Principles of Geology Part 67

To some accident of this kind we may refer the position of the skeleton of a whale, seventy-three feet long, which was found at Airthrey, on the Forth, near Stirling, imbedded in clay twenty feet higher than the surface of the highest tide of the river Forth at the present day. From the situation of the Roman station and causeways at a small distance from the spot, it is concluded that the whale must have been stranded there at a period prior to the Christian era.[1108]

Other fossil remains of this cla.s.s have also been found in estuaries known to have been silted up in recent times, one example of which has been already mentioned near Lewes, in Suss.e.x.

[Ill.u.s.tration: Fig. 106.

Fossil eggs of turtles from the Island of Ascension.[1109]]

_Marine reptiles._--Some singular fossils have lately been discovered in the Island of Ascension, in a stone said to be continually forming on the beach, where the waves threw up small rounded fragments of sh.e.l.ls and corals, which, in the course of time, become firmly agglutinated together, and const.i.tute a stone used largely for building and making lime. In a quarry on the N. W. side of the island, about 100 yards from the sea, some fossil eggs of turtles have been discovered in the hard rock thus formed. The eggs must have been nearly hatched at the time when they perished; for the bones of the young turtle are seen in the interior, with their shape fully developed, the interstices between the bones being entirely filled with grains of sand, which are cemented together, so that when the egg-sh.e.l.ls are removed perfect casts of their form remain in stone. In the single specimen here figured (fig. 106), which is only five inches in its longest diameter, no less than seven eggs are preserved.[1110]

To explain the state in which they occur fossil, it seems necessary to suppose that after the eggs were almost hatched in the warm sand, a great wave threw upon them so much more sand as to prevent the rays of the sun from penetrating, so that the yolk was chilled and deprived of vitality. The sh.e.l.ls were, perhaps, slightly broken at the same time, so that small grains of sand might gradually be introduced into the interior by water as it percolated through the beach.

[Ill.u.s.tration: Fig. 107.

One of the eggs in fig. 106, of the natural size, showing the bones of the foetus which had been nearly hatched.]

_Marine testacea._--The aquatic animals and plants which inhabit an estuary are liable, like the trees and land animals which people the alluvial plains of a great river, to be swept from time to time far into the deep; for as a river is perpetually shifting its course, and undermining a portion of its banks with the forests which cover them, so the marine current alters its direction from time to time, and bears away the banks of sand and mud against which it turns its force. These banks may consist in great measure of sh.e.l.ls peculiar to shallow and sometimes brackish water, which may have been acc.u.mulating for centuries, until at length they are carried away and spread out along the bottom of the sea, at a depth at which they could not have lived and multiplied. Thus littoral and estuary sh.e.l.ls are more frequently liable even than freshwater species, to be intermixed with the exuviae of pelagic tribes.

After the storm of February 4, 1831, when several vessels were wrecked in the estuary of the Forth, the current was directed against a bed of oysters with such force, that great heaps of them were thrown _alive_ upon the beach, and remained above high-water mark. I collected many of these oysters, as also the common eatable whelks (_Buccina_), thrown up with them, and observed that, although still living, their sh.e.l.ls were worn by the long attrition of sand which had pa.s.sed over them as they lay in their native bed, and which had evidently not resulted from the mere action of the tempest by which they were cast ash.o.r.e.

From these facts we learn that the union of the two parts of a bivalve sh.e.l.l does not prove that it has not been transported to a distance; and when we find sh.e.l.ls worn, and with all their prominent parts rubbed off, they may still have been imbedded where they grew.

_Burrowing sh.e.l.ls._--It sometimes appears extraordinary, when we observe the violence of the breakers on our coast, and see the strength of the current in removing cliffs, and sweeping out new channels, that many tender and fragile sh.e.l.ls should inhabit the sea in the immediate vicinity of this turmoil. But a great number of the bivalve Testacea, and many also of the turbinated univalves, burrow in sand or mud. The Solen and the Cardium, for example, which are usually found in shallow water near the sh.o.r.e, pierce through a soft bottom without injury to their sh.e.l.ls; and the Pholas can drill a cavity through mud of considerable hardness. The species of these and many other tribes can sink, when alarmed, with considerable rapidity, often to the depth of several feet, and can also penetrate upwards again to the surface, if a ma.s.s of matter be heaped upon them. The hurricane, therefore, may expend its fury in vain, and may sweep away even the upper part of banks of sand or mud, or may roll pebbles over them, and yet these Testacea may remain below secure and uninjured.

_Sh.e.l.ls become fossil at considerable depths._--I have already stated that, at the depth of 950 fathoms, between Gibraltar and Ceuta, Captain Smith found a gravelly bottom, with fragments of broken sh.e.l.ls, carried thither probably from the comparatively shallow parts of the neighboring straits, through which a powerful current flows. Beds of sh.e.l.ly sand might here, in the course of ages, be acc.u.mulated several thousand feet thick. But, without the aid of the drifting power of a current, sh.e.l.ls may acc.u.mulate in the spot where they live and die, at great depths from the surface, if sediment be thrown down upon them; for even in our own colder lat.i.tudes, the depths at which living marine animals abound is very considerable. Captain Vidal ascertained, by soundings made off Tory Island, on the northwest coast of Ireland, that Crustacea, Star-fish, and Testacea occurred at various depths between fifty and one hundred fathoms; and he drew up Dentalia from the mud of Galway Bay, in 230 and 240 _fathoms_ water.

The same hydrographer discovered on the Rockhall Bank large quant.i.ties of sh.e.l.ls at depths varying from 45 to 190 fathoms. The sh.e.l.ls were for the most part pulverized, and evidently recent, as they retained their colors. In the same region a bed of fish bones was observed extending for two miles along the bottom of the sea in eighty and ninety fathoms water. At the eastern extremity also of Rockhall Bank, fishbones were met with, mingled with pieces of fresh sh.e.l.l, at the depth of 235 fathoms.

a.n.a.logous formations are in progress in the submarine tracts extending from the Shetland Isles to the north of Ireland, wherever soundings can be procured. A continuous deposit of sand and mud, replete with broken and entire sh.e.l.ls, Echini, &c., has been traced for upwards of twenty miles to the eastward of the Faroe Islands, usually at the depth of from forty to one hundred fathoms. In one part of this tract (lat. 61 50', long. 6 30') fish-bones occur in extraordinary profusion, so that the lead cannot be drawn up without some vertebrae being attached. This "bone bed," as it was called by our surveyors, is three miles and a half in length, and forty-five fathoms under water, and contains a few sh.e.l.ls intermingled with the bones.

In the British seas, the sh.e.l.ls and other organic remains lie in soft mud or loose sand and gravel; whereas, in the bed of the Adriatic, Donati found them frequently inclosed in stone of recent origin. This is precisely the difference in character which we might have expected to exist between the British marine formations now in progress and those of the Adriatic; for calcareous and other mineral springs abound in the Mediterranean and lands adjoining, while they are almost entirely wanting in our own country. I have already adverted to the eight regions of different depths in the aegean Sea, each characterized by a peculiar a.s.semblage of sh.e.l.ls, which have been described by Professor E. Forbes, who explored them by dredging. (See above, p. 649.)

During his survey of the west coast of Africa, Captain Sir E. Belcher found, by frequent soundings between the twenty-third and twentieth degrees of north lat.i.tude, that the bottom of the sea, at the depth of from twenty to about fifty fathoms, consists of sand with a great intermixture of sh.e.l.ls, often entire, but sometimes finely comminuted.

Between the eleventh and ninth degrees of north lat.i.tude, on the same coast, at soundings varying from twenty to about eighty fathoms, he brought up abundance of corals and sh.e.l.ls mixed with sand. These also were in some parts entire, and in others worn and broken.

In all these cases, it is only necessary that there should be some deposition of sedimentary matter, however minute, such as may be supplied by rivers draining a continent, or currents preying on a line of cliffs, in order that stratified formations, hundreds of feet in thickness, and replete with organic remains, should result in the course of ages.

But although some deposits may thus extend continuously for a thousand miles or more near certain coasts, the greater part of the bed of the ocean, remote from continents and islands, may very probably receive, at the same time, no new accessions of drift matter, all sediment being intercepted by intervening hollows, in which a marine current must clear its waters as thoroughly as a turbid river in a lake. Erroneous theories in geology may be formed not only from overlooking the great extent of simultaneous deposits now in progress, but also from the a.s.sumption that such formations may be universal or coextensive with the bed of the ocean.

We frequently observe, on the sea beach, very perfect specimens of fossil sh.e.l.ls, quite detached from their matrix, which have been washed out of older formations, const.i.tuting the sea-cliffs. They may be all of extinct species, like the Eocene freshwater and marine sh.e.l.ls strewed over the sh.o.r.es of Hampshire, yet when they become mingled with the sh.e.l.ls of the present period, and buried in the same deposits of mud and sand, they would appear, if upraised and examined by future geologists, to have been all of the same age. That such intermixture and blending of organic remains of different ages have actually taken place in former times, is unquestionable, though the occurrence appears to be very local and exceptional. It is, however, a cla.s.s of accidents more likely than almost any other to lead to serious anachronisms in geological chronology.

CHAPTER L.

FORMATION OF CORAL REEFS.

Growth of coral chiefly confined to tropical regions--Princ.i.p.al genera of coral-building zoophytes--Their rate of growth--Seldom flourish at greater depths than twenty fathoms--Atolls or annular reefs with lagoons--Maldive Isles--Origin of the circular form--Coral reefs not based on submerged volcanic craters--Mr.

Darwin's theory of subsidence in explanation of atolls, encircling and barrier reefs--Why the windward side of atolls highest--Subsidence explains why all atolls are nearly on one level--Alternate areas of elevation and subsidence--Origin of openings into the lagoons--Size of atolls and barrier reefs--Objection to the theory of subsidence considered--Composition, structure, and stratified arrangement of rocks now forming in coral reefs--Lime, whence derived--Supposed increase of calcareous matter in modern epochs controverted--Concluding remarks.

The powers of the organic creation in modifying the form and structure of the earth's crust, are most conspicuously displayed in the labors of the coral animals. We may compare the operation of these zoophytes in the ocean, to the effects produced on a smaller scale upon the land by the plants which generate peat. In the case of the Sphagnum, the upper part vegetates while the lower part is entering into a mineral ma.s.s, in which the traces of organization remain when life has entirely ceased.

In corals, in like manner, the more durable materials of the generation that has pa.s.sed away serve as the foundation on which the living animals continue to rear a similar structure.

The stony part of the lamelliform zoophyte may be likened to an internal skeleton; for it is always more or less surrounded by a soft animal substance capable of expanding itself; yet, when alarmed, it has the power of contracting and drawing itself almost entirely into the cells and hollows of the hard coral. Although oftentimes beautifully colored in their own element, the soft parts become when taken from the sea nothing more in appearance than a brown slime spread over the stony nucleus.[1111]

The growth of those corals which form reefs of solid stone is entirely confined to the warmer regions of the globe, rarely extending beyond the tropics above two or three degrees, except under peculiar circ.u.mstances, as in the Bermuda Islands, in lat. 32 N., where the Atlantic is warmed by the Gulf stream. The Pacific Ocean, throughout a s.p.a.ce comprehended between the thirtieth parallels of lat.i.tude on each side of the equator, is extremely productive of coral; as also are the Arabian and Persian Gulfs. Coral is also abundant in the sea between the coast of Malabar and the island of Madagascar. Flinders describes a reef of coral on the east coast of New Holland as having a length of nearly 1000 miles, and as being in one part unbroken for a distance of 350 miles. Some groups of coral islands in the Pacific are from 1100 to 1200 miles in length, by 300 or 400 in breadth, as the Dangerous Archipelago, for example, and that called Radack by Kotzebue; but the islands within these s.p.a.ces are always small points, and often very thinly sown.

[Ill.u.s.tration: Fig. 108.

_Meandrina labyrinthica_, Lam.]

Of the numerous species of zoophytes which are engaged in the production of coral banks, some of the most common belong to the Lamarckian genera Astrea, Porites, Madrepora, Millepora, Caryophyllia, and Meandrina.

_Rate of the growth of Coral._--Very different opinions have been entertained in regard to the rate at which coral reefs increase. In Captain Beechey's late expedition to the Pacific, no positive information could be obtained of any channel having been filled up within a given period; and it seems established, that several reefs had remained for more than half a century, at about the same depth from the surface.

Ehrenberg also questions the fact of channels and harbors having been closed up in the Red Sea by the rapid increase of coral limestone. He supposes the notion to have arisen from the circ.u.mstance of havens having been occasionally filled up in some places with coral sand, in others with large quant.i.ties of ballast of coral rock thrown down from vessels.

[Ill.u.s.tration: Genera of Zoophytes most common in coral reefs.

Fig. 109.

_Astrea dipsacea_, Lam.]

[Ill.u.s.tration: Fig. 110.

Extremity of branch of _Madrepora muricata_, Lin.]

[Ill.u.s.tration: Fig. 111.

_Caryophyllia fastigiata_, Lam.]

[Ill.u.s.tration: Fig. 112.

_Porites clavaria_, Lam.]

[Ill.u.s.tration: Fig. 113.

_Oculina hirtella_, Lam.]

The natives of the Bermuda Islands point out certain corals now growing in the sea, which, according to tradition, have been living in the same spots for centuries. It is supposed that some of them may vie in age with the most ancient trees of Europe. Ehrenberg also observed single corals of the genera Meandrina and Favia, having a globular form, from six to nine feet in diameter, "which must (he says) be of immense antiquity, probably several thousand years old, so that Pharaoh may have looked upon these same individuals in the Red Sea."[1112] They certainly imply, as he remarks, that the reef on which they grow has increased at a very slow rate. After collecting more than 100 species, he found none of them covered with parasitic zoophytes, nor any instance of a living coral growing on another living coral. To this repulsive power which they exert whilst living, against all others of their own cla.s.s, we owe the beautiful symmetry of some large Meandrinae, and other species which adorn our museums. Yet Balani and Serpulae can attach themselves to living corals, and holes are excavated in them by saxicavous mollusca.

At the island called Taaopoto, in the South Pacific, the anchor of a ship, wrecked about 50 years before, was observed in seven fathoms water, still preserving its original form, but entirely incrusted by coral.[1113] This fact would seem to imply a slow rate of augmentation; but to form a correct estimate of the average rate must be very difficult, since it must vary not only according to the species of coral, but according to the circ.u.mstances under which each species may be placed; such, for example, as the depth from the surface, the quant.i.ty of light, the temperature of the water, its freedom from sand or mud, or the absence or presence of breakers, which is favorable to the growth of some kinds and is fatal to that of others. It should also be observed that the apparent stationary condition of some coral reefs, which according to Beechey have remained for centuries at the same depth under water, may be due to subsidence, the upward growth of the coral having been just sufficient to keep pace with the sinking of the solid foundation on which the zoophytes have built. We shall afterwards see how far this hypothesis is borne out by other evidence in the regions of annular reefs or atolls.

In one of the Maldive islands a coral reef, which, within a few years, existed on an islet bearing cocoa-nut trees, was found by Lieutenant Prentice, "_entirely covered with live coral and madrepore_." The natives stated that the islet had been washed away by a change in the currents, and it is clear that a coating of growing coral had been formed in a short time.[1114] Experiments, also, of Dr. Allan, on the east coast of Madagascar, prove the possibility of coral growing to a thickness of three feet in about half a year,[1115] so that the rate of increase may, under favorable circ.u.mstances, be very far from slow.

It must not be supposed that the calcareous ma.s.ses termed coral reefs are exclusively the work of zoophytes: a great variety of sh.e.l.ls, and, among them, some of the largest and heaviest of known species, contribute to augment the ma.s.s. In the South Pacific, great beds of oysters, mussels, _Pinnae marinae_, _Chamoe_ (or _Tridacnae_), and other sh.e.l.ls, cover in profusion almost every reef; and on the beach of coral islands are seen the sh.e.l.ls of echini and broken fragments of crustaceous animals. Large shoals of fish are also discernible through the clear blue water, and their teeth and hard palates cannot fail to be often preserved although their soft cartilaginous bones may decay.