It is more difficult to trace the downward movement of the land, for the evidence of each successive sea-margin is carried down, and washed away or covered up. Never theless, the fact of subsidence can be satisfactorily established by the following kinds of proofs: (1) The results of mere erosion by the sea and those of actual depression of the level of the land cannot always be distinguished without some care. The encroachment of the sea upon the land, involving, it may be, the disappearance of suc cessive fields, roads, houses, villages, and even whole parishes, does not necessarily indicate a sinking of the land. Such destruction of the coast-line may. indeed, be in progress without any actual change of level. Should the sea, however, rise to the level of roads and buildings which it never used to touch; should former half-tide rocks cease to show even at low-water, and should rocks, previously above the reach of the highest tide, be turned first into shore-reefs, then into hummocks and islets, we infer that the coast-line is sinking. Such kind of evidence is found in Scania, the most southerly part of Sweden. Streets, built of course above high-water mark, now lie below it, with older streets lying lower than they, so that the subsidence is of some antiquity. A stone. the position of which had been exactly determined by Linnmus in 1749, was found after 87 years to be 100 ft. nearer the water's edge. The w. coast of Green land, for a space of more than 600 m., is perceptibly sinking. It has there been noticed that over ancient buildings on low shores, as well as over entire islets, the sea has risen. The Moravian settlers have been more than once driven to shift their boat-poles inland, some of the old poles remaining visible under water. (2) As the land is brought down within reach of the waves, its characteristic surface-features are apt to be effaced, so that the submerged area, which passes down beneath the sea, may retain little or no evidence of its having been a land-surface. It will be covered, as a rule, with sea-worn sand or silt. Hence, no doubt, the reason why, among the marine strata which form so large a part of the stratified portion of the earth's crust, and where there are many proofs of depression, actual traces of land-surfaces are comparatively rare. It is only under very favorable circumstances,.as, for instance, where the area is sheltered from prevalent winds and waves, and where, therefore, the surface of the land can sink tranquilly under the sea, that fragments of that surface may be completely preserved under overlying marine accumulations. It is in such places that "submerged forests" occur. These are stumps or roots of trees still in their positions of growth in their native soil. Beds of peat, full of tree stumps, hazel-nuts, branches, leaves, and other indications of a terrestrial surface, are often found in similar situations. Sir Henry de la Beebe has described, around the shores of Devon, Cornwall, and western Somerset, a vegetable accumulation, consisting of plants of the same species as those which now grow freely on the adjoining land, and occurring as a lied at the mouths of valleys, at the bottoms of sheltered bays, and in front of and under low tracts of land, the sea ward side of which dips beneath the present level of the sea. Over this submerged land-surface sand and silt containing estuarine shells have generally been deposited. whence we may infer that in the submergence the valleys first became estuaries, and then sea-bays. If now, in the course of ages, a series of such submerged forests should be formed one over the other, and if, finally, they should, by upheaval of the sea bottom, be once more laid dry, so as to be capable of examination by boririg, well -sinking, or otherwise, 'they would prove a former long-continued depres sion, with intervals of rest. In such a case, the intervals of pause would be. marked by the buried forests, and the progress of the depression by the strata of sand and mud lying between them. In short, as to a former protracted elevation followed by a long pause, the evidence would be strictly on a parallel with that furnished by a succession of raised beaches. 3. An interesting kind of proof of an extensive depression of the north-west of Europe is furnished by the deep fjords or sea-locks by which that region is indented. A fjord is a long, narrow, and often singularly deep inlet of the sea, which terminates inland at the mouth of a glen or valley. The word is Norwegian, and'in Norway fjords are characteristically developed. The English word "firth," however, is the same, and the western coast of the British isles furnish many excellent examples of fjords. In Scotland they are usually called lochs, as loch bourn, loch Nevis, loch Fyne, Gareloch ; in Ireland they are sometimes known by the name of loughs, as lough Foyle, but more commonly by that of bays, as Dingle bay, Bantry bay. There can be little doubt that, though now filled with salt water, fjords have been originally land valleys. The long inlet was first excavated as a land-valley or glen. This valley exactly corresponds in form and character with the hollow of the fjord, and must be regarded as merely its inland prolongation. That the glens have been excavated by mib-aerial agents is a conclusion borne out by a great weight of evidence. If, therefore. we admit the sub-aerial origin of the glen, we must also grant a similar origin to its s:-, ward prolongation. Every fjord will thus mark the site of a submerged valley. This inference is confirmed by the fact that fjords do not, as a rule, occur singly. Like the glens of the land, they lie in groups: so that when they life found intersecting a long line of coast like that of the Iv. of Norway, or the w. of Scotland, we conclude that the land has there sunk down so as to permit the sea to run far up and fill the submerged glens. 4. Evidence of widqhpread depression over the area of the Pacific ocean is f urnished by the numerous atolls, 146r coral islands, scattered through that vast expanse of water. Mr. Darwiu ascertained that the reef-building corals do not live at a greater depth than about 15 or 20 fathoms. Yet reefs and circular islets of coral rise with nearly perpen dicular sides from a depth of 2,000 ft. and upwards, until they reach the surface of the sea.. As the corals could not have begun to grow upwards from such vast depths, Mr. Darwin first suggested that the sites of these coral reefs had undergone a progressive subsidence, the rate of upward growth of the reefs keeping pace, on the whole, with the depression. A fringing reef would first be formed fronting the land within the limit of the 20-fathom line. Growing upward until it reached the surface of the water, it would be exposed to the dash of the waves, which would break off pieces of the coral and heap them upon the reef. In this way islets would be formed which, by' successive accumu lations of materials thrown up by the breakers, or brought by the winds, would remain permanently above water. On these islets palms and other plants, whose seeds might be drifted from the adjoining land, would take root and flourish. Inside the reef there would be a shallow channel of water, communicating through gaps in the reef, with the main ocean outside. Fringing reefs of this character are of common occurrence at the present time. In the case of a continent they front its coast for a long'distance, but they may entirely surround an island. If the site of a fringing reef undergoes depression at a rate sufficiently slow to allow the corals to keep pace with it, the reef will grow upward as the bottom sinks downward. The lagoon channel inside will become deeper and wider, while, at the same time the depth of the water outside will increase. In this way a barrier reef will be formed. Continued slow depression must continually diminish the area of the land inclosed within one of these rings of coral reef, while the reef itself retains much the same size and position. At last the final peak of the original island disappears under the lagoon, and an atoll, or true coral island, is formed. Should any more rapid or sudden downward movement take place, it might carry the atoll down beneath the surface, as seems to have happened at .the Great Chagos bank in the Indian ocean, which is a submerged atoll. It has recently been suggested that barrier reefs do
not necessarily prove subsidence, seeing that they may grow outward from the land, upon a talus of their own debris broken down by the waves, and may thus appear to consist of solid coral, which had grown upward from the bottom during depression, although only the upper layer, 20 fathoms or thereabouts in thickness, is composed of solid, unbroken, coral growth. The explanation may doubtless account for some barrier-reefs, an for the way in which the steep seaward face of all such reefs is formed and maintained. But it does not elucidate the existence of submerged atolls, the pres ence of gaps in atolls answering to gaps in the fringing reefs opposite to the mouths of rivers: and the difficulty of supposing that, in a coral archipelago,,there should have been scores of submerged peaks so nearly of the same height as to rise within 20 fathoms of the surface, and yet so seldom actually to tower above it. According to the simple and luminous theory of Mr. Darwin, every stage in the progress of the changes is open to observation, from the incipient fringing reef to the completed and submerged atoll. Every observed fact fits in harmoniously with the others, and we reach the mpressive conclusion that a vast area of the Pacific ocean, fully 6,000 geographical miles from 'e. to tv., has undergone a recent subsidence, and may be slowly sinking still. It by no means follows, however, as some writers have imagined, that the presentPacilic ocean occupies the site' of a vast submerged continent. imagined, the coral islands seem tohave been built on volcanic peaks. ,Wherever any non-ea]careous rock appears it is of volcanic origin. We must therefore conceive of these oceanic islands as detached vol canic eminences rising out of a wide area of subsidence, and doubtless as deriving their existence from the results of that subterranean movement.
These movements, without question, we must again trace back to consequences of the original heat of the earth. There are various ways in which the heat may have acted. Titus a considerable accession of heat expands rocks; and, on the other hand, a loss of heat causes them to contract. We may suppose, therefore, that, during the subterranean changes, a veal extent of the crust underneath a tract of land may have its temperature slowly raised. The effect of this increment would be to cause a slow uprise of the ground above. The gradual transference of the heat to another quarter might produce a steady subsidence. Such variations in subterranean temperature, how ever, could give rise at the most to very insignificant elevations or depressions. A far more important and generally effective cause is to be sought in the secular contraction of the globe. If our planet has been steadily losing heat by radiation into space, it must have progressively diminished in volume. The cooling implies contraction. According to Mr. Mallet, the diameter of the earth is less by at least '189 ra, since the time when the planet was a mass of liquid. But the contraction has not manifested itself uniformly over the whole surface of the planet. The crust varies much in struc ture, in thermal resistance, and in the •position.of its isogeo-thermid lines, As the hinter nucleus contracts more rapidly by cooling than the cooled and hardened crust, the latter must sink down by its own weight, and in so doing must accommodate itself to a con- • tinually diminishing descent of the crust gives rise to enormous tangential pressures. The rocks are crushed, crumpled, and broken in many places. Subsidence must have been the general rule,'but every general subsidence would doubtless be accom panied with local upheavals of a more limited kind. The positions of these upheaved tracts would largely depend upon the original structure of the crust. The action would occur in lines which, once taken as lines of weakness or relief from the intense strain, would probably be used again and again at successive paroxysms or more tranquil periods of contraction. Mr. Mallet has ingeniously connected these movements with the linear direction of mountain chains, volcanic vents, and earthquake shocks.
Mountains may arise from three causes: 1, from the corrugation of the earth's crust due to the effects of secular contraction; 2, from accumulation of materials poured out volcanic orifices; and, 3, from isolation of elevated masses of ground, owing to the removal, by denudation, of the materials originally connecting them, and to the conse quent formation of.valleys. Mountains formed in the volcanic way are almost always conical, and are either solitary, as Etna, or occur in linear groups, like the volcanoes of Java. Those formed by denudation are of minor dimensions, and deserve rather the name of hills. Mountain-chains, on the other hand, which are the dominant features of the earth's surface, though they may have lines of volcanic vents along their crests, are not formed essentially of volcanic materials, but of. the sedimentary and crystalline rocks of the crust been ridged up into.vast folds. If the continental lands may be compared to great undulations of the solid surface of the globe, the mountain chains may be likened to the breaking crests of such wave-like movements. In their internal structure, mountain-chains bear witness to the intense crumbling of the rocks during the process of upheaval. As a consequence of the uprise of two or more paral lel ranges of mountains, lines of longitudinal valleys must be produced. But no sooner is a mass of land raised above the sea than it is exposed to the attacks of air, rain, frost, springs, glaciers, or other meteoric agents of disintegration. Its surface is then worn down, the flow of water down its sides cuts out gulleys, ravines, and valleys, so that eventually a very rugged surface is produced, on which, probably, no portion of the original surface of upheaval may remain, but where new lines of minor ridge and val ley may appear as the combined result of internal geological structure and atmospheric denudation. During the movements by which mountain masses have been upheaved, the stratified rocks have been so compressed as to occupy, in many cases, but a small proportion of thd horizontal extent over which they originally extended. They have adjusted themselves to this diminished area by undergoing intense plication, and thus acquiring a much greater vertical depth. On the other Claud, they have been abund antly fractured, some portions of their mass being pushed up, others being let down, so that the crust is traversed with a kind of complicated network of fissures. • The greater part of the geological changes are produced by agencies active at the earth's surface. These agents are material and visible, and we • can see and feel their action. The movements of the air; evaporation from land and rain, hail, and snow; the flow of rivers and glaciers; the tides, waves, and currents of the ocean; the growth and decay of organized existence ou land and sea; the whole circle of move ment now in progress must come into view. Epigene is suggested as a convenient term for this visible action, antithetical to hypogene, or subterranean action, already consid ered. A simple arrangement of this part of geological dynamics will be in three sec tions: 1. Air—the influence of the atmosphere in forming and destroying rocks; 2. Water—the geological functions of the action of the sea and of the circulation of water through the air, and between sea and land; 3. Life, or the part taken by plants and animals in preserving, destroying, and reproducing geological formations. The words destructive, reproductive, and conservative, employed in describing the operations of the epigene agents, do not necessarily imply that anything useful to man is destroyed, reproduced or preserved. On the contrary, the destructive action of the atmosphere may turn barren rock into rich soil, while its reproductive effects sometimes turn rich land into barren desert. Again, the conservative influence of vegetation has sometimes for centuries retained as barren morass what might otherwise have become rich meadow or luxuriant woodland. The terms are used in it strictly geological sense, to denote the removal and reproduction of material and its agency in preserving what lies henenth it.