The hollows formed on the surface of the earth by the ground sinking or rising, earthquakes, streams of lava, the intersection of strata, and those that occur along the edges of the different formations, are generally filled with water, and constitute systems of lakes, some salt and some fresh. Many of the former may be remnants of an ancient ocean left in the depression of its bed during its retreat as the continents arose.

Almost all lakes are fed by springs in their beds, and they are occasionally the sources of the largest rivers. Some neither receive tributaries nor have outlets: the greater number do both. The quantity of water in lakes varies with the seasons everywhere, especially from the melting snow on mountain chains and high latitudes, and between the tropics from periodical rains. Small lakes occur in mountain passes, formed by water which runs into them from the commanding peaks: they are frequently, as in the Alps, very transparent, of a bright green or azure hue. Large lakes are common on tablelands and in the valleys of mountainous countries, but the largest are on extensive plains. The basin of a lake comprehends all the land drained by it; consequently it is bounded by an imaginary line passing through the sources of all the waters that fall into it.

There are more lakes in high than in low latitudes, and in this respect there is a great analogy between the northern plains of the two principal continents.