feet from the surface, is one degree Fahr., which would give us, at a depth of 125 miles, sufficient heat to melt most of the rocks. This intense internal heat has generated, in times long gone by, enormous forces, by which rocks of all ages have been raised and depressed, twisted and distorted, broken and forced out of position, and forcibly compressed, so as to eventually cause most important changes of surface level. The next class of rock-formation is totally different from the Plutonic, or unstratified series, in that it is the result of the wear and tear of the surface when acted upon by air and water, and is laid down, in the first instance, by water, as sediment. Water, in the forms of seas, rivers, rain, and ice, has been the chief agent in the arrangement of all the stratified rocks, the determination of the earth’s contour, the direction of valleys, and, in fact, the regulation of the whole physical geography of the visible portion of the earth. With the help of this mighty agent, so soon as the earth had become sufficiently cool to permit condensation to take place in its vapoury envelope, the ceaseless wear and tear of the Plutonic—and, subsequently, of all other—rocks, which has accumulated so vast a mass of material, commenced. Large volumes of water were gradually deposited, without intermission, until permanent seas and rivers had become established, and the new process of stratification, which was henceforth destined to shape the crust of the earth and to provide the conditions of life, commenced to operate. This action is taking place daily in rivers and seas, as we may observe at any time. On the tops of mountains the same action is in operation, though under different conditions, snow and ice splitting fragments from the rocks to be borne away as grit into the valleys by impetuous torrents and deposited in other places. Within the Polar circles ice on a grander scale is levelling down the land; glaciers, covering thousands of square miles, are slowly sliding down the valleys, grinding their surfaces still deeper—forming sands, clays, and gravels, and forcing these down to the sea-shore; and icebergs, many miles in circumference, are carried by currents along coasts and against cliffs like huge ploughs, completely altering the face of the rocks beneath. This wear and tear results in the formation of immense quantities of detritus, which is deposited in layers at the bottom of seas and rivers, and consolidated by pressure, being frequently assisted by lime, iron, or silica as a cement. The coarser-textured rock has been laid down in rapidly-moving, shallow water; and the finer-textured in still, deep water. Thus, through many long ages—probably millions of years—the surface of the earth underwent continual change from the constant deposition of stratified rock, each layer of which completely buried beneath it the various life forms of the previous period, which circumstance enables us to ascribe to