Although we have for convenience termed the earth a flattened spheroid, it is only such in a very general sense. It has an infinite number of minor irregularities which it is the province of the geographer to trace and that of the geologist to account for. In the first place, its surface is occupied by a great array of ridges and hollows. The larger of these, the oceans and continents, first deserve our attention. The difference in altitude of the earth's surface from the height of the continents to the deepest part of the sea is probably between ten and eleven miles, thus amounting to about two fifths of the polar flattening before noted. The average difference between the ocean floor and the summits of the neighbouring continents is probably rather less than four miles. It happens, most fortunately for the history of the earth, that the water upon its surface fills its great concavities on the average to about four fifths of their total depth, leaving only about one fifth of the relief projecting above the ocean level. We have termed this arrangement fortunate, for it insures that rainfall visits almost all the land areas, and thereby makes those realms fit for the uses of life. If the ocean had only half its existing area, the lands would be so wide that only their fringes would be fertile. If it were one fifth greater than it is, the dry areas would be reduced to a few scattered islands. From all points of view the most important feature of the earth's surface arises from its division into land and water areas, and this for the reason that the physical and vital work of our sphere is inevitably determined by this distribution. The shape of the seas and lands is fixed by the positions at which the upper level of the great water comes against the ridges which fret the earth's surface. These elevations are so disposed that about two thirds of the hard mass is at the present time covered with water, and only one third exposed to the atmosphere. This proportion is inconstant. Owing to the endless up-and-down goings of the earth's surface, the place of the shore lines varies from year to year, and in the geological ages great revolutions in the forms and relative area of water and land are brought about.