It is therefore probable that for the last 96,000,000 years the rate of increase of temperature under ground has gradually diminished from 1/10 to about 1/50 of a degree Fahrenheit per foot, and that the thickness of the crust through which any stated degree of cooling has been experienced has gradually increased during that period from 1/5 of its present thickness to what it now really is."[150] We freely admit our inability to sit in judgment on the validity of Sir William Thomson's conclusions. There are eminent geologists who entertain the opinion that the secular cooling of the earth has proceeded with much greater rapidity. It is, however, sufficient for our purpose that the most distinguished physicists of the day are agreed in teaching that _the existing terrestrial economy had a beginning_. There are other terrestrial changes which engage the attention of the geologist, and which force upon him the conclusion that the existing terrestrial order had a beginning and must have an end. The surface of the earth has at intervals undergone great changes in the disposition of its land and water. That which is now dry land was once the ocean-bed, and the ocean waves now roll and murmur over what was once dry land. Sudden, or comparatively sudden, catastrophes have extinguished the then existing creations, and the earth has been repeopled by new orders of life. Changes are now in progress which are gradually reducing the populous regions of the earth to the condition of the Sahara of Africa and the Desert of Arabia. Upper and Lower Mesopotamia, the seat of the ancient monarchies of Chaldæa, Assyria, and Babylonia, now present "vast tracts of arid plain--yellow, parched, and sapless--which have now become a bare and uninhabited desert." That ancient continent drained by the Colorado, once as fertile as the Valley of the Mississippi, is now the Great American Desert. "Every freshet burdens the streams with a load of sediment; and the Mississippi bears daily to the Gulf material sufficient for a cotton plantation. From the slopes of the Alleghanies and the Rocky Mountains, from the broad acres over which the Mississippi and the Ohio reach their silver fingers to filch from the land, the sediments are stolen and carried away to the sea. The Western States are slowly traveling toward the Gulf. The hills are melting, and even the mountain cliffs are lowering under the ceaseless conflict with storm and frost. The summits of the Alleghanies have come down 3000 feet from their original altitudes. Give time enough, and the inequalities of the land will disappear. The ocean will be filled, and again assert a triumph over the continents which in the beginning were wrested from his dominion." Thus by the storms of heaven, the erosion of the atmosphere, the blasting power of frost, the gnawing of the tidal wave, the mountains are being leveled, and the rocks and soils carried onward by the rivers to fill up the basin of the sea. The headlong rush of the