three - quarters of the rain that falls sinks into the ground, where it is used by the plants and in part makes its way by underground channels till it reaches an outlet where it can escape to the surface again as a spring, or descends lower and lower and becomes absorbed in rockforming minerals.
Rivers. — If we imagine a continent to have newly risen above sea level and for the first time to have been exposed as dry land, the surface will be a featureless plain and the first rain that falls will flow away in broad sheets of water to the ocean. As time goes on, the inequalities of the surface will be taken advantage of, and the flowing water will gradually follow definite channels, which in course of time it will deepen by carrying away the mud and sand, so that the course of the river will become more or less fixed. The course of the early rivers will be a meandering one, as there will not be much fall to induce the water to rush straight to the sea. In general the rivers will form systems leading by common main channels to the ocean, the smaller branches being called tributaries of the larger channels. The area of one stream system is a drainage area, or if regard be had to the rain that falls it may be called the catchment area. The ridge surrounding one catchment area is called the watershed. The level of the sea forms a base-level below which the river cannot cut, so that if the supposed newly formed continent is only elevated slightly above the sea, and remains stationary for a long while, the rivers will become stagnant and tend to form lakes, marshes, and great winding curves in their courses. If now the continent rises again, the rivers immediately will tend to cut downwards. The rivers will still follow the courses deter-
