Besides the sudden movements due to earth-shocks, the crust of the earth undergoes, in many places, oscillations of an extremely quiet and uniform character, sometimes of an elevatory, sometimes of a subsiding nature. So tranquil are these changes that they produce from day to day no appreciable alteration in the aspect of the ground affected. Only after the lapse of several generations, and by careful measurements, can they be proved. Indeed, in the interior of a country nothing but a series of accurate levelings from some unchanged datum-line might detect the change of level, unless the effects of this terrestrial movement showed themselves in altering the drainage. It is only along the sea-coast that a ready measure is afforded of any such movement. In popular lan guage it is usual to speak of the sea as rising or sinking relatively to the land. But so long as the volume of the ocean remains the same, the general sea-level can neither rise nor fall, unless by some movement of the solid globe underneath it. And, as we can not conceive of any possible augmentation of the oceanic waters, nor of any diminution, save what may be due to the extremely slow process of abstraction by the hydration of minerals, or absorption into the carth's interior, we are con pellcd to regard the sea level as furnishing a practically constant datum-surface, Any deviation from which, in the apparent heights of sea and land, must be due to movement of the land and not of the sea. There are, indeed, certain comical causes which may affect the relative levels of sea and land. Thus the accumulation of immense masses of snow and ice as an ice cap at one of the poles would tend to displace the earth's center of gravity, and as a consequence, to raise the level of the ocean in the hemisphere so affected, and to dimin ish it in a corresponding measure elsewhere. The return of the ice into the state of water would produce the opposite effect. Dr. Croll has also drawn attention' to the fact that, as a consequence of the diminution of the centrifugal force, owing to the retardation of the earth's rotation caused by the tidal wave, the sea-level must have a tendency to subside at the equator and rise at the poles. A larger amount of land need not ultimately be laid bare at the equator, for the change of level resulthig from this cause would be so slow that the general degradation of the surface of the land might keep pace with it, and diminish the terrestrial area as much as the retreat of the ocean tended to increase it. Dr. Croll has further pointed out that the waste of the equatorial land, and the deposition of the detritus in higher latitudes, must still further counteract the effects of retardation and the consequent change of ocean-level. . Such widespread general causes of change must produce equally far-reaching effects. But in examining the changes of level between land and sea, we find them to he eminently local and vari able in character, pointing to some local and unequally acting cause—so that, while admitting these cosmical and widespread influences to be part of the general system of geological change, we must yet hold the sea-level, for all practical purposes, to be inva riable, any apparent oscillations of that level upon the land being due to terrestrial move mauls.
Various maritime tracts of the land have been ascertained to have undergone in recent times, or to be still undergoing, a gradual elevation above the sea. Thus the coast of Siberia for 600 m. to the e. of the river Lena, the western tracts of South America, and the Scandinavian peninsula, with the exception of a small area at its southern apex, have been proved to have been recently upheaved. The proofs of this change of level chiefly to be relied on 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 animals on its surface would not necessarily prove anything; for it might be cast up by a storm; but a line of large bowlders, which had evidently not been moved since the cirripedes and mollusks 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 laud. The amount of the upheaval might be determined with sufficient accuracy 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 evidences of recent siprise, since caves of this kind are hollowed out only by the waves between tide-marks. (3) One of the most striking proofs of upheaval is furnished by what are termed " raised beaches." A beach is the space between tide-marks, where the sea is constantly busy depositing sand and gravel, mingled with the remains of shells and other organisms, sometimes piling the deposits up, sometimes sweeping them away into the more open water. The terrace or platform thus formed is a well-marked feature of coast-line skirting tidal seas. When the land rises with sufficient rapidity to carry the line of littoral deposits above the reach of the waves, the fiat 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 uplifted one. Raised beaches abound along many parts of the coast-line of Britain. Some excellent examples occur iu Cornwall and Devon. The coast-line on both sides of Sootland is fringed with raised beaches, sometimes four or five occurring in succession at heights of 25, 40, 60, 75, and 100 ft. above the present high-water mark. Such beaches can be traced also in the valley of the Connecticut river in western Massachusetts. Each terrace marks a for mer lower level of the land with regard to the sea, and probably a lengthened stay of the land at that level, while the differences of level indicate 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 (lid not materially change. (4) Any stratum of rock contain ing marine orgatiiiifm-s;,.wliicil th1r direct where tfabir remains now lie, must be held to prove upheafal of the land. In 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 Himalaya mountains, undoubted marine shells occur in the solid rocks. (5) In countries which have beeu 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 construction. Piers and harbors, if now found to stand above the upper limit of high-water, furnish indisputable evidence of a rise of land since their erection.