Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/270

Rh 256 apex, have been proved to have been recently nplreaverl. ! The proofs of this change of level chiefly to be relied on I are the following‘ :— (1.) The position of rocks covered with barnacles or other littoral adherent animals, or pierced by lithodomous shells. A single stone with these creatures on its surface would not necessarily prove anything, for it might be cast up by a storm ; but a line of large boulders, which had evidently not been moved since the cirripedes and rnollnscs lived upon them, and still more a solid cliff with these marks of littoral or sub-littoral life upon its base, now raised above high- water mark, would be sufficient to demonstrate a rise of land. The amount of the upheaval might be pretty accurately. determined by measuring the vertical distance between the upper edge of the barnacle zone upon the upraised rock, and the limit of the same zone on the present shore. (2.) A line of sea-caves, now standing at a distance above high water-mark beyond the reach of the sea, would afford I evidence of recent uprise, since caves of this kind are only hollowed out by the waves between tide—1narks. One of the most striking proofs of upheaval is frrrnished by what are termed 7-aiserl beaches. A beach is the space between tide-marks, where the sea is constantly busy producing sand and gravel, rningliug with them the remains of shells and other organisms, sometimes piling the deposits up, sometimes sweeping them away out into opener water. The terrace or platform thus formed is a well- marked feature of coast-lines skirting tidal seas. When the land rises with sufficient rapidity to carry up the line of littoral deposits above the reach of the waves, the ﬂat terrace thus elevated is known as a raised beach. The former high-water mark then lies inland, and while its ' sea-worn caves are in time hung with ferns and mosses, it furnishes itself an admirable platform, on which meadows, ﬁelds, and gardens, roads, houses, villages, and towns spring up, while a new beach is made below the margin of the uplifted one. Raised beaches abound round many parts of the coast-line of Britain. Some excellent examples occur in Cornwall and Devon. The coast—line on both sides of Scotland is fringed with raised beaches, sometimes four or : ﬁve occurring above each other, at heights of 25, 40, 60, 75 and 100 feet above the present high-water mark. Each terrace marks a former lower level of the land with regard I to the sea, and probably a lengthened stay of the land at that level, while the intervals between them represent the vertical amount of each successive uplift of the land, and show that the land in its upward movement did not remain long enough at intermediate points for the formation of terraces. A succession of raised beaches, rising above the present sea-level, may therefore be taken as pointing to a former prolonged upheaval of the country, interrupted by long pauses, during which the general level did not materially change. (4.) Any stratum of rock containing marine organisms, which have manifestly lived and died where their remains now lie, rnrrst be held to prove upheaval of the land. In I this way it can be shown that most of the solid land now visible to us has once been under the sea. Even high on the peaks of the cliffs and the ﬂanks of the Ilirnalaya mountains, undoubted marine shells occur in the solid rocks (5.) In countries which have been long settled by a human population, it is sometimes possible to prove, or at least to render probable, the fact of recent uprise of the land by reference to tradition, to local names, and to works of human (-.onslr‘ru-linn, Piers and luulmurs, if new found to stand above the upper limit of high-water, furnish indeed indisputable evidence of a rise of land since their erec- tion. 1 “Eartllfillakes and Volcano?-9," by A. Glflkie, Chambers's .l[r's- GEOLOGY cellan y of Tracts. [rrr. DY‘.'A.IIC.-L. § 2. Jlovenzcnls Q/' Subsidence. 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. Neverthe- less, the fact of subsidence can be satisfactorily established by the following kinds of pro0f'3:—— (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 disappear- ance of successive ﬁelds, roads, houses, villages, aml 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 skerrics and islets, we infer that the coast-line is sinking. Such kind of evi- dence is found in Scania, the rrrost southerly part of Sweden. Streets, built of course above high-water mark, now lie below it, with older streets lying beneath them, so i that the subsidence is of some antiquity. A stone, the position of which had been exactly determined by Linnaeus ir1 1749, was found after 87 years to be 100 feet nearer the water’s edge. The west coast of Greenland, for a space of more than 600 miles, is pereeptibly 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 surfaee—features are, of course, 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 favourable 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 rrnder 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, f rrll of tree-stnrnps, hazel- nuts, branches, leaves, and other indications of a terrestrial surface, are often found in similar situations. Sir Henry de la Beche has described, round 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 bed at the mouths of valleys, at the bottoms of sheltered bays, and in front of and under low tracts of land, the seaward 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 subrnergence 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-bottorn, be once more laid dry, so as to be capable of examination by boring, well—sinking, or otherwise, they 2 Ibrll.