Myriads of animals that form calcareous shells live and die in the ocean. The shells of the dead animals are falling like a perpetual shower on certain ocean bottoms, year after year, so that immense accumulations of calcareous substances occur there, which will also in time be consolidated into limestone rocks, and uplifted as dry land. Deltas are forming at present at the mouths of certain rivers, and estuaries at others. Lands are now gradually emerging from the sea in some places and at others sinking into the sea. The entire coast of Scandinavia, both on the Baltic and Atlantic sides, is rising out of the sea, and has been doing so for a long time. It is rising at the rate of more than two feet in a hundred years. During an immense period of time there has been a gradual elevation of all the southern part of the South American continent. Sometimes a large elevation takes place rapidly. In 1822, and again in 1835, the southwest coast of South America, after severe earthquakes, was elevated several feet along a distance of several hundred miles. It is known that the coast of Greenland, for five hundred miles, is subsiding. From a study of coral barriers and atolls it is believed that an area of the mid-Pacific sea bottom covering over ten million square miles is sinking and has been doing so for a long time. The foregoing facts tend to illustrate the truth that nothing is permanent in the environment of living creatures at present. All surroundings are perpetually changing, though apparently ever so slowly. The changes that are now going on were also taking place yesterday, last week, last century, last æon, and so on throughout geologic time. Gradual oscillations of the earth’s crust on a grand scale and affecting whole continents, but usually so slowly as to escape popular observation, have been taking place ceaselessly through inconceivable ages. These oscillations have produced all the great inequalities of the earth’s surface, such as ocean basins, continents and mountain chains. The oscillations are probably due to the slow cooling and unequal shrinking of the whole earth which has been progressive through all geologic time. The state of the contest between the eroding and the uplifting agencies of the world at any time determines the height of mountains and continents, the depths of seas, the distribution of land and water, for that period. [Illustration: FIG. 11.—Archæan North America. The white part of the drawing indicates the emerged land; the dark shading indicates the submerged land covered by a shallow sea; the light shading indicates the deep sea. From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.]