CHANGES IN THE LEVEL OF THE LAND. Among the most far-reaching results of geological study is the proof that the earth's surface is not stable at the present time, and that a similar condition has existed in all periods of the past. Again and again stratigraphic geology tells of changes in land-level of stupendous nature; and studies in dynamic geology have proved that similar changes are now in progress in many parts of the world. In some places the movement is an up rising of the land, in others a downsinking; and these movements in some cases affect broad areas of the crust in a slow uprising or downsinking, while in other cases the movement is localized and spasmodic. These latter movements are usually associated with mountain growth, earth quakes, or volcanic eruptions, and over a limited area the level of the land may change several inches, or even feet, in a few minutes. The move ments affecting large areas are so slow that care ful study is necessary to prove their existence.
Many instances of land movement now in prog ress might be given. The coast-line of New Jersey is sinking at the rate of about two feet a century; the coast of Labrador is rising at an unknown rate; the coast of West Greenland is sinking; in Sweden, records of 150 years show that the region south of Stockholm is sinking, while to the north the land is rising, in one place having risen seven feet in that period. Local rapid movements of the land were observed in Japan during the earthquake of 1891; changes of level, both uprising and downsinking, have oc curred in the Bay of Naples; the coast of Chile has been uplifted during earthquakes in the last century. Evidence of changes of level in past ages is furnished by elevated beaches, raised beaches that are no longer horizontal, and sub merged forests. The irregular coast-line of parts of continents, as in Northeastern America, is in terpreted as a drowned coast, where, by land sinking, sea-water has been allowed to enter the valleys, forming fiords. In some cases the con
tinuation of the land valleys may be traced along the sea bottom, as in the case of the Hudson River (q.v.).
The question has naturally been raised as to whether these changes are due to land movement or to changes in sea-level. Some of them, as in Sweden, where the movement is differential, and the spasmodic movements in limited areas, are certainly due to land movement. With regard to others, the conclusion is not so certain, though the geological evidence all points toward a change in the land rather than of the sea.
The cause for the instability in the crust ia believed to be the heated condition of the earth's interior. Various hypotheses have been proposed to account for the exact manner in which this heated condition causes change in level. The most currently accepted hypothesis is that of contraction, by which it is held that, through loss of heat, the interior is shrinking, and the crust, in accommodating itself to the shrinking interior, is caused to move. Another hypothesis is that of isostasy. This explains crust movement by assuming that variations in the load on the crust cause movements. The reduction of load by denudation of the land and the increase of load in places of sedimentation necessitate an isostatic readjustment, causing sinking is one place and rising in another, as there would be in a pile of wax of irregular height. This hypothesis as sumes a plasticity for the earth which many geologists are not willing to accept. Other hy potheses have also been proposed, but space for bids their discussion here.