Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/271

Rh UPIIEAVAL AND DErnEssIoN.] would prove a former long-continued depression, 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 sind and mud lying between them. In short, the evidence would be strictly on a parallel with tl1at furnished by a succession of raised beaches as to a former protracted elevation with lo11g pauses. An interesting kind of proof of an extensive depres- sion of the north-west of Europe is furnished by the deep fjords or sea—lochs 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 coasts of the British Isles furnish many excellent examples of fjords. In Scotland they are usually called lochs, as Loch Hourn, Loch Nevis, Loch Fyne, Gareloch; in Ireland they are sometimes known by the name of loughs, as Lough F oyle, but more commonly by that of bays, as Dingle Bay, Bantry Day. 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 exca- vated by subaerial agents is a conclusion borne out by a great weight of evidence. If, therefore, we admit the subaerial origin of the glen, we must also grant a similar origin to its seaward prolongation. Every fjord will thus mark the site of a submerged valley. This inference is Cl nﬁrmed by the fact that fjords do not, as a rule, occur singly. Like the glens on the land they lie in groups ; so that when they are found intersecting a long line of coast like that of the west of Norway, or the west 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 widespread depression over the area of the Paciﬁc Ocean is furnished by the numerous atolls or coral islands scattered throughout that vast expanse of water. Mr Darwin ascertained that the reef-building corals do 11ot live at a greater depth than about 15 or 20 fathoms. Yet reefs and circular islets of coral rise with nearly per- pendicular sides from a depth of 2000 feet and upwards, until they reach the surface of the sea. As the corals could not have begun to grow upward 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 11ntil 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 accumula- tions of materials thrown up by the breakers or brought by 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 GEOLOGY 257 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 enclosed 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 the top of 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 thick- ness, is composed of solid, unbroken, coral growth. The explanation may doubtless account for some barrier-reefs, and 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 presence of gaps in atolls answering to gaps in the fringing reefs opposite to the mouths of rivers ; and the diﬂiculty of supposing that, in a coral archipelago, there should have been scores of sub- merged 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 impres- sive conclusion that a vast area of the Paciﬁc Ocean, fully 6000 geographical miles from east to west, has undergone a recent subsidence, and may be slowly sinking still. It by no means follows, however, as some writers have imagined, that the present Paciﬁc Ocean occupies the site of a vast submerged continent. All the coral islands seem to have been built on volcanic peaks. Wherever any non- calcareous rock appears it is of volcanic origin. We must therefore conceive of these oceanic islands as detached volcanic eminences rising out of a wide area of subsidence, and doubtless as deriving their existence from the results of that subterranean movement. § 3. Causes qf Seczclar U-pkecu.-a.l and Depression. These movements, without question, we must again trace back to consequences of the original internal heat of the earth. There are various ways in which the heat may have acted. Thus a considerable accession of heat expands rocks, and, on the other hand, a loss of heat causes them to con- tract. Ve may suppose therefore that, during the sub- terranean changes, a great 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, however, could give rise at the most to but 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 miles since the time when the planet was a mass of liquid.‘ But the con- traction has not manifested itself uniformly over the whole 1 Phil. Trans, 1873, p. 250. _ X. — 33