Popular Science Monthly/Volume 1/July 1872/Corals and Coral Architecture
|CORALS AND CORAL ARCHITECTURE.|
THERE are forests and gardens in the depths of the sea. Stately, tree-like structures inextricably branched, and a multitudinous shrubbery, delicate in form, and crowned with brilliant perennial blossoms, constitute a world of life and beauty in the obscurities of the ocean, where the eye of man but rarely penetrates.
But what kind of life is that which produces such wonderful growths? The similarity of the appearances to a garden was too striking not to suggest an answer, for what but vegetal forces can produce growing branches covered with flowers? And so Theophrastus, the old Greek botanist, described these sea-structures as of vegetal origin, and this opinion prevailed for 2,000 years. It began at length to be suspected that the old notion was wrong, and in 1751 Peyssonnel sent to the Royal Society an elaborate memoir, in which he maintained that these ocean-forests are really formed by little animals. This, as a matter of course, was indignantly disputed, and was pronounced by the great Reaumur too absurd to be discussed. To ascribe to "poor, little, helpless, jelly-like animals" the skill and power necessary to build such stately and beautiful structures, looked like a wanton appeal to human credulity, and the point was hence warmly controverted. Linnaeus, however, proposed a compromise. He would admit the animal, but would not deny the vegetable He therefore assumed that these little toilers of the sea were of an intermediate nature, and named them zoophytes, animal-plants. But the coral-polype is now known to be as truly an animal as a cat or a dog. The apparent flower is a little sac-like creature, and the wreath of colored petals its arms or tentacles. These are arranged around its circular mouth to seize and draw in the food upon which it lives and grows, while the structures which it produces are not perishable wood but enduring stone.
But, lowly organized and apparently insignificant as these animals are, the part they play in the operations of Nature is in the highest degree imposing. Not only do they produce these exquisite arborescent forms, but they build gigantic structures with caverns, grottoes, and mighty arch-work, and raise rocky walls, which rival in extent and massive grandeur the noblest mountain scenery upon the land. Though these constructions proceed but slowly, yet in numbers that are inconceivable, and through ages that are incalculable these tiny beings have been engaged in the work of rock-manufacture, until they now rank with earthquakes, the rising and sinking of continents, and other stupendous agencies by which the crust of the globe has been shaped. Multitudes of islands, hundreds and thousands of feet above the surface of the sea, and multitudes of others sunk thousands of feet below it; stony reefs, along which the navigator sails hundreds of miles; the sheet of rock through which Niagara is slowly cutting its way, and extensive beds of limestone scattered over the continents—all have a common origin—all have been extracted from sea-water and secreted by animals low in structure, and chiefly by these jelly-form polypes, many kinds of which are so minute as to be hardly visible to the naked eye. Living, working, multiplying, and dying like ourselves, building blindly but grandly in the final result, perhaps here also not unlike Fig. 1. Fresh-water hydra. ourselves, these humble creatures illustrate the method of Nature, and their works and ways are of inexhaustible interest. Their instructive story has just been told by Mr. Dana, with the fascination of romance and the fidelity of science, in his charming book on "Corals and Coral Islands." In the present brief presentation of some of the facts of the subject we shall chiefly follow Prof. Dana, and we are indebted to the courtesy of his publishers for the accompanying illustrations from his work.
The animal kingdom is divided into several sub-kingdoms, one of which comprises numerous species of animals termed Radiates, because their parts are arranged radially round a centre. One division of the radiates is known as polypes, and they have the faculty of secreting a stony frame or skeleton, which is termed coral. The polypes are the most important coral-making animals, but this substance is produced also by other radiates, by some of the lowest tribes of mollusks, and a kind of coral is made by lime-secreting sea-weeds.
There is a group of radiates termed Hydroids. One of these, the fresh-water hydra, is represented in Fig. 1, as it is often seen attached to the under surface of a floating leaf. Prof. Dana says: "It is seldom over half an inch long; it has the form of a polype, with long, slender tentacles, but no special organs except a mouth and tubular stomach. Like the fabled hydra, if its head be cut off, another will grow out, and any fragment will, in the course of a short time, be- come a perfect hydra, supplying head or tail, or whatever is wanting, and hence the name given to the genus by Linnaeus."
Some of the hydroids are coral-makers. Fig. 2 represents the kind of work done by one of them. It certainly looks like a plant, and, in allusion to its delicate plumes, it is called Plumularia. Along the branches are minute cells (indicated by the fine dots in the woodcut), each of which was the seat of one of the little hydra-like animals (in this case not more than the fourth of a line long), and usually with short tentacles spread out star-like.
"We will now pass to the true polypes. These may be divided into those which secrete coral and those which do not. The latter
|Fig. 3.||Fig. 4.|
|An Horizontal Section of a Polype showing the Internal Arrangement of the Folds and Compartments.||Coral from the West Indies, showing the Structure of the Cells.|
have soft, leathery bodies, and live attached to stones and other substances upon the sea-bottom, by a basal, sucker-like disk. They have the power of locomotion by contraction and expansion of the muscles of the disk. But the coral-making polypes are fixed to the stone which they create, and which is part of themselves. The polype is the living part of the coral, the gelatinous mass which
fills the radiating cells upon the coralline surface. It consists of a sac or stomach, and an enveloping membrane. An opening from the stomach outward is the animal's mouth. This is surrounded by tentacles, which by their motion aid in bringing to it currents of water in which floats its food, and of the solid matters of which it constructs its calcareous skeleton." In the polype, the stomach or digestive sac, with its appendages, constitutes the whole animal. Into the stomach the sea-water passes freely, and carries the digested food through the internal cavities. There is no other circulation of fluids;
Fleshy folds or partitions extend from the upper to the lower end of the animal, and give to the polype the appearance of a little balloon of tissue-paper crumpled up. These radial folds in the animal's structure are illustrated in Fig. 3, page 260, which is an ideal sectional view of one of this class of animals. The radiating partitions or folds are seen to be arranged in pairs. In the coral-secreting polypes these pairs are six or five, or multiples of six or five. The space between each of these partitions opens into the tubular tentacle at the top of the animal. The tentacles of the polypes moving freely and with
considerable muscular strength, seem sufficient to supply the animal's wants, but it has a formidable armature in the stinging barbs which cover its tentacles, mouth, and stomach, and which produce torpor and death in any small animal brought in contact with them. These are called lasso-cells. The cells in which the lasso or barbs are located measure from 1/350 of an inch to 1/5000 of an inch in length. From these the lasso is projected, inflicting, in some species, upon even a human hand, painful and serious injury. Owing to this peculiarity of certain jelly-fishes, they have been appropriately named sea-nettles.
Between the fleshy partitions of the polype's body are thin, stony plates. These, with the other hard portions, make up the coral skeleton, and are wholly secreted by the polype. Fig. 4, page 260, represents a group of polypes (Phyllangia Americana) from the West Indies, and illustrates well the radial structure of the cells.
The secretions occur around and underneath the polype, never in its interior, which would interfere with its functions. It constructs its skeleton by secretion, as an oyster does its shell, or as the tissues of a vertebrate animal do its bones; an act wholly involuntary in both cases.
Therefore, when we speak of the labors or architecture of the coral animal, we do not imply outside mechanical work as the bee in constructing its comb, but simply the operation of a vital function. "This process of secretion," says Prof. Dana, "is one of the first and most common of those that belong to living tissues. It belongs eminently to the lowest kinds of life. These are the best stone-makers, for in their simplicity of structure they may be almost all stone, and still carry on the processes of nutrition and growth."
The young polype in the reef-building species arises by a process of budding from the parent animal. It was from this curious operation that early observers strengthened their argument in favor of the vegetable nature of corals. "The bud," says Dana, "commences as a slight
prominence on the side of the parent. The prominence enlarges, a mouth opens, a circle of tentacles grows out around it, and increase continues until the young finally equals the parent in size. Since in these species the young do not separate from the parent, this budding produces a compound group."
From this it is obvious that, while the polypes exist as individuals, they are nevertheless connected by intervening tissues which form a thin sheet of animal matter covering the surface of the coral, and through which fluids circulate. This sheet of animal matter unites the polypes, but does not destroy their individuality. The budding process
takes place in some species by a spontaneous division of the parent polype. So that in the same cell a new polype forms side by side with the old one (Fig. 5, page 260), and begins an individual life, but the results are essentially the same.
The reef-building corals comprise all those with a stony skeleton, yet the great work is carried on mainly by a few of the principal groups. Of these the following are the most conspicuous and familiar:
The Astræas, so called from their star-shaped cells (Fig. 6, page 261). They grow in huge hemispherical masses, often twenty feet or more in diameter. The brain-coral, covered with meandering furrows and ridges resembling cerebral convolutions (Fig. 1, page 282). The masses are large, and are shaped like the astræa. The Porites, often branched,
sometimes massive and covered with exceedingly minute cells, are represented in Fig. 8, page 263. Other species are branching or lamellar, as the Millipores (Fig. 9), which contribute largely to the material of the West-India reefs. The animal, however, in this case, is not a true polype, but belongs to the group of acalephs, or jelly-fishes. Then there are the beautifully branched, tree-like Madrepores (Fig. 10, page 205). Fig. 11, page 266, represents one of the most beautiful of the corals produced by the alcyonoid polypes. Almost all of these are flexible, and sway with the moving waters. Some kinds are too flexible to stand erect, and they hang from the coral ledges, as in the coral caves, in gorgeous clusters of scarlet, yellow, and crimson. All these corals are covered with cells, and each represents the dwelling place of an individual polype. In some of these the diameter of the expanded rays or tentacles of the polype is about one-eighth of an inch; in others, as the astræa, nearly an inch. The rays, when expanded, closely resemble the petals of flowers, and coral flowers is, with many persons, a more familiar term than "rays," and equally expressive. The astræas have sometimes nearly a hundred petals or tentacles to a single animal. Others, as madrepores and porites, have twelve rays each; in
still other species a larger or smaller number is found. The rays or tentacles readily fold inward over and into the animal's mouth, and upon a slight jar of a mass of coral the waving tentacles close, and all motion or evidence of life disappears from its surface.
The number of polype-cells upon some species of coral is immense. A dome of astræa, twelve feet in diameter, with a cell to each half-inch of its surface, would contain 100,000 individuals. Prof. Dana remarks that a porites of the same size would number 5,500,000 polypes. But Agassiz states that he has estimated 14,000,000 individuals in a mass of porites not more than twelve feet in diameter.
Notwithstanding the enormous mass of some coral formations, they are dead and deserted throughout, excepting a thin crust upon the surface. This, in different species, may vary in thickness from 1/8 to 1/16 of an inch to half an inch, and constitutes the living portion of the coral where the work of growth goes on. The inner or dead portions constitute the stony mass on which new material is secreted, but is no more essential for that purpose than a rock or a sandy shore. Prof. Dana observes that, if the living portion could be separated, it would form a hemispherical shell about half an inch thick. As the higher orders of trees increase in size by additions of new wood at the outer margin of the trunk, long after the heart-wood is dead, so the coral is alive and grows only on its surface.
It is obvious that the increase of the coral will continue without
limit except from surrounding conditions. Thus, if the dome reaches the surface of the water, the polypes die, and growth ceases in that direction; but it may increase in diameter, forming some remarkable structures, which we will presently notice. But, if the coralline mass be continually sinking by a subsidence of the land on which it rests, the conditions of growth continue, and reefs of tremendous mass and thickness are formed.
The dead coral is always more or less porous, until the pores and polype-cells are filled by comminuted substance or the infiltration of carbonate of lime. Aided by chemical changes, the mass becomes solid coral-rock, and finally compact limestone, with few traces remaining of the coral structure. In many of the branching, tree-like corals the stems are formed nearly solid as they grow, and are of great strength. In some of the massive species the surface cells occupied by the living animals are very shallow, measuring from one-sixteenth to one-fourth of an inch in depth. Underneath the polype is a floor or partition of coral secreted by the animal, and which separates the new from the old cell. Hence many corals, when split vertically, show a coarse cellular structure. In the life-and-death process of the polypes, animal matters remain confined in the old cells.
As the coral masses increase in size, it is evident that there must have occurred a simultaneous increase in the number of polypes. This fact is the more interesting, when it is known that a great coral dome may have arisen from a few, or perhaps from a single parent individual. During the long period of its growth, reaching through thousands of years, how enormous is the number of builders of it that have lived and died!
The rate at which corals grow is an interesting question, but not fully determined, for want of sufficient data. A single mass standing in clear water would increase more rapidly than corals in a reef. If at the rate of an inch in six years, a dome 20 feet in diameter would require about 1,400 years. Some species seem to grow more rapidly than this, but the increase of reefs is slower, notwithstanding additions from shells and other sources. On a coral-plantation, as a reef may be called, a portion is always unproductive. There are barren areas on the reef where sands or sediment destroy the polypes, and retard its growth. The investigations of Prof. Agassiz, at Key West, indicate a growth of about six inches in 100 years. He says: "If we allow twice that rate of growth, not less than 7,000 years would be required for the formation of the great reef at that place, and hundreds of thousands of years for the coral growths which form the peninsula of Florida."
After a careful estimate, Prof. Dana concludes that the growth of reefs, from increase of their corals, may be from 1/44 to 1/80 of an inch per year, and adds that, "whatever the uncertainties of calculation, is evident that a reef increases with extreme slowness." It is a reasonable calculation that more than 1,000,000 years have elapsed since the foundations were laid of some of the great Pacific reefs.
An opinion prevailed formerly that the different species of corals occur in a reef in a uniform order of superposition—that each flourishes at certain depths of water, and not above or below that plane. The general fact is known that no important reef-building coral grows at a depth greater than 120 feet. Above that plane, all the work of coral architecture is carried on. Prof. Agassiz supposes that the range of different corals in depth is in part limited by pressure of the waters. At 32 feet depth, the animal is under a pressure of two atmospheres, and of more than four atmospheres at 120 feet. In this connection he states that the astræas occur at the bottom of the reef. Next in order, the brain-coral, porites, madrepores, and on the surface the light, branching varieties which are the shrubbery of the coral world. This arrangement may occur where the bottom on which the reef stands is immovable, or has remained without change of level since the reef commenced. But how it could occur in a coral mass 2,000 feet thick, growth being limited to a depth of 120 feet, is not entirely obvious. In order to explain the enormous depth of coral-reefs upon the submerged lands of the Pacific, it is necessary to consider the further well established fact, first suggested by Charles Darwin, that the lands and ocean bed have gradually subsided. The subsidence has been often at the same slow rate that coral-reefs have increased by upward growth. It seems inevitable from this that the builders are at work on the upper portions of the reef; certainly it is here only that the work of elevation can go on. Summing up this subject, Prof. Dana says: "Reef-building corals of the different groups grow together promiscuously at different depths up to low-tide level. The largest astræas, mændrines (brain-coral), porites, and other kinds, have been seen by the author, constituting the upper part of the growing reef." The coral polype flourishes only in the belt of warm waters which lies in and near the tropics. A temperature lower than 68° Fahrenheit is fatal to them. The great reefs abound and grow with greatest vigor in the zone of greatest heat.
Surrounding most of the tropical islands are two principal reefs, one fringing the shore; the other, called the barrier-reef, lying seaward, sometimes more than 15 miles from the land. The intervening space is often filled with minor reefs and a gorgeous wealth of coral vegetation.
Here lie immense platform-reefs, a shell of coral covering the bottom beneath the shallow waters. These together make up the coral-reef ground of the island. West of the two larger Feejee Islands are 3,000 square miles of reef-ground. New Caledonia has a reef along its western shores a distance of 250 miles. The great Australian barrier, lying east and northeast of that island, forms a broken reef, 250 miles in length. On these outer reefs the waves forever break, and here, where the plunge of the surf is most furious and persistent, the polypes flourish with greatest vigor, and open their many-colored petals to the life-giving waters, as do thirsty flowers to the welcome rain. On every dead space delicate moss-like and lichen-like corals quickly form their thin, hard crusts.
Outside the reefs there occurs, in many places, a coral growth alike curious and interesting. In isolated patches are found immense mushroom-shaped masses called coral-heads. One is described by Whipple, cited by Dana, standing in water 50 feet deep, near Turk's Island. Its trunk is about 15 feet in diameter, supporting a tabular mass 100 feet in diameter, the top being bare at low tide. When these corals reach the surface, growth in that direction ceases, but may continue indefinitely around the margins. Thus, in many places, the tops of adjacent trunks have joined together, forming a coral floor resting upon arches and pillars built without axe or sound of hammer. In some parts of the reef-region, such united coral-heads cover large areas. A magnificent scene would be presented should the waters recede and leave bare these arches and columns. The ruins which "sentinel the desert" would not rival them in grandeur. To thread their avenues and sounding aisles would be the labor of a lifetime.
The illustration, Fig. 12, after a sketch by Prof. C. F. Hartt, in his "Geology of Brazil," is of an area of the sea off that coast, abounding in coral-heads similar to those described. "The corals," says Prof. Hartt, "grow in the open sea, and often rise 40 or 50 feet, and form what the natives call chapeiroes (signifying 'big hats').
They are abundant on one part of the coast over an area of 40 square miles. A vessel running on the top of one of these chapeiroes would remain perched like a weathercock on the top of a tower, to the great amazement of the captain who finds deep water all around." Inside the outer or barrier reef the water is smooth as in an inland bay. If free from sediment, and not freshened by discharge from rivers, it is the paradise of the smaller corals. The beautiful fungia lie with innumerable shells upon the bottom. The feathery and fan corals grow in clusters, and, amid their delicate plumes, fishes, which rival them in gayety, glide through the transparent water.
It is a peculiarity of coral-reefs that the outer side is usually nearly perpendicular, while the inner side is a gentle declivity. The cause of this may be better understood if we follow the development of the reef from its beginning. Upon a shore of sand or rock, and at a depth of less than 120 feet, the reef-builders attach themselves and commence to grow. The reef rises, but, as before remarked, corals grow most rapidly in the purest waters, and thus it is that reefs often seem to crowd against the waves without and assume a wall-like aspect. But, owing to the wash of the land within, and the discharge of streams in some instances, the polypes are less healthy and their growth more precarious. These causes modify the form of the reef. But, however modified, the reef fringes or encircles the land (Fig. 13). We have already remarked that coral-reefs can attain no greater thickness than about 120 feet, unless there occurs with their growth a simultaneous subsidence of the land on which they rest; but, with that coincidence, the growth of the reef may continue so long as the subsidence goes on.
It is obvious that, with this sinking of the land, the area of the island must diminish, the sea and its accompanying corals gradually encroaching upon its shores. At last the land disappears. Then we have a lake or lagoon over its former site, surrounded by coral-reefs—for the builders have not been at rest. All the features of coral growth continue, but the land with its wealth of vegetation is buried. A coral floor has formed over it. This is the history of hundreds, perhaps thousands of former islands in the Pacific alone; and the great reefs, from which the surf sends up an incessant wail, are the monuments of this ocean-cemetery.
The encircling reefs, with the lagoon, are called an atoll, which is only another name for a coral-island. This effect is well shown in Fig. 14, page 271.
Moreover, the force of waves and lifting-power of water have broken fragments of coral and coral-rocks, and thrown them upon the reef. The mass may have been already weakened by the perforations of in- numerable boring worms and mollusks which burrow in the reefs. Thus in places beaches have been formed 10 or 12 feet in elevation above the ocean. They are composed of coral sand made fine and drifted by waves and winds, fragments of shells, bones of fishes, and other matters drifted thither by the sea.
The beauty of the completed atoll must be given in Prof. Dana's words: "When first seen from the deck of a vessel, only a series of dark points is descried just above the horizon. Shortly after, the points enlarge into the plumed tops of cocoa-nut trees, and a line of green, interrupted at intervals, is traced along the water's surface. Approaching still nearer, the lake and its belt of verdure are spread out before the eye, and a scene of more interest can scarcely be imagined. The surf, beating loud and heavy along the margin of the reef, presents a strange contrast to the prospect beyond. There lie the white coral-beach, the massy foliage of the grove, and its embosomed lake with its tiny islets. The color of the lagoon-water is often blue as the ocean, although but 10 or 20 fathoms deep, yet shades of green and yellow are intermingled." In some instances there is a ship-channel through the reefs into the lagoon, in others only a shallow passage, in others none at all.
By a series of soundings, we have some idea of the depth of water near the ocean-side of many of the great reefs.
Seven miles from Clermont Tonnerre, of the Panmotus group, bottom was not found at 6,870 feet. From another point of the same island, only 1,500 yards from shore, the lead touched at 2,100 feet, then dropped off (probably from a projecting coral), and descended 3,600 feet without finding bottom. In another instance, about a cable's length from the island of Ahii (Peacock Island), in the same group, the lead struck at 900 feet, fell off and touched bottom at 1,800 feet. Off Whitsunday, 500 feet from the shore, no bottom was found at 1,500 feet. Deep soundings in the immediate vicinity of coral-islands is almost universal. Should these submerged islands of the Pacific be again elevated until their gigantic coral crowns should be lifted above the waves, an immense area of the Pacific would be converted again into an archipelago, not indeed of verdure-covered land as before, but of hills and mountains of coral-rock, bristling with crags, sublime with precipices and stupendous walls.
The great coral-bearing area of the Pacific is about 12,000,000 square miles in extent, nearly as large as the continent of Africa, or of Europe and North America combined. It extends from the southern side of the Hawaiian Islands to Pitcairn's Island to the southeast, thence 2,000 miles broad and 6,000 miles in length to the Pelew Islands, north of New Guinea in the Polynesian seas. In this area are 204 islands, very few of which are high or with land still above the sea. Southward of this area are many mountainous islands, or with highlands, surrounded with reefs, evidently beyond the line of greatest subsidence.
The evidence is satisfactory that the depressed area has gone down, in comparatively recent geologic time, many thousands of feet, and yet the subsidence may have been less than the elevation of lands elsewhere; for we have the elevation of the Rocky Mountains, Andes, Alps, and Himalayas, modern events in geologic history. It is more than probable that great subsidence in one section is correlated by elevations elsewhere. And the depression of the Pacific area may correspond to the elevation of northern lands which probably caused the cold and glaciers of the glacial age. The movement was one of those great secular changes of the earth's crust which dates far back in its history.
There is evidence that the Pacific subsidence has ceased, or nearly so, and that local elevations have long since commenced. In about 40 instances, Pacific coral-islands have been elevated since reefs were formed upon them. Many of these elevations are a few feet only, others, a few hundred feet; as many as 600 feet in one or two instances. These elevated masses of coral-rock have the perpendicular walls and configuration before described. Metia, or Aurora Island (Fig. 15), is one of the Panmotu group; its walls of coral-limestone are 250 feet high, and resemble the Palisades on the Hudson.
Along the outer margins of the elevated islands are deep caverns, showing, by their contour, the wearing and wasting action of waves. The Bermudas are remarkable for their caverns; the coral-made land being in places 260 feet above the level of the sea. On the island of Oahu, the Rev. John "Williams entered one by a descent of 20 feet, and wandered a mile in one of its branches. Innumerable openings presented themselves on all sides. The roof, a superb stratum of coral-rock, 15 feet thick, was supported by stalactitic columns, and thickly hung with stalactites.
We cannot dismiss this subject without considering the coral-island, or completed atoll, in its relations to life. Upon an area so limited and so uniform, there may be much beauty, but little variety. On many of them there are less than a dozen species of plants, and not an animal higher in the scale than fishes, except a few migratory birds. Twenty-nine species of plants were found upon one island. There, as elsewhere, on the dry rocks, black lichens grow in patches. The germs of this class of plants seem to be present everywhere within the geographical limits of life. On some of the more favored islands are some tropical birds, a few rats and mice, and perhaps other animals introduced by man. The drift of the sea may convey to it various organic germs.
The coral-made land is ocean-born; its palm-groves were planted by the waves; and here too is man, savage, swarthy, unclothed, filthy, barbarous. With him degradation is an inheritance, and physical conditions hold him with relentless grasp. With occasional surfeit, he is in danger daily of starvation. He is driven to infanticide in self-defence. The taste which adorns our New-England landscapes can never develop here. In the land of the elm and the oak, rather than beneath the shade of the pandanus and the cocoa-nut palm, we must look for the conditions which mould manhood in the common struggle for life.
We quote again, and lastly, from Prof. Dana's work: "A coral-island, even in its best condition, is but a miserable place for human development, physical, mental, and moral. There is poetry in every feature, but the natives find this a poor substitute for the bread-fruit and yams of more favored lands. How many of the various arts of civilized life could exist in a land where shells are the only cutting instruments—fresh water barely enough for household purposes—no streams, nor mountains, nor hills? How much of the poetry and literature of Europe would be intelligible to persons whose ideas had expanded only to the limits of a coral-island, who had never conceived of a surface of land above half a mile in breadth—of a slope higher than a beach, or of change in seasons beyond a variation in the prevalence of rain?"
Such are coral-islands—beautiful gems of the ocean; delightful as a subject of study, equally in their aspects and development, their geological importance and in their relations to life.