable mud. Thus, as the current sweeps along in its

course, the sediment which it bears away from the land will be in a

manner sorted, and three distinct layers of different materials will

be deposited in the bed of the ocean;--first, nearest to the shore,

a layer of pebbles and coarse gravel, then a layer of sand, and last

of all a layer of fine mud or clay. This is the first step in the

construction of stratified rock. To complete the work nothing more

is necessary than the consolidation of these loose and incoherent

materials. If this could be accomplished, then we should have a solid

stratum of Conglomerate, a solid stratum of Sandstone, and a solid

stratum of Shale formed in the bed of the ocean.

With regard to this operation, however, we cannot hope for the

advantage we have hitherto enjoyed, of actual observation. The process

of consolidation, if it take place at all, is going on in the depths of

the Sea. But though it is thus removed beyond the reach of our senses,

it is not beyond the reach of our intelligence. We may borrow the torch

of Science, and search even into the hidden recesses of Nature’s secret

laboratory.

In the first place, a partial consolidation of clay and sand, and

even of gravel, may take place under the influence of pressure alone.

Many of us are familiar with this truth, but few, perhaps, are aware

how extensively it is illustrated in the practical arts of life. Here

are some curious and interesting examples. The minute fragments of

coal which are produced by the friction of larger blocks against one

another, and which may be obtained abundantly in the neighborhood of

every coal mine, are now manufactured into a solid patent fuel by the

simple process of forcible compression. Again, the dust and rubble

of black lead, formerly cast aside as useless, are now carefully

collected, and by no other force than pressure are converted into a

solid mass, fit to be employed in the manufacture of lead-pencils. “The

graphite or black lead of commerce,” says Sir Charles Lyell, “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 as 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 a thousand 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.”[35] An instance yet more to our

purpose occurs in the experiments made to try the force of gunpowder.

Leathern bags filled with sand are put into the mortar that is to

receive the cannon-ball at a distance of fifty feet from the mouth of

the gun; and the sand is often compressed by the percussion of the

ball into a solid mass of Sandstone.[36] Now the deposits of which we

are speaking cannot fail to be subjected to a very powerful and a very

constant compressing force. For, since the process of deposition is

always going on, the matter which is deposited to-day will to-morrow be

covered with a new layer, and in the course of ages it may lie beneath

an immense pile of mineral matter, hundreds or even thousands of feet

in thickness.

But in fact there is another and more important agent at work. When

the harder and more compact blocks of Conglomerate and Sandstone are

subjected to a close analysis in the laboratory of the chemist, it is

found that they are strongly cemented together, sometimes by a solution

of lime filling up the interstices between the grains or pebbles,

sometimes by a solution of silica, sometimes by a solution of iron. Now

this discovery affords a useful clue when we come to study the present

operations of Nature. It is to the agency of a mineral cement we must

look for the perfect consolidation of Mechanical Rocks. Let us see if

such a cement can be found.

It is well known that the water of rivers, lakes, and springs, is

more or less charged with carbonic acid gas; and therefore, when it

comes in contact with limestone, it dissolves a portion of the lime

and holds it in solution. Hence it follows that in every part of the

world there exists an abundant store of calcareous cement. Again, our

readers must have observed the brownish, rusty color sometimes produced

by streams on the surface of rocks and herbage. This is the result of

the iron with which the streams are impregnated: and we are informed by

scientific inquirers that water containing a solution of iron prevails

very generally in almost all countries. The solution of silica in water

is not so common; because pure silica cannot be dissolved by water

except at a very high temperature. Nevertheless, it has been clearly

demonstrated by observation, that silica, where it occurs in certain

combinations with other mineral substances, may be dissolved readily

enough: for instance, in the decomposition of felspar, and of all rocks

in which felspar is an ingredient, silica is carried off in a state of

solution.[37] And since these rocks are very numerous, and distributed

over every part of the earth, we may fairly conclude that a solution of

silica exists very abundantly in nature.

Now when we bear in mind that we have on the one hand in the Crust of

the Earth, solid strata of Conglomerate and Sandstone, exhibiting the

evident operation of these mineral cements; and on the other hand, near

the surface, the loose materials of Conglomerate and Sandstone as if

ready to be cemented, and close at hand the cementing mineral itself in

a convenient form, it is not unreasonable to assume that the process

should actually take place;--that water highly charged with iron, or

lime, or silica, should filter through the loose gravel and sand,

depositing its mineral cement as it passes along, and converting the

newly-formed strata into compact and solid rock.

But this conclusion does not rest upon antecedent probability alone.

We have proof unquestionable that a process such as we have described

is actually going on. In the dredging of the river Thames large masses

of solid Conglomerate are found from time to time, firmly compacted

together by a ferruginous cement. And there is internal evidence that

the process of solidification has been effected by natural causes

within historic times; for it happens not unfrequently that Roman coins

and fragments of pottery are found embedded in the solid block of

stone. Similar discoveries were made in deepening the bed of the river

Dove in Derbyshire, about the year 1832. Thousands of silver coins were

found about ten feet under the surface, firmly cemented into a hard

Conglomerate. Several of these coins bear dates of the thirteenth and

fourteenth centuries; and therefore the pebbles which form the rock

must have been deposited and converted into a solid mass since that

time. But we must not suppose that so long an interval is necessary for

the consolidation of rocks. In the early part of the present century

a vessel called the Thetis was wrecked off cape Frio on the coast of

Brazil. A few months afterward, when an attempt was successfully made

to recover the dollars and other treasures which had gone to the

bottom with the wreck, they were found completely enveloped in solid

masses of quartzose Sandstone. The materials of the newly-formed stone

were in this case manifestly derived from the granite rocks of the

Brazilian coast.[38]

In many parts of the Mediterranean, and along its shores, this process

is known to be going on with equal rapidity. “The new-formed strata

of Asia Minor,” writes Sir Charles Lyell, “consists of stone, not of

loose, incoherent materials. Almost all the streamlets and rivers,

like many of those in Tuscany and the south of Italy, hold abundance

of carbonate of lime in solution, and precipitate Travertine, or

sometimes bind together the sand and gravel into solid Sandstones

and Conglomerates; every delta and sandbar thus acquires solidity,

which often prevents streams from forcing their way through them, so

that their mouths are constantly changing their position.”[39] In the

Museum at Montpelier is exhibited a cannon embedded in a crystalline

calcareous rock which was taken up from the bed of the Mediterranean

near the mouth of the Rhone.[40]

To these examples of the solidification of rock within recent times

we are tempted to add one more, taken from a Memoir published by the

late Dr. Paris in the Transactions of the Royal Geological Society of

Cornwall. “A sandstone occurs in various parts of the northern coast

of Cornwall, which affords a most instructive example of a recent

formation, since we here actually detect Nature at work in converting

loose sand into solid rock. A very considerable portion of the northern

coast of Cornwall is covered with calcareous sand, consisting of minute

particles of comminuted shells, which in some places has accumulated

in quantities so great, as to have formed hills of from forty to

fifty feet in elevation. In digging into these sand-hills, or upon

the occasional removal of some part of them by the winds, the remains

of houses may be seen; and in places where the churchyards have been

overwhelmed, a great number of human bones may be found. The sand is

supposed to have been originally brought from the sea by hurricanes,

probably at a remote period. It first appears in a state of slight but

increasing aggregation on several parts of the shore in the Bay of St.

Ives; but on approaching the Gwythian River it becomes more extensive

and indurated.... It is around the promontory of New Kaye that the

most extensive formation of Sandstone takes place. Here it may be seen

in different stages of induration, from a state in which it is too

friable to be detached from the rock on which it reposes, to a hardness

so considerable that it requires a very violent blow from a sledge to

break it. Buildings are constructed of it; the church of Cranstock is

entirely built with it; and it is also employed for various articles of

domestic and agricultural uses.”

No reasonable doubt can therefore remain that the loose beds of gravel,

sand, and clay, which, as we have already seen, are deposited from day

to day, and from year to year, and from century to century, beneath

the waters of the ocean, may be converted in the course of time by

natural agents into solid rocks of Conglomerate, of Sandstone, and of

Shale. But this is not enough. It yet remains for us to explain how

these solid rocks come to be arranged in a series of distinct layers

or strata. The reader will remember that the supply of materials in

any given area of the ocean is not fixed and continuous, but, on the

contrary, variable and intermittent. During the periodical rains within

the tropics, and during the melting of the snows in high latitudes or

in mountain regions, the rivers become enormously swollen, and carry

down a far greater quantity of sediment than at other seasons. The

waste of cliffs, too, by the action of the waves, is much greater in

winter than in summer. Thus, while at one season a particular river

or current may be comparatively free from sediment, at another it

will carry along in its turbid course an almost incredible freight of

mineral matter. We have a notable example in the case of the Ganges.

The bulk of earthy matter which this river discharges into the sea

during the four months of rain, averages about 50,000,000 of cubic feet

per day; whereas the daily discharge during the three months of hot

weather is considerably less than one hundredth part of that amount.[41]

Besides this variety in the quantity of materials carried, there is

also a great variety in the velocity both of rivers and of currents;

and therefore they will not always carry the same materials to the same

distance; for the less rapid the stream, the sooner will the sediment

fall to the bottom. We may add that currents, as is well known, often

change their direction from various causes, and thus at different times

they will carry the waste of the land to different parts of the ocean.

From these considerations two conclusions may be fairly deduced:

First, that the process of deposition may often go on very rapidly

for a time over a given area, and then altogether cease, and after an

interval begin again. In this way time may be allowed for one deposit

to acquire more or less consistency before the next is superimposed;

and thus a succession of distinct beds will be produced. Secondly, we

may infer that the same precise materials will not always be deposited

over the same area; at one time it will be sand, at another gravel,

at another clay, at another some combination of these or other

mineral substances. And thus it may happen that the strata deposited

in successive periods of time shall not only be distinct one from the

other, but composed of different materials;--that there shall be,

in fact, as we so often see that there are, beds of Conglomerate,

Sandstone, Clay, Marl, and other rocks, succeeding one another in every

variety of order.

[Illustration]

[Illustration]

_CHAPTER VI._

STRATIFIED ROCKS OF MECHANICAL ORIGIN--FURTHER ILLUSTRATIONS.

_Impossible to witness the formation of stratified rocks in the

depths of the ocean--On a small scale examples are exhibited

by rivers and lakes--Alluvial plains--Their extraordinary

fertility--Great basin of the Nile--Experiments of the Royal

Society--The Mississippi and the Orinoco--Some rivers fill

up their own channels--Case of the river Po--Artificial

embankments--Large tract of alluvial soil deposited by the

Rhone in the Lake of Geneva--Deltas--The delta of the Ganges

and Brahmapootra--Delta of the Nile._

The argument set forth in the last chapter is simple, ingenious,

and persuasive. Nay, we must fairly confess that to us it seems

conclusive. We do not mean to say that it amounts to a rigorous

demonstration. But it affords at least a strong presumption that the

process of deposition, the process of consolidation, and the process of

stratification, are going on to a vast extent beneath the waters of the

ocean; and that, in these latter ages of the world’s history, Aqueous

Rocks are slowly growing up under the influence of natural causes,

which resemble in every important feature those that are now attracting

so much attention within the Crust of the Earth. We are therefore

prepared to accept this conclusion, if it be not found at variance with

any well-established fact, or with any known and certain truth. But in

matters of physical science the evidence of our senses is, after all,

the most satisfactory argument. And our readers, no doubt, would like

to witness, if possible, with their eyes, the building up of Stratified

Rocks. Now, though it is not given to us to see this process in all its

colossal magnitude as it goes on within the depths of the mighty ocean,

it is yet possible to behold it exhibited, as it were, in miniature, in

certain cases where the sediment of rivers is deposited within reach of

observation.

Every one is familiar with the fact that many rivers overflow their

banks at certain seasons, and spread themselves out over a wide area,

sometimes reaching to the foot of the hills that bound the valleys

through which they flow. This is the origin of those Alluvial Plains

so remarkable for their surpassing richness and fertility. In each

successive year a thin film of sediment is deposited on the surface

of the land; and thus in the course of ages a soil is formed capable

of producing, season after season, the most luxuriant crops without

manifesting any symptoms of exhaustion. The soil of the Alluvial Plain

near St. Louis, on the Mississippi, is thus spoken of by a modern

traveller: “As to the quality of the land, any given number of crops

might be grown off it. Corn has been raised on it for a hundred years

together--as far back as the settlement is known. To inquire about the

system of farming in the West is not productive of information which

would be of service on the continent of Europe. There is no system: the

farmer scratches the ground and throws in the seed, and his bountiful

harvests come up year after year without further thought or trouble.

Thousands of centuries have made the soil for him, and it defies him to

make too heavy demands upon it. It gives him all he asks, and is never

known to disappoint or fail.”[42]

The great basin of the Nile offers an admirable example of an

Alluvial Plain on a scale of considerable magnitude. Even in the days

of Herodotus, Egypt was regarded as the “gift of the Nile:” and the

correctness of this opinion has been placed beyond all reasonable

doubt by the investigations of modern science. The river bears along

in its current, especially during the flood season, a large quantity

of fine earthy sediment obtained by the process of Denudation from the

mountains of central Africa. Once a year, between the months of July

and November, it overflows its banks, and this sediment is deposited on

the adjoining plains. Thus a new layer of rich soil is spread out every

year over the existing surface; and the whole country is, in a manner,

growing upward at the average rate, according to a rough estimate, of

about six inches in the century. Near Cairo, where excavations have

been made, the successive layers of annual deposit are distinctly

visible to the eye. And it is worthy of remark that, although each one

of these is no thicker than a sheet of paste-board, the stratum of

alluvial soil which overlies the sands of the desert, and which to all

appearance has come into existence by the very same process, is often

forty, fifty, and even sixty feet in depth.

A series of interesting observations and experiments have been recently

made under the auspices of the Royal Society, which afford some useful

information on this subject. The colossal statue of Rameses, near

Memphis, was found to be partly embedded in a stratum of mud which

had gradually accumulated around it. Upon sinking a shaft, it was

discovered that from the present surface of the plain to the base of

the pedestal is a distance of nearly ten feet. Now, Rameses flourished,

according to Lepsius, about one thousand three hundred and sixty years

before the Christian Era; and therefore, since that time, or within a

space of 3200 years, it is pretty clear that a thickness of ten feet

has been added at this spot to the Alluvial Plain of the Nile. It is

hard to resist the conclusion that the next stratum of ten feet as we

proceed downward, which, in every respect, resembles the first, must

have been produced in the same way by natural causes; and so on till we

reach the barren sand of the desert, which is here just forty-two feet

below the present level of the plain.[43]

It should seem, therefore, that Egypt is nothing more than a great

Alluvial Plain, slowly built up in the long lapse of ages, by the

annual inundations of the Nile. Vast tracts of the same kind are to

be found in other parts of the world. The Mississippi, which drains

about one-seventh of the whole North American continent, has formed an

Alluvial Plain more than a thousand miles in length, and from thirty

to eighty in breadth. And in South America, the Orinoco once a year

spreads out its swollen and turbid waters over an area not unfrequently

seventy miles broad; leaving behind, when it subsides, a substantial

layer of muddy sediment to enrich the soil.[44] It would be easy to

accumulate examples. But we shall be content with having referred

the reader to the Great Basin of the Nile, which affords special

opportunities for the study of alluvial phenomena; being illustrated at

once by the historical monuments of remote antiquity and the scientific

researches of recent times.

There is another process by which Alluvial Plains are formed. It often

happens that a river fills up the channel in which it has been moving

for years, and is forced to shift its course and seek a new passage

to the sea. In progress of time this channel is filled up like the

former and deserted, and then a third, and then a fourth. At each

change a new stratum is formed, almost always distinguished for its

extraordinary fertility. This phenomenon is chiefly to be looked for

when an extensive and almost level plain lies between some lofty range

of mountains and the sea. In such a case, the river which bears away

the waste of the mountains, will move onward in its course with a

sluggish current, and will, of necessity, deposit the greater part of

its burden on the way. There is scarcely a country in the world that

does not abound in formations of this kind; and we could point to many

notable instances in which herds of cattle are now grazing on the very

spot where, within quite recent times, the turbid waters of some great

stream flowed sullenly along.

The river Po, which receives through a thousand mountain torrents

an enormous quantity of mineral sediment from the Alps, affords an

instructive example. Since the beginning of the fifteenth century it

has many times changed its course, often committing great devastations,

and always leaving behind unmistakable traces of its movements. Several

towns that once stood on the left bank of the river are now on the

right. In some instances parish churches and religious houses were

pulled down when the devouring stream was seen slowly to approach,

and then rebuilt with the same materials at a greater distance. An

old channel may be easily recognized at the present day near Cremona,

which bears the name of Po Morto, and another called Po Vecchio, in the

territory of Parma.

It may be interesting to our readers to learn that these movements have

been checked in modern times. By a system of artificial embankment the

waters of the river are now confined within definite and narrow limits:

thus the velocity of the current is increased and a very considerable

portion of the sediment is carried on to the sea. Nevertheless, much

is still deposited in the bed of the river, which is, in consequence,

raised higher and higher each successive year. Hence it has become

necessary, in order to prevent inundations, to add every season to

the height of the embankments, so that the river now presents the

appearance of an enormous aqueduct, of which some idea may be formed

from the fact that, in the neighborhood of Ferrara, the surface of

the stream is higher than the roofs of the houses. This system of

embankment is carried on very extensively in Northern Italy to check

the overflowing of rivers, and to prevent them from changing their

courses. It is as old as the time of Dante, who tells us that the

inhabitants of Padua erected barriers along the Brenta when the snows

began to melt and the season of the floods was approaching,

“Per difender lor ville e lor castelli,

Anzi che Chiarentana il caldo senta.”

_Inferno_, Canto xv.

As a river sometimes fills up its own channel, so too may it fill up

a lake through which it flows, and convert it likewise into a great

Alluvial Plain. Thus it is said several extensive lakes have been

transformed into dry land in modern times near Parma, Piacenza, and

Cremona. Elsewhere the process may be seen in actual operation. The

Rhone when it enters the lake of Geneva is a turbid discolored stream;

the natural consequence of the immense quantity of earthy sediment

with which it is charged. But as it slowly moves along, the sediment

falls to the bottom, and when, at length, “by Leman’s waters washed,”

it emerges at the town of Geneva, and shoots beneath the magnificent

bridge that joins the opposite shores, it has already assumed that

beautiful azure blue which travellers love to gaze on, and poets love

to sing. The sediment left behind goes to form a great alluvial tract

which is slowly but steadily advancing into the lake. An ancient town

called Port Vallais, which, eight centuries ago, stood at the water’s

edge, is now a mile and a half inland. And if the world were to last

long enough, and the natural agents at present in operation were to

remain unchanged, the time would come, we can scarcely doubt, when the

whole lake of Geneva would have been converted into an Alluvial Plain

of vast extent and inexhaustible fertility.

This last example leads us on to the phenomenon of Deltas, which

afford, perhaps, the best opportunity of observing the actual formation

of stratified rocks. Some large rivers, as we have already seen, enter

the sea with such extreme velocity as to bear away their sediment to a

distance of several hundred miles from the land. But in other cases the

onward rush of the stream is much sooner arrested, and the sediment,

if it be not caught up by ocean currents, is deposited near the mouth

of the river, and forms a triangular tract of alluvial land. This kind

of deposit is called a Delta, from the resemblance it bears to the

letter (Δ) of that name in the Greek Alphabet. The apex of the triangle

points up the stream, the base is toward the sea. Hence, when a Delta

is formed the river naturally divides into two branches, one flowing to

the right, the other to the left. In progress of time new channels are

almost always made, and the great stream empties itself into the sea by

many mouths.

The Delta formed in the Bay of Bengal by the two great rivers of

India, the Ganges and the Brahmapootra, offers an illustration of

this phenomenon on a scale of unusual magnitude. Indeed, strictly

speaking, it is not one Delta only, but rather two Deltas lying side

by side; the one deriving its origin from the Ganges, the other from

the Brahmapootra. This double Delta extends its base for two hundred

and fifty miles along the Bay of Bengal, and stretches inward into the

continent of India to an almost equal distance. Here, then, is a vast

tract of country manifestly composed of earthy sediment, obtained by

the process of Denudation from the Himalayan mountains, and afterward

transported to its present site by the agency of moving water. But

the deposition of earthy matter does not suddenly come to an end when

we reach the present line of the coast. The sea is visibly discolored

by the sediment far beyond the actual base of the Delta; and a sloping

bank of mud is found to stretch beneath the waters of the Bay to a

distance of a hundred miles.

Even within the short period of a man’s life the domain of dry land

is often visibly enlarged. Sandbanks are first formed in some of

those numerous winding channels through which the two rivers find

their way to the sea. The sandbanks, receiving fresh accessions

during each succeeding flood, in a short time become islands; and

the islands have been known, in a few years, to attain a superficial

extent of many square miles. Then begins to appear a wild and

luxuriant vegetation--reeds, long grass, shrubs, and trees; and those

impenetrable thickets are formed, to which the buffalo, the rhinoceros,

and the tiger soon resort for shelter. A very extensive tract of this

kind, adjoining the sea-coast, and known as the Sunderbunds, is said to

be as large as the principality of Wales.

The Delta of the Nile, though not quite one-half as large as the

Delta of the Ganges, presents nevertheless some features of peculiar

interest. In many places where a vertical section is exposed to view,

the phenomenon of stratification may be distinctly recognized. The

upper part of the deposit belonging to each year is composed of earth

of a lighter color than the lower part; and the whole forms a distinct

layer of hardened clay, which may be easily separated from those above

and below. This formation, therefore, corresponds exactly with those

strata of shale which we so often meet with in the Crust of the Earth.

Again, many of the old channels through which the Nile made its way

to the sea in ancient times, have been since filled up and converted

into solid land. The two extreme arms of the river, which formerly

enclosed the Delta, were two hundred miles apart where they entered

the Mediterranean. But these channels are now Alluvial Plains, and the

base of the Delta is but ninety miles in length. Hence, though the

quantity of land which has been formed by the sediment of the Nile is

much greater now than it formerly was, the size of the Delta properly

so called has not been increased but diminished.

If we turn to the great continent of America, we are met by results

not less striking and important. The Delta of the Mississippi is two

hundred miles in length, and one hundred and forty in breadth. This

vast stratum of mud is between five and six hundred feet thick, and

covers an area twelve thousand square miles in extent. Each year it

receives from the great _Father of Rivers_ a new accession of sediment

which is computed at 3,700,000,000 of cubic feet. And besides this

annual deposit of inorganic matter, we must not omit from our estimate

the countless trees of various species and of gigantic size, which

are torn up by the floods, carried along by the impetuous stream, and

buried at last with the bones of animals, and works of human art,

and other spoils of the land, in the mud of the Delta at the river’s

mouth.[45]

[Illustration]

[Illustration]

_CHAPTER VII._

STRATIFIED ROCKS OF CHEMICAL ORIGIN.

_Chemical agency employed in the formation of mechanical

rock--But some rocks produced almost exclusively by the

action of chemical laws--Difference between a mixture

and a solution--A saturated solution--Stalactites and

Stalagmites--Fantastic columns in limestone caverns--The grotto

of Antiparos in the Grecian Archipelago--Wyer’s cave in the

Blue Mountains of America--Travertine rock in Italy--Growth of

limestone in the Solfatara Lake near Tivoli--Incrustations of

the Anio--Formation of travertine at the baths of San Filippo

and San Vignone._

The Aqueous Rocks of which we have spoken in the last two chapters are

called by Geologists Mechanical; inasmuch as they owe their existence

chiefly to the agency of Mechanical force. It should be observed,

however, that a very considerable share in the production of these

rocks must be ascribed, not unfrequently, to Chemical influence.

Chemical action helps to prepare the materials of which they are

composed; and Chemical action likewise furnishes the calcareous,

siliceous, and other mineral cements by which they are, in a great

measure, consolidated. There is, however, a second class of Aqueous

Rocks which are produced almost exclusively by the operation of

Chemical laws, and which we have accordingly denominated Stratified

Rocks of Chemical Origin. It is of these that we purpose to speak in

the present chapter. They constitute a much smaller proportion of the

Earth’s Crust than either the Mechanical or the Organic Rocks. But the

history of their formation is curious and instructive. We shall confine

ourselves to one or two simple and familiar illustrations.

In the course of these illustrations we shall have a good deal to say

about Carbonate of Lime in a state of solution; and it may perhaps be

useful to explain, first of all, what is meant by a solution, in the

technical language of Chemistry. If a spoonful of salt is put into a

tumbler of water, the particles of salt, after a little time, cease to

cohere together, and become so diffused through the water as to be no

longer visible to the eye, although their presence in every part may be

easily discerned by the taste. The salt is then said to be _dissolved_,

and the water in which it is dissolved is called a _solution_ of salt.

It is important to distinguish the case of a solution from the case of

a mere mechanical mixture. If, instead of the salt, we were to put into

the tumbler of water a spoonful of very fine sand, then we should have

a _mixture_ but not a _solution_. By stirring briskly the contents of

the tumbler we might, indeed, effect a very close union between the

particles of water and the particles of sand: but this union would be

altogether different in kind from the union that was observed in the

former case between the particles of water and the particles of salt.

First, the sand would remain visible to the eye, making the water

turbid and discolored; whereas the salt entirely disappeared, leaving

the water limpid and transparent as before. Again, if the water be

allowed to rest, the sand will in time fall to the bottom, whereas the

salt will not.

But there is a limit to the capacity of water for holding salt in

solution. If spoonful after spoonful be added, it will be found, when a

certain point has been reached, that the water can at length dissolve

no more. It is then called a _saturated solution_ of salt. If, in

this case, a portion of the water were to pass away by evaporation,

it is clear, we should have the same quantity of salt as before, in

a smaller quantity of water. The consequence would be that _all_ the

salt could not then be held in solution, and some of it would fall to

the bottom; or, in chemical language, a precipitate of salt would be

formed on the bottom of the tumbler. Now, according to the theory of

Geologists, many rocks, hundreds of feet thick, and solid enough to

form the walls of our palaces, our churches, and our castles, have been

produced in the Crust of the Earth by just such a process as this.

In support of their theory we are about to show that the process is

actually going on in our own time, and is open to the examination of

all who may desire to study it for themselves.

We shall begin with the formation of Stalactites and Stalagmites. The

mode in which these singular masses of rock are brought into existence

is very clearly explained, and the picturesque appearance they so often

present to the eye is very graphically described, by Dr. Mantell,

in his Wonders of Geology, from which the following passages are

taken:--“One of the most common appearances in limestone caverns is the

formation of what are called Stalactites, from a Greek word signifying

distillation or dropping. Whenever water filters through a limestone

rock it dissolves a portion of it; and on reaching any opening, such as

a cavern, oozes from the sides or roof, and forms a drop, the moisture

of which is soon evaporated by the air, and a small circular plate or

ring of calcareous matter remains; another drop succeeds in the same

place, and adds, from the same cause, a fresh coat of incrustation. In

time, these successive additions produce a long, irregular, conical

projection from the roof, which is generally hollow, and is continually

being increased by the fresh accession of water, loaded with calcareous

or chalky matter: this is deposited on the outside of the Stalactite

already formed, and, trickling down, adds to its length by subsiding

to the point, and evaporating as before; precisely in the same manner

as, during frosty weather, icicles are formed on the edges of the eaves

of a roof. When the supply of water holding lime in solution is too

rapid to allow of its evaporation at the bottom of the Stalactite, it

drops on the floor of the cave, and drying up gradually, forms in like

manner a Stalactite rising upward from the ground, instead of hanging

from the roof; this is called for the sake of distinction Stalagmite.

“It frequently happens, where these processes are uninterrupted, that a

Stalactite hanging from the roof, and a Stalagmite formed immediately

under it from the super-abundant water, increase until they unite, and

thus constitute a natural pillar, apparently supporting the roof of the

grotto. It is to the grotesque forms assumed by Stalactites and these

natural columns, that caverns owe the interesting appearances described

in such glowing terms by those who witness them for the first time. One

of the most beautiful stalactitic caverns in England is at Clapham,

near Ingleborough. In the Cheddar Cliffs, Somersetshire, there has been

discovered a similar cave richly incrusted with sparry concretions.

There are others in Derbyshire.

“The grotto of Antiparos in the Grecian Archipelago, not far from

Paros, has long been celebrated. The sides and roof of its principal

cavity are covered with immense incrustations of calcareous spar, which

form either Stalactites depending from above or irregular pillars

rising from the floor. Several perfect columns reaching to the ceiling

have been formed and others are still in progress, by the union of the

Stalactite from above with the Stalagmite below. These, being composed

of matter slowly deposited, have assumed the most fantastic shapes;

while the pure, white, and glittering spar beautifully catches and

reflects the light of the torches of the visitors to this subterranean

palace, in a manner which causes all astonishment to cease at the

romantic tales told of the place--of its caves of diamonds and of its

ruby walls; the simple truth, when deprived of all exaggeration, being

sufficient to excite admiration and awe.

“Sometimes a linear fissure in the roof, by the direction it gives to

the dropping of the lapidifying water, forms a perfectly transparent

curtain or partition. A remarkable instance of this kind occurs in a

cavern in North America called Wyer’s Cave. This cave is situated in

a ridge of limestone hills running parallel to the Blue Mountains.

A narrow and rugged fissure leads to a large cavern, where the most

grotesque figures, formed by the percolation of water through beds of

limestone, present themselves, while the eye, glancing onward, watches

the dim and distant glimmers of the lights of the guides--some in the

recess below, and others in the galleries above. Passing from these

recesses, the passage conducts to a flight of steps that leads into

a large cavern of irregular form and of great beauty. Its dimensions

are about thirty feet by fifty. Here the incrustations hang just like

a sheet of water that was frozen as it fell; there they rise into a

beautiful stalactite pillar; and yonder compose an elevated seat,

surrounded by sparry pinnacles. Beyond this room is another more

irregular, but more beautiful; for besides having sparry ornaments in

common with the others, the roof overhead is of the most admirable and

singular formation. It is entirely covered with Stalactites, which

are suspended from it like inverted pinnacles; and they are of the

finest material, and most beautifully shaped and embossed. In another

apartment an immense sheet of transparent Stalactite, which extends

from the floor to the roof, emits, when struck, deep and mellow sounds

like those of a muffled drum.

“Farther on is another vaulted chamber, which is one hundred feet

long, thirty-six wide, and twenty-six high. Its walls are filled with

grotesque concretions. The effect of the lights placed by the guides

at various elevations, and leaving hidden more than they reveal, is

extremely fine. At the extremity of another range of apartments, a

magnificent hall, two hundred and fifty feet long, and thirty-three

feet high, suddenly appears. Here is a splendid sheet of rock-work

running up the centre of the room, and giving it the aspect of two

separate and noble galleries. This partition rises twenty feet above

the floor, and leaves the fine span of the arched roof untouched.

There is here a beautiful concretion, which has the form and drapery

of a gigantic statue; and the whole place is filled with stalagmitical

masses of the most varied and grotesque character. The fine perspective

of this room, four times the length of an ordinary church, and the

amazing vaulted roof spreading overhead, without any support of pillar

or column, produce a most striking effect. In another apartment, which

has an altitude of fifty feet, there is at one end an elevated recess

ornamented with a group of pendant Stalactites of unusual size and

singular beauty. They are as large as the pipes of a full-sized organ,

and ranged with great regularity: when struck they emit mellow sounds

of various keys, not unlike the tones of musical glasses. The length of

this extraordinary group of caverns is not less than one thousand six

hundred feet.”

In the case of Stalactites and Stalagmites the actual formation of

limestone by the influence of Chemical action is brought home forcibly

to the mind, and, in a manner, made palpable to the senses. We shall

now pass to other examples in which the process is scarcely less

open to observation, and in which the limestone assumes a somewhat

more massive and rock-like form. Every one who has been in Italy is

familiar with the limestone rock called Travertine. It is seen in the

ancient walls and the venerable temples of Pæstum, which have withstood

unharmed the wasting hand of time for upward of twenty centuries.

In Rome, too, this stone is associated in our minds as well with the

enduring monuments of antiquity, as with the imposing splendor of

Christian art. The Coliseum, the most stupendous of ruins, and St.

Peter’s, the most sublime of temples, are built of Travertine. In fact

it seems to have been, in every age, the chief building stone employed

in the architecture of the Eternal City; and the quarries from which

it was taken in ancient times may still be seen at Ponte Lucano, near

Tivoli. Now it is an interesting fact, that close to this very spot,

at the Solfatara lake on the one side, and at Tivoli itself on the

other, the formation of Travertine is going on in our own time, by the

precipitation of lime from a state of solution.

The Solfatara lake, situated about fourteen miles from Rome, on the

road to Tivoli, is supplied with an unfailing stream of tepid water,

impregnated with carbonic acid gas and saturated with carbonate of

lime. The amount of carbonate of lime which the water is capable of

holding in solution depends chiefly on three things: first, on the

presence of carbonic acid; secondly, on the high temperature of the

water; and thirdly, on its quantity. Now the carbonic acid is ever

rising in bubbles to the surface and passing away; the temperature

of the water is lowered by contact with the cooler atmosphere; and

its quantity is diminished by evaporation. Thus the capacity which

the water at first had for holding the carbonate of lime in solution

is notably diminished, and a part of the lime is precipitated to the

bottom in a solid form, or clings to the vegetable matter with which it

comes in contact.

A very simple and interesting experiment, made in the early part of

the present century by Sir Humphrey Davy, will illustrate the rapidity

with which the formation of solid stone is even now taking place. In

the month of May he fixed a stick in the bed of the lake, and left

it standing until the following April, when he found that it was

covered with an incrustation of limestone several inches thick.[46] In

precisely the same way new layers of Travertine are annually deposited

in the bed of the lake, and incrusted on its rocky margin; and so the

lake itself is becoming smaller and smaller from year to year. We are

told that in the middle of the seventeenth century it was a mile in

circuit, and now it is a little more than a quarter of a mile.[47]

Here, therefore, we have an immense mass of compact limestone rock,

built up by natural agents within the last two centuries.

At Tivoli, about four miles beyond the Solfatara, and two miles from

the quarries of Ponte Lucano, phenomena of the same kind are exhibited.

The waters of the Anio, which are saturated with carbonate of lime,

form incrustations of Travertine on the banks of the river; and at the

celebrated falls, where the whole volume of the stream leaps at a bound

from a height of three hundred and twenty feet, the most beautiful

stalactites are formed by the foam.

The formation of Travertine is going on with no less activity in

other parts of the Italian Peninsula. At the baths of San Filippo, in

Tuscany, there are three warm springs which contain a very large amount

of mineral matter in solution. The water which supplies the baths falls

into a pond, where it has been known to deposit a solid stratum of rock

_thirty feet thick_ in twenty years. In the same neighborhood are the

mineral baths of San Vignone. The source from which the water flows is

situated on the summit of a hill not more than a few hundred yards from

the high road between Sienna and Rome; and so rapid is the formation

of stone, that half a foot of solid Travertine is deposited every year

in the pipe that conducts the water to the baths. At this spot we have

a very good illustration of the argument we are now considering. As

the stream of water flows down the slopes of the hill, a thin layer

of Travertine rock is produced on the surface of the earth, almost

before our eyes; and so it was previous to our own time, and so it has

been for ages, as history and tradition testify. The quantity produced

in each year and in each century is comparatively small, but we can

have no doubt that it _has_ been produced by the means described.

Now, beneath the surface of the Earth, immediately below these modern

formations, of which we have so clearly ascertained the origin, we find

strata of the same kind, composed of the same materials, and arranged

in the same way, layer resting upon layer, down to a depth of two

hundred feet: and the Geologist accounts for the formation of the one

according to the same laws which he has seen at work in the production

of the other.[48]

[Illustration]

[Illustration]

_CHAPTER VIII._

STRATIFIED ROCKS OF ORGANIC ORIGIN--ILLUSTRATIONS FROM ANIMAL LIFE.

_Nature of organic rocks--Carbonate of lime extracted from the

sea by the intervention of minute animalcules--Chalk rock--Its

vast extent--Supposed to be of organic origin--A stratum of

the same kind now growing up on the floor of the Atlantic

ocean--Coral reefs and islands--Their general appearance--Their

geographical distribution--Their organic origin--Structure

of the zoophyte--Various illustrations--Agency of the

zoophyte in the construction of coral rock--How the sunken

reef is converted into an island and peopled with plants and

animals--Difficulty proposed and considered--Hypothesis of Mr.

Darwin--Coral limestone in the solid crust of the earth._

We now pass to the third division of Aqueous Rocks, those, namely,

which are believed to have come into existence chiefly through the

agency of animal and vegetable life, and are therefore called Organic.

The study of these rocks has been prosecuted with no inconsiderable

ardor during the last thirty years; and the facts which have been

brought to light are certainly amongst the most curious and interesting

in the whole range of physical science. Indeed we are convinced that

a simple narrative of the researches which have recently been made

upon this subject, and the discoveries to which these researches

have led, would be no less attractive, and scarcely less wonderful,

than a fairy tale. But it is not for us to wander at large over this

vast and tempting field of inquiry. We must be content with one or

two examples, which may help to illustrate the process of inductive

reasoning upon which the general principles of geological science are

founded.

It is argued, then, that the present operations of Nature afford

the best key for the interpretation of her works in bygone times.

We observe various beds of rocks now in course of formation on the

surface of the Earth; and within the Crust of the Earth we discover

corresponding strata of the self-same rock already complete, and laid

by, as it were, in Nature’s storehouse. Side by side, therefore, we

may study and compare the finished work and the work that is yet in

progress; and if, on a close examination, they are found to agree in

all essential characters, we have doubtless a strong presumption,

that the same causes which are now producing the one, must in former

times have produced the other. This line of argument we have already

considered in reference to those two classes of Aqueous Rocks, which

are said to be respectively of Mechanical and of Chemical origin. We

now proceed to show that it is no less applicable to those which are

called Organic. And although we may not hope to unfold all the secret

wonders of Nature’s laboratory, that have come to light in recent

times, yet we may afford a passing glimpse at her operations, which can

scarcely fail to be interesting and instructive.

We have shown how strata of solid rock are sometimes formed in lakes by

the precipitation of lime from a state of solution. Now this process

cannot take place in the sea; for though lime is present in the sea,

the quantity of carbonic acid with which it is there associated, is

far more than sufficient to render its precipitation impossible.[49]

But Nature has another contrivance for gathering together the solid

elements of her building. The depths of the ocean are teeming with

life; and countless tribes of minute animals are furnished with the

power of extracting the lime from the waters they inhabit, and of

reproducing it under a new form. Sometimes, through this mysterious

operation of organic life, the lime is converted into a calcareous

shell, like that of the oyster; sometimes into a stony skeleton, as

in the case of the numerous families of coral-producing animalcules.

After death the soft, fleshy substance of these animals melts away

and disappears; but the limestone shells and skeletons remain,

accumulating during the long course of ages to an almost incredible

extent. And, if we are to believe Geologists, out of these accumulated

materials, sometimes preserving their original form and structure,

sometimes altered more or less by chemical action, sometimes broken

up into fragments by mechanical force, has been produced a very large

proportion of the limestone rocks which occur so abundantly in the

Crust of the Earth.

No better illustration can be found than the white earthy limestone,

familiar to every one under the name of chalk. An undulating stratum

of Chalk Rock, attaining not unfrequently a thickness of one thousand

feet, may be said, speaking roughly, to underlie the southeastern half

of England. Sometimes it appears at the surface: sometimes it dips

downward, and forms a kind of great basin, over which are regularly

spread out various other groups of Stratified Rocks. On the southern

coast it rises to a height of several hundred feet above the level of

the sea in a line of perpendicular cliffs, conspicuous from a distance

by their dazzling whiteness. But the White Chalk of England is only an

insignificant part of a great rock-formation, which may be traced over

extensive areas throughout all Europe, from Ireland to the Crimea, from

the Baltic Sea to the Bay of Biscay; and which everywhere preserves in

a remarkable degree the same mineral character, and presents to the eye

the same general appearance.

Now it had often been suggested by Geologists that this wide-spread

formation derived its existence chiefly from the accumulated remains

of organic life. For in many instances the broken shells of minute

animalcules could be distinctly observed to constitute a part of the

rock. And even where the organic structure could not be so clearly

traced, the carbonate of lime composing the Chalk presented just

that appearance which would naturally result from the decomposition

of such shells. This theory, however, was long put forward with

diffidence and received with incredulity. Even scientific men found

it hard to persuade themselves that a solid rock of such great extent

and thickness could have been the work of agents apparently so

insignificant. But it has been confirmed and illustrated in a very

interesting and unexpected manner within the last few years.

When the project of connecting Europe and America by a telegraph

cable was first set on foot, it became necessary to ascertain, as far

as possible, the general configuration of the ocean bottom and the

exact nature of the bed on which the cable was to lie. Accordingly

in the year 1857 an expedition was fitted out for this purpose under

the command of Captain Dayman; and a careful series of soundings was

taken between Valentia, on the West Coast of Kerry, and Trinity Bay on

the shores of Newfoundland. It was found that the floor of the ocean

between Ireland and America is a vast irregular plain, and that by

far the greater part is covered over with a kind of soft mud or ooze.

Samples of this ooze were scooped up, even at the most profound depths,

by means of an ingenious apparatus attached to the sounding-lines, and

brought undisturbed to the surface. Afterward they were carried home to

England and submitted for examination to Professor Huxley. The result

has been to show that the materials of a limestone rock, resembling in

every essential feature the White Chalk of Europe, are being spread out

at the present day over an area of immense extent on the floor of the

Atlantic Ocean.

With the permission of our readers we shall allow Professor Huxley, as

far as may be, to tell his own story.[50] As to the ocean floor itself,

“It is,” he says, “a prodigious plain--one of the widest and most even

plains in the world. If the sea were drained off, you might drive a

wagon all the way from Valentia to Trinity Bay. And, except upon one

sharp incline about two hundred miles from Valentia, I am not quite

sure that it would even be necessary to put the skid on, so gentle are

the ascents and descents upon that long route. From Valentia the road

would lie down hill for about two hundred miles to the point at which

the bottom is now covered by 1700 fathoms of sea-water. Then would come

the central plain, more than a thousand miles wide, the inequalities

of the surface of which would be hardly perceptible, though the depth

of water upon it now varies from 10,000 to 15,000 feet; and there are

places in which Mont Blanc might be sunk without showing its peak

above water. Beyond this the ascent on the American side commences,

and gradually leads for about three hundred miles, to the Newfoundland

shore.”

The central plain here described, which has been since found to extend

many hundred miles north and south of the cable line, is covered almost

everywhere by that soft, mealy sort of mud of which we have already

spoken; and this, it is now confidently believed, is nothing else than

a stratum of Chalk Rock in an early stage of formation. When thoroughly

dried it assumes a whitish color, and exhibits a texture which even

to the superficial observer appears closely to resemble fine chalk.

Nay, we are told that if so disposed, one may take a bit of it in his

fingers and write with it upon a blackboard. Like chalk, too, when

chemically analyzed it is found to be almost pure carbonate of lime.

But there is a yet more striking analogy between the mud of the

Atlantic and the White Chalk of Europe. Both have been submitted to

the magnifying power of the Microscope; and, after an examination

conducted with scrupulous care, a wonderful and almost startling

identity of mineral, or rather we should say of organic, composition

has been established between them. To the naked eye Chalk is simply a

soft, earthy sort of stone. But when a thin transparent slice is placed

under the Microscope, the general mass is found to be made up of very

minute particles, in which are embedded a vast number of other bodies

possessing a well-defined form and structure. These are of various

sizes, but on a rough average may be said not to exceed a hundredth

of an inch in diameter. Hundreds of thousands of them are sometimes

contained in a cubic inch of Chalk, together with countless millions of

the more minute granules.

Professor Huxley succeeded in separating these bodies from the mass of

granules in which they were embedded, and by examining them apart, he

has ascertained still more fully their exact structure and composition.

“Each one of them,” he says, “is a beautifully constructed calcareous

fabric, made up of a number of chambers communicating freely with one

another. They are of various forms. One of the commonest is something

like a badly-grown raspberry, being formed of a number of nearly

globular chambers of different sizes congregated together. It is called

Globigerina; and some specimens of Chalk consist of little else than

Globigerinæ and granules.”

Previous to 1857 the Globigerinæ of the Chalk were a matter of no small

controversy among Geologists and Naturalists. Some contended that

they were the organic remains--the shells or skeletons--of ancient

animalcules. Others were disposed to regard them simply as aggregations

of lime, which, so to speak, chanced to assume the form of these

little chambered bodies; though it was not easy to explain, on this

hypothesis, how these chance concretions, however much they varied

in size, preserved over the whole of Europe the same exact form and

structure. But the controversy is now at an end. The specimens of the

Atlantic ooze brought home by Captain Dayman, when examined under the

higher powers of the Microscope, are found, like Chalk, to be composed

almost entirely of Globigerinæ. And that no doubt may remain as to

their organic origin, a portion of the fleshy integument of the little

animalcules is seen, in many cases, still adhering to the calcareous

skeleton.

“Globigerinæ of every size,” we are told, “from the smallest to the

largest, are associated together in the Atlantic mud, and the chambers

of many are filled by a soft animal matter. This soft substance is,

in fact, the remains of the creature to which the Globigerina shell,

or rather skeleton, owes its existence--and which is an animal of

the simplest imaginable description. It is, in fact, a mere particle

of living jelly, without defined parts of any kind--without a mouth,

nerves, muscles, or distinct organs; and only manifesting its vitality

to ordinary observation by thrusting out and retracting, from all

parts of its surface, long filamentous processes which serve for arms

and legs. Yet this amorphous particle, devoid of everything which, in

the higher animals we call organs, is capable of feeding, growing,

and multiplying; of separating from the ocean the small proportion of

carbonate of lime which is dissolved in sea-water; and of building up

that substance into a skeleton for itself, according to a pattern which

can be imitated by no other known agency.”

That the same process is going on in other parts of the ocean appears

by observations made by Sir Leopold M’Clintock during the cruise of

the Bulldog in 1860. He discovered that a calcareous ooze having the

consistency of putty is spread out over extensive areas between the

Faroe Islands and Iceland, and also between Iceland and Greenland. Of

this mud about ninety-five per cent. is composed of Globigerinæ, which

in some instances were brought up actually living to the surface, and

busily engaged in secreting, by their vital powers, carbonate of lime

from the waters of the sea.[51]

Professor Huxley goes yet one step further in following out the

resemblance between the Chalk Rock that exists in the Crust of the

Earth and the stratum of Chalk that is now growing up in the depths

of the Atlantic. Not only are the Globigerinæ, of which the one is in

great part composed, identical with the animalcules that make up about

nine-tenths of the other, but even the minute granules that constitute

the residue of each formation, correspond in a very remarkable manner.

“In working over the soundings collected by Captain Dayman, I was

surprised to find that many of what I have called the Granules of

that mud were not, as one might have been tempted to think at first,

the mere powder and waste of Globigerinæ, but they had a definite

form and size. I termed these bodies Coccoliths, and doubted their

organic nature. Doctor Wallich verified my observation, and added the

interesting discovery that, not unfrequently, bodies similar to these

Coccoliths were aggregated together into spheroids, which he termed

Coccospheres. So far as we knew, these bodies, the nature of which is

extremely puzzling and problematical, were peculiar to the Atlantic

soundings.

“But a few years ago Mr. Sorby, in making a careful examination of the

Chalk by means of thin sections and otherwise, observed, as Ehrenberg

had done before him, that much of its granular basis possesses a

definite form. Comparing these formed particles with those in the

Atlantic soundings, he found the two to be identical; and thus

proved that the Chalk, like the soundings, contains these mysterious

Coccoliths and Coccospheres. Here was a further and a most interesting

confirmation, from internal evidence, of the essential identity of the

Chalk with modern deep-sea mud.”

We may, therefore, set it down as certain, first, that the formation

of Chalk Rock is going on very extensively at the present day; and

secondly, that the chief agency employed in its production is no other

than the vital action of minute animalcules. This is no longer merely

a plausible theory or an ingenious hypothesis: it is simply a matter

of fact ascertained by direct observation. If then it is just and

philosophical to ascribe like effects to like causes, the conclusion is

plain that the White Chalk of Europe came into existence in some far

distant age by just such a process as that which is now in operation on

the bed of the Atlantic Ocean.

From the Chalk mud of the Atlantic we will now pass to the Coral Reefs

that are growing up beneath the waters of the Pacific and the Indian

Oceans. Every one has heard of Coral Reefs and Coral Islands; yet we

fancy many persons have but vague and indefinite notions about them.

We shall, therefore, in the first place, give a brief account of their

general appearance, their extent, and their geographical distribution.

Afterward we shall give some of the evidence which goes to show that

these huge masses of rock owe their existence to the organic powers of

minute living animalcules.

The Coral Reef is familiar to the navigator of tropical seas under a

great variety of forms, and in many different stages of development.

In one case it is a chain of hidden rocks rising not quite to the

level of the sea; in another it appears just above the waters, but is

washed over by each returning tide; while in another it rises up beyond

the reach of the waves, is clothed with luxuriant vegetation, and

inhabited by various species of animals, even by man himself. Again

there is great diversity of outline among these rocks, whether they are

sunk beneath the surface of the waters or lifted above them. But all

may be reduced to four classes, of which we propose to give a short

description.

First is the Atoll, or lagoon island. It is a circular strip of

limestone rock enclosing a shallow lake within, and surrounded by a

deep and often unfathomable ocean without. The scene presented by

some of these circular reefs is described by travellers as equally

striking for its singularity and its beauty. “A strip of land a few

hundred yards wide is covered by lofty cocoa-nut trees, above which

is the blue vault of heaven. This band of verdure is bounded by a

beach of glittering white sand, the outer margin of which is encircled

with a ring of snow-white breakers, beyond which are the dark heaving

waters of the ocean. The inner beach encloses the still clear water

of the lagoon, resting in its greater part on white sand, and, when

illuminated by a vertical sun, of a most vivid green.”

These lagoon islands are often found in groups stretching, with little

interruption, for many hundred miles across the ocean. The Maldives,

for example, which lie a little distance to the southwest of Hindostan,

form a continuous chain, running due north and south, four hundred and

seventy miles in length and fifty miles in breadth. Each successive

link in this chain does not consist, as might be supposed, of a single

circular reef, but it is rather a ring of small coral islets, sometimes

more than a hundred in number, each of which is itself a perfect Atoll

or lagoon island such as we have just described. Of these miniature

islets many are from three to five miles in diameter; while the larger

rings of which they form a part are from thirty to fifty. The Laccadive

islands, a little more to the north, exhibit a similar arrangement,

and indeed would seem to be a continuation of the same group. In the

Pacific are found some chains of coral islands yet more extensive;

as for instance the Dangerous Archipelago, which is upward of eleven

hundred miles in length, and from three to four hundred in breadth; but

the islands within these spaces are thinly scattered, and insignificant

in size.

Sometimes the annular strip of coral rock encloses within itself a

lofty island, which rises up from the centre of the lagoon. In this

case it is called an Encircling Reef; the lagoon being simply a broad

channel surrounding the island in the centre, and encompassed itself by

the coral rock. An example occurs in the island of Vanikoro, celebrated

for the shipwreck of La Peyrouse, where the Encircling Reef runs at a

distance of two or three miles from the shore, the channel between it

and the land having a general depth of between two and three hundred

feet. The well-known mountainous island of Tahiti in the South Pacific

Ocean is also encompassed by an Encircling Reef, from which it is

separated by a broad belt of tranquil water.

A third class of Coral Reefs consists of those which run parallel to

the shores of continents or great islands, from which they are cut off

by a broad channel, to which the sea has free access through certain

open passages in the rock. They are called Barrier Reefs; and differ

from the former only in this, that they do not surround the land, but

run parallel to it at a distance of some miles. The Great Barrier Reef

of Australia offers a noble example. It has been described as a huge,

massive, submarine wall or terrace, fronting the northeastern coast

of that continent, varying from ten to ninety miles in breadth, and

extending, with some trifling interruptions, to a length of 1250 miles.

Another reef of the same kind, 400 miles in length, faces the western

coast of the long narrow island of New Caledonia.

When a chain of Coral rocks approaches close to the shore, so as to

leave no intervening channel of deep water, they are called Fringing

Reefs; and these constitute the fourth and last class of the Coral

formation. They prevail everywhere in tropical regions, and appear as

banks of Coral encrusting the rocky shores of islands and continents.

As regards the geographical distribution of Coral Reefs, the first

circumstance that claims our notice, is that they are exclusively

confined to the warmer regions of the globe. They exist in great

profusion within the tropics, and are rarely to be found beyond the

thirtieth parallels of latitude on each side of the Equator. The only

remarkable exception is in the case of the Bermuda Islands in 32° north

latitude; but here, it is to be observed, the ocean is warmed by the

waters of the Gulf Stream. Another singular fact is the almost total

absence of Coral Reefs from the Atlantic Ocean. In fact, the Bermudas,

we believe, constitute here again the only exception. The Pacific,

on the contrary, is wonderfully productive of coral; also the Indian

Ocean, the Persian and Arabian Gulfs, and the Red Sea.

It may gratify, perhaps, the curiosity of some readers, if we add a

word on the Red Coral which is now so favorite an ornament in the

fashionable world. Though it never attains to the magnitude of those

reefs and islands we have been describing, it partakes nevertheless of

the same peculiar structure; and no doubt is entertained that, like

them, it derives its existence from animal life, in the manner we shall

presently explain. It is produced chiefly in the Mediterranean, in the

Red Sea, and in the Persian Gulf; and is brought up from the great

depths by means of a grappling apparatus attached to boats. The largest

pieces have a shrub-like branching form, and are supposed to grow to

the height of one foot in about eight years.[52]

So much for the existence of the Coral Formation. Next comes the

question of its origin, with which, of course, we are chiefly

concerned. It is now the received belief of all distinguished

Naturalists, that these huge and wide-spread masses of limestone rock,

against which the breakers of the ocean are ever thundering in vain,

are the work of tiny marine animalcules, and chiefly of those seemingly

insignificant creatures known by the name of Polyps or Zoophytes. The

Zoophyte, they tell us, is a mason who himself produces the stones

that he employs in his building. “He has neither plane, nor chisel,

nor trowel; there is no sound of hammer in his city. He erects mighty

and enduring edifices, yet has no mechanical power by which to raise

his rocks to their summits. He can answer thee nothing--no tongue, no

eyes, no hands, no brains has he--yet from the caves of old ocean has

he raised that which fills you with admiration.”[53] Surely if all this

be true, these countless myriads of animalcules call aloud to us from

the depths of the ocean in language that cannot be mistaken: “Know

ye that the Lord He is God; it is He that hath made us, and not we

ourselves.”[54]

The Zoophyte belongs to the simplest form of the animal creation. Its

body consists merely of a pouch or stomach, with tentacles arranged

round the margin, which it can extend at pleasure to supply itself with

food. In many species the individuals grow together on a common stem,

from which new members are constantly shooting forth like buds from

the branches of a tree. Hence the origin of the name Zoophyte, which

literally means a plant-like animal. The common stem on which they grow

is sometimes composed of a horny substance, but more generally it is

pure carbonate of lime, which they secrete by the powers of organic

action from the waters of the sea. It forms, therefore, a kind of

internal skeleton or framework, to which the soft, gelatinous parts of

the animal adhere, pretty much as, in the case of other animals, the

flesh adheres to the bones. Thus we have, as it were, a community of

living creatures, growing together upon one common stony framework,

called a Polypidom or Polyp edifice, which they themselves build by the

very fact of living.

[Illustration: Fig. 4.--Campanularia Gelatinosa.]

[Illustration: Fig. 5.--Gorgonia Patula.]

The peculiar structure of these wonderful little communities may

perhaps be made more intelligible by the aid of a few illustrations.

Figure 4 exhibits the branching skeleton and, at the extremities of

the branches, the several Polyps by whose vital action the skeleton

has been constructed. Some of the animalcules are shown in a state of

activity, with their tiny arms spread out in search of food: others

are withdrawn within their cells, and appear in a state of repose.

This species of Zoophyte, which is highly magnified in the figure,

flourishes abundantly on the shores of Ireland and England. It has

received the name of Campanularia, from the bell-like form of its

cells. Our next cut represents a Gorgonia from the Mediterranean,

which is also considerably magnified. The fleshy integument of this

specimen is of a brilliant red color: the Polyps are arranged in rows

on each side of the stem, and are shown in a state of expansion.

[Illustration: Fig. 6.--Frustra Pilosa.]

[Illustration: Fig. 7.--Madrepora Plantaginea.]

A mass of Coral animalcules, which are known by the name of Frustra

Pilosa, is represented of the natural size in Figure 6. To the

naked eye it seems like a piece of fine net-work, disposed around a

fragment of sea-weed, which may be observed protruding in the upper

part of our illustration. With the aid of an ordinary magnifier the

net-like surface is seen to abound in minute pores arranged with much

regularity. Each of these pores is the cell of a Zoophyte. And if

a fragment of Frustra be examined with a powerful microscope, when

immersed in sea-water, the curious little inhabitants themselves may be

seen darting in and out of their cells, expanding and contracting their

long feelers, and exhibiting altogether a wonderful activity. In the

adjoining woodcut, Figure 7, is shown another interesting species of

the arborescent Zoophyte. It belongs to the family of Madrepores, and

abounds in almost all Coral Reefs. Alive under water it appears clothed

in a gelatinous coating of rich and varied hues. But when removed from

its native element this gelatinous coating, which is the living animal

substance, quickly melts away; and, in some instances, runs off from

the calcareous skeleton in a kind of watery slime.

[Illustration: Fig. 8.--Corallium Rubrum.]

A good idea of the celebrated red and pink Coral of commerce, so much

admired for its brilliant color, and the high polish of which it is

susceptible, may be gathered from our next illustration. As in the

other species to which we have referred, the calcareous skeleton is

enveloped in a living gelatinous substance, from which the Zoophytes

seem to shoot out like buds from the bark of a tree. Several of these

animalcules are exhibited in our figure, in the active enjoyment of

life; gathering in, with their expanded tentacles, the elements of

their stony edifice from the surrounding waters. After death the fleshy

integument is wasted away by the action of the sea; and the framework

that remains behind, washed ashore by the waves, or hooked up by the

coral fisherman, is wrought into brooches, bracelets, necklaces, and

other ornaments of various kinds.

Not a few varieties of the Coral-producing Zoophytes are to be found

in actual living reality on our own coasts, where the curious student

may examine for himself their habits and general structure. But it

is in the warmer regions of the Earth that they are developed in the

greatest numbers, and decked in the brightest hues. Those who have seen

them through the crystal waters of tropical seas, swarming in countless

multitudes on the clear white sand below, speak with enthusiasm of

their luxuriant profusion and of their striking beauty. Combining to

a picturesque elegance of form a rich variety and pleasing harmony of

colors, they present to the eye a scene which has been compared to

a magnificent garden, laid out in diverse beds of rare and splendid

flowers.

So far we have spoken only of the Polypidom, that is to say, the

community of Polyps living together on a common stem of their own

construction. Now this Polypidom is the first element of the Coral

Reef. In some species of Zoophytes, the Red Coral for instance, the

calcareous stem never attains a size greater than that of a diminutive

shrub. But in others, and they are very numerous, especially in

tropical seas, there seems to be no limit to the growth of the solid

stony framework. As the existing generation of Zoophytes is dying out,

new individuals are ever budding forth, which continue unceasingly to

secrete carbonate of lime, as their predecessors had done before them,

from the waters of the ocean; and thus the tree-like form spreads its

branching arms on every side, growing upward and outward day by day.

The soft gelatinous parts of those generations that have passed away

are, in a short time, dissolved, and the stony skeleton alone remains

behind. Ages roll on: the calcareous framework, ever increasing in

size, becomes at length a formidable rock; and this rock is the Coral

Reef.

Let it not be supposed we are here advancing a theory: we are only

stating a fact that has been established by close and repeated

observations. All the phenomena exhibited in the development of the

Polypidom, are exhibited no less plainly in every Coral Reef that

has yet been examined. On the surface of the Reef are the living

Zoophytes, clinging to the calcareous skeleton which is ever growing

larger through the unconscious action of their vital functions; while

immediately beneath may be seen the same stony skeleton, already

divested of its fleshy integument, and beginning to assume the

appearance of compact and massive rock. We can behold, therefore, the

mason at work on the upper story of his building, and the structure

already finished below. And so we have little less than ocular

demonstration that the Coral Reef is the work of the Zoophyte.

It must not be supposed, however, that in every part of the Coral Reef,

the form and outline of the stony skeleton are exactly preserved.

Fragments of the rock are broken off by the force of the waves, and

mixed up with the comminuted shells of oysters, mussels, and other

crustaceous animals inhabiting the same waters. In this way a sort of

calcareous gravel, sometimes a calcareous paste, is formed, which fills

up the interstices, and connects the tree-like coral into a compact

rock.

We have yet to explain how the Coral Reefs come, in many cases, to

rise above the surface of the ocean, and to form dry land: for it has

been found that the reef-building Zoophytes require to be continually

immersed in salt water, and therefore, by their own efforts, they

cannot raise their structure above the ordinary level of the sea.

This question was for a long time involved in obscurity; but it has

been cleared up by the actual observations of Naturalists in modern

times. The following description, which is given to us by Chamisso,

the companion of Kotzebue on his voyages, will convey a good idea of

the process by which a sunken reef is often converted into a smiling,

fruitful island. “When the reef is of such a height that it remains

almost dry at low water, the corals leave off building. Above this

line a continuous mass of solid stone is seen, composed of the shells

of mollusks and echini, with their broken-off prickles and fragments

of coral, united by calcareous sand, produced by the pulverization of

shells. The heat of the sun often penetrates the mass of stone when it

is dry, so that it splits in many places, and the force of the waves is

thereby enabled to separate and lift blocks of coral, frequently six

feet long and three or four in thickness, and throw them upon the reef,

by which means the ridge becomes at length so high that it is covered

only during some seasons of the year by spring tides. After this the

calcareous sand lies undisturbed, and offers to the seeds of trees

and plants cast upon it by the waves, a soil upon which they rapidly

grow, to overshadow its dazzling white surface. Entire trunks of trees,

which are carried by the rivers from other countries and islands, find

here at length a resting place after their long wanderings: with these

come some small animals, such as insects and lizards, as the first

inhabitants. Even before the trees form a wood, the sea-birds nestle

here; stray land-birds take refuge in the bushes; and, at a much later

period, when the work has been long since completed, man appears and

builds his hut on the fruitful soil.”[55]

Another question that seems to call for some explanation is suggested

by the well-known habits of the Zoophytes themselves. From the

observations of Kotzebue and Darwin it appears that those species

which are most effective in the construction of Reefs cannot flourish

at a greater depth than twenty or thirty fathoms; whereas the coral

rocks rise up in many cases from the bottom of an unfathomable

ocean. How, then, it may be asked, have the foundations of these

wonderful structures been laid? This question opens a wide field for

philosophical speculation; and we freely admit that no theory of

Coral Reefs can be regarded as complete and satisfactory, which does

not furnish a reasonable answer. But so far as the purpose of our

argument is concerned, it is quite sufficient if a stratum of solid

limestone, twenty fathoms thick, has been formed mainly through the

agency of these minute animalcules. And this conclusion, so abundantly

demonstrated by facts, is left quite untouched by the difficulty to

which we now refer.

It will be interesting, however, to notice in passing the explanation

of this phenomenon first suggested by Mr. Darwin, and now very

generally accepted. He maintains that the whole Coral Reef--foundations

and superstructure alike--is, in most cases, the result entirely of

organic agency. The reef-building Zoophyte always begins his labors in

water that is comparatively shallow. But as he is building upward, it

often happens that the bed of the sea is sinking downward in pretty

nearly the same proportion; and thus the reef is ever increasing in

height from its original base, while the living mass of Zoophytes on

its upper surface remains in about the same depth of water as when the

building first began.

This theory is supported by a vast amount of curious and ingenious

reasoning. In the first place, there is nothing more remarkable in the

physical conformation of the Globe, than the immense predominance of

water over land throughout those extensive tracts of ocean where Coral

Reefs abound. Now this is just what we should naturally expect if the

hypothesis of Mr. Darwin were admitted; for wherever the Crust of the

Earth has been subsiding for many ages on a large scale, the domain of

the sea must of necessity have been considerably enlarged, and that

of the land contracted in proportion. Again, this hypothesis will be

found to harmonize most perfectly with all the phenomena of Fringing

Reefs, Barrier Reefs, Encircling Reefs, and Lagoon Islands. The

Fringing Reef represents, as it were, the first stage of progress. The

building operations have just commenced near the shore of some island

or continent, and but little space intervenes between the land and the

incrusting wall of coral. Then, as the Crust of the Earth gradually

subsides, the water encroaches on the land, and forms a channel between

it and the reef. Meanwhile the Zoophytes are at work, and the coral

rock is growing upward as the foundation on which it rests is sinking

downward: each year it is higher from the bed of the sea, and yet no

nearer to the surface of the waters. And when at length the channel,

which is ever growing wider and wider, has reached a certain limit, the

Fringing Reef becomes a Barrier Reef, or if it encompasses an island,

an Encircling Reef. Lastly, the Encircling Reef will finally become

a Lagoon Island, when the highest peaks of the land it encloses have

slowly disappeared beneath the surface of the waters.

In confirmation of this reasoning Mr. Darwin has pointed out numerous

examples to illustrate each intermediate stage through which, according

to his hypothesis, the Coral Reef must pass in the progress of its

construction. He traces the gradual transition from the low bank of

coral incrusting a rocky shore to the Encircling Reef that compasses

round a lofty island, like Tahiti, with a broad channel between.

Then he shows how this channel insensibly becomes wider and wider,

encroaching more and more upon the land, until at length only a few

high peaks remain above water. Finally he leads us on to the case of a

perfect Atoll, within which no trace of land remains to be seen; and

the channel, now become a lagoon, is encompassed by a Reef of Coral

Rock that rises steeply from an unfathomed ocean.

We do not mean to dwell upon this ingenious speculation, which would

carry us too far from the object at which we are aiming. It seems to

us, however, that the arguments in its favor are at least deserving of

careful consideration; and we may add that they receive new strength

from the facts we shall have occasion hereafter to bring forward, when

we come to speak of the undulating movements to which the Crust of the

Earth has been subject at many different times, and in many different

localities, even within the historic period.

The formation and structure of existing Coral Reefs being once fairly

established, Geologists have little difficulty in ascribing a similar

origin to many of the limestone strata that are found in the Crust

of the Earth. For though the internal texture has been considerably

modified in the long course of ages, by chemical and other influences,

nevertheless the stony skeletons of the reef-building Zoophytes can be

distinctly recognized in great abundance. Indeed it is not an uncommon

thing to meet with limestone rock exhibiting plainly to the eye all

the appearance of Coral Reefs lifted up from the bed of the ocean.

“The Oolite,” says Doctor Mantell, “abounds in corals, and contains

beds of limestone which are merely coral reefs that have undergone

no change but that of elevation from the bottom of the deep, and the

consolidation of their materials. The Coral-rag of Wilts presents in

fact all the characters of modern reefs: the polypifera belong chiefly

to the Astræidæ, the genera of which family principally contribute to

the formations now going on in the Pacific. Shells, echinoderms, teeth,

and bones of fishes, and other marine exuviæ, occupy the interstices

between the corals, and the whole is consolidated by sand and gravel,

held together in some instances by calcareous, in others by siliceous

infiltrations. Those who have visited districts where the Coral-rag

forms the immediate subsoil, and is exposed to view in the quarries

or in natural sections, must have been struck with the resemblance of

these rocks to modern coral banks.”[56]

Even in many of our finest marbles the coral skeletons may be traced

distinctly enough, and contribute not a little to that variegated color

which is so much admired. Nay, it is recorded by Mr. Parkinson that he

discovered in a piece of solid marble, the _animal membrane itself_ by

which the lime was originally abstracted from the sea. He immersed the

marble in dilute muriatic acid; and he relates with delight how, as

the calcareous earth dissolved, and the carbonic acid gas escaped, he

observed the animal tissue begin distinctly to appear in the form of

light, elastic membranes.[57]

[Illustration]

[Illustration]

CHAPTER IX.

STRATIFIED ROCKS OF ORGANIC ORIGIN--ILLUSTRATIONS FROM VEGETABLE LIFE.

_Origin of coal--Evident traces of plants and trees in

coal-mines--Coal made up of the same elements as wood--Beds

of coal found resting upon clay in which are preserved

the roots of trees--Insensible transition from wood to

coal--Forest-covered swamps--Accumulations of drift-wood in

lakes and estuaries--Peat bogs--Beds of Lignite--Seams of

pure coal with half-carbonized trees, some lying prostrate,

some standing erect--Summary of the argument hitherto

pursued--Objection to this argument from the Omnipotence of

God--Answer to the objection._

As animals, by organic action, extract lime from the waters of the

ocean they inhabit, which, being converted in the first instance into

minute shells, or stony skeletons, afterward passes into a compact

and solid rock, so in like manner do plants and trees extract carbon

from the atmosphere in which they vegetate, and convert it into coal.

No reasonable doubt can now be entertained that coal derives its

existence, almost entirely, from the woody tissue of sunken swamps and

forests. Though the nature of the process by which this transformation

takes place, is yet but imperfectly understood, and is, indeed, at

the present moment a subject of much discussion and controversy,

nevertheless the _fact_ that the change _has_ taken place is fully

accepted by all as an established truth, and is supported by an

accumulation of evidence which it is not easy to resist.

The first circumstance to which we shall call attention, is the

wonderful profusion of vegetable life that is always associated with

coal. Every one who has descended at any time into a coal mine, or

who has examined the specimens usually exhibited in a well-furnished

museum, must have been struck by the countless forms of trees and

plants, which still remain vividly impressed on this black and

unsightly mineral. Dr. Buckland has described this phenomenon with

much vigor and beauty in his celebrated Bridgewater Treatise: “The

finest example I have ever witnessed is that of the coal mines of

Bohemia just mentioned. The most elaborate imitations of living foliage

upon the painted ceilings of Italian palaces, bear no comparison with

the beauteous profusion of extinct vegetable forms with which the

galleries of these instructive coal mines are overhung. The roof is

covered as with a canopy of gorgeous tapestry, enriched with festoons

of most graceful foliage, flung in wild irregular profusion over every

portion of its surface. The effect is heightened by the contrast of

the coal-black color of these vegetables with the light ground-work

of the rock to which they are attached. The spectator feels himself

transported, as if by enchantment, into the forests of another world;

he beholds trees of forms and characters now unknown upon the surface

of the earth, presented to his senses almost in the beauty and vigor of

their primeval life; their scaly stems and bending branches, with their

delicate apparatus of foliage, are all spread forth before him, little

impaired by the lapse of countless ages, and bearing faithful records

of extinct systems of vegetation, which began and terminated in times

of which these relics are the infallible historians.”

[Illustration:

Fig. 9.--Pecopteris Adiantoides. Fig. 10.--Sphenopteris Affinis.

Fossil Ferns found in the Coal Measures of Europe and America.]

The next important fact that points to the vegetable origin of

Coal is, that wood and Coal are both composed of the same ultimate

elements--carbon, hydrogen, and oxygen. This analogy is the more

remarkable when we are told that no other rock except Coal exhibits

anything approaching to this composition. It is true that the elements

just enumerated do not exist in the same proportions in wood and in

Coal. But the difference, when rightly understood, rather tends to

confirm our theory that the one is derived from the other. There is

more Carbon in Coal than in wood; while there is less oxygen and less

hydrogen. To explain how this may have come to pass during the process

of transition, we must call in the assistance of the chemist. It

appears from the researches of Liebig that, when vegetable matter is

buried in the earth, exposed to moisture, and partially or entirely

excluded from the air, the process of decomposition sets in, and that

under this process carbonic acid gas and carburetted hydrogen gas are

slowly evolved. At the same time a portion of the oxygen when set free

would naturally enter into a new combination with a portion of the

hydrogen, and form water. The result of these several changes would

necessarily be, that the accumulation of vegetable matter buried in the

earth would part, in course of time, with no small share of its carbon,

its hydrogen, and its oxygen, but not with all in the same proportions:

for the new combinations would use up more of the oxygen than of the

hydrogen, and more of the hydrogen than of the carbon.[58] In other

words, if the process should have gone on for a sufficient lapse of

ages, these elements would no longer exist together in the proportions

which are necessary to constitute wood, but would rather exist in the

proportions which are found to constitute coal.[59]

This explanation is confirmed by a fact with which our readers are

no doubt familiar. According to the explanation, carbonic acid and

carburetted hydrogen are evolved during the process by which coal is

produced from wood. We should therefore expect to find these gases

closely associated with Coal. If they are _not_ so associated, their

absence is a serious objection against our theory; but if they _are_

so associated, their presence is a strong evidence in its favor. Now

on this point, as every one knows, practical miners bear testimony

that the fact corresponds exactly with our theory. They tell us that

reservoirs of Choke-damp, which is carbonic acid, and of Fire-damp,

which is carburetted hydrogen, are found very commonly pent up in the

crevices and cavities of coal beds, and are the cause, when tapped, of

many of the accidents which take place. They even assure us that some

beds of coal are so saturated with gas that, when cut into, it may

be heard oozing from every pore of the rock, and the coal is called

_singing coal_ by the colliers.[60]

To sum up, then, what we have said on this point: it appears, first,

that the same constituent elements are found in wood and Coal;

secondly, though they do not exist in the same proportions in the two

substances, the difference is fully accounted for by the changes which

we should naturally expect to take place when large accumulations of

vegetable matter are buried in the earth; thirdly, in the hypothesis of

these changes, carbonic acid and carburetted hydrogen would certainly

be developed; and in point of fact, these gases are found intimately

associated with Coal all over the world.

There is another remarkable fact which fits in most admirably with our

theory. Coal is found at the present day in the Crust of the Earth,

disposed in thin seams or beds, and each bed is almost uniformly

found to rest upon a stratum of fine clay, sometimes several feet in

thickness. This is just what our theory would lead us to expect. If

coal is produced from plants and trees, these plants and trees must

have grown upon some suitable soil; and, therefore, in this hypothesis

we should expect, ordinarily speaking at least, to find a bed of clay

beneath every bed of coal. But this is not all. When we examine more

closely the stratum on which the coal reposes, we find the roots and

stems of trees mingled with the clay in the greatest profusion. In

the Welsh coal field, in a depth of twelve thousand feet, there are

from fifty to a hundred beds of coal, each lying on a stratum of clay

abounding in these remains.[61]

We now come to an argument of a practical kind which appeals to common

sense and common experience. Let us suppose that a person wholly

unacquainted with the art of manufacturing paper, were to enter a

paper-mill when the workmen are away, and the process of manufacture

for a time suspended. At first sight he would probably find it

difficult to persuade himself, that the piles of clean white paper,

which attract his notice at one end of the building, are produced

from the heaps of filthy rags which he sees accumulated at the other.

But if he be a sagacious observer, he will soon find evidence to

convince him that this is really the case. For he will perceive, upon

close examination, that the self-same material is exhibited in every

intermediate state of progress from one extreme to the other. First,

there is the great chest with its numerous compartments, in which the

rags are seen carefully sorted, according to their various degrees of

quality and texture. Next comes the fulling-mill, where they are washed

and bleached. Then the revolving cylinder, furnished on the exterior

surface with sharp blades or cutters; and the vat in which it moves is

filled with the rags, which now assume the form of a thin liquid pulp.

Advancing still further he will see this pulp evenly spread out upon

a wire-gauze frame, and now at last it is beginning to exhibit some

likeness to the form and substance of paper. Further on it is seen

pressed and dried; and last of all cut into sheets and laid aside in

lofty piles.

Now it seems to us that we are placed in somewhat of the same

position, as regards the manufacture of Coal. We cannot observe the

process actually going on; for though, in this process, the work is

never suspended, the workmen never at rest, yet extending as it does

over a space of many centuries, it is too slow to be sensible; and

besides it is conducted in great part beneath the surface of the

Earth. Nevertheless, we can trace the progress of change through

each intermediate stage of the transition, from one extreme to the

other,--from the primeval swamps and forests through the numerous

varieties of the Peat and Lignite to the richest beds of pure Coal.