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They can cut only shallow valleys in the weaker mass, for its waste will be shed so rapidly that a steep slope is needed by the streams to carry the waste away. The contrast between the two areas is strengthened if the region of harder structure has a plentiful rainfall, and the region of weaker structure has a light rainfall. All of these points of difference are with difficulty stated, if the streams are said to have reached baselevel when their carrying power is reduced to equality with their load.

In certain cases, it seems to be possible for a stream to cut down its profile to a gentler grade in its early adolescence than is suitable to later adolescence and maturity. If we conceive that the load offered by the waste from the valley slopes continues to increase after the grading of the stream has been reached, then the grade must be steepened again by the deposition of the excess of load; thus increasing carrying power and decreasing load, and maintaining an equilibrium. Local examples of this relation are often seen in valleys among mountains, where a lateral stream is depositing an alluvial fan in the larger valley that it enters. The larger valley was deepened before the lateral valley had gained a considerable area of wasting slopes; but as the lateral valley grows headwards and discharges an increasing volume of waste, it cannot all be carried by the main stream, and hence the main valley is clogged up, and its grade is somewhat increased.

Stages in the cycle of geographic development.—Following the terminology of organic growth, it is convenient to speak of the successive stages in the geographical cycle as infancy, youth, adolescence, maturity, old age, and perhaps second childhood. Let us consider particularly the activities of the drainage system as determined by the topographic form of a region in its different stages.

In infancy, the rainfall is slowly concentrated from the broad constructional surface; it is only gradually collected into streams; it is often delayed in lakes. Much of it is lost by evaporation, and the ratio of discharge at the river mouth to rainfall over the river basin is relatively low. The initial streams simply adopt the courses offered to them, without the least consideration or