Page:Popular Science Monthly Volume 54.djvu/488

470 accomplished. It is to be hoped that our observations have made plain the processes of rock disintegration and water transportation; that in the oceans all these materials are eventually deposited in beds horizontally arranged, composed of such products of decay in the condition of sand and mud. We have only to point out the proof that great land masses, composed of water-deposited materials, have been lifted from the ocean to become continents and mountain ranges.

As the ocean deposits slowly accumulate in layers to beds of many thousands of feet in thickness, the lower parts are gradually subjected to greatly increased pressure produced by the overlying beds. During this time waters of a varying temperature, carrying, chemically dissolved, great quantities of lime, silica, and iron oxide, are allowed free circulation through them. These conditions promote chemical change: much silica (the mineral quartz), lesser amounts of carbonate of lime (the mineral calcite), and iron oxide are precipitated about the loose sand grains, firmly cementing them together into a solid rock. A cycle has thus been completed; the dense rocks composing a continent have passed by the process of weathering into incoherent sand and clay, which, when transported to the ocean floor, become again converted into solid rock.

Historical records prove that during the last three thousand years there have taken place many changes in the ocean's level. Old islands have disappeared; new ones have emerged above the surface of the water. Great stretches of seacoast exist at the present time which within the historical period have been covered by the ocean. Even at the present writing we are witnessing the gradual submergence of some parts of the earth and the rising of others; terraces on the northern Atlantic coast may be seen along the hillsides many feet above the present level of the ocean—all of which go to show that the relationship of the land to the water is an unstable one. These are the evidences of continental growth and depressions from the historical standpoint, and the validity of the data upon which the belief is founded can not be shaken. The evidence from the geological side is -overwhelming, but before we speak of this it will be well once more to say a word as to the causes of continental uplift.

From an original fluid globe possessing a high temperature, the earth has now cooled down to a degree sufficiently low to permit the formation of a thick rock crust. Underneath this crust an approach to the old surface temperatures is still maintained, and the existence of a certain degree of fluidity is demonstrated to us from time to time by the phenomenon of volcanism. Successive zones of cooling took place. The outer part could only conform to a shrinking interior by wrinkling, folding, or bodily lifting considerable areas above the general level. An adjustment of strains thus set up would take place