78 THE JOURNAL OF GEOLOGY. 



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 con- 

 tinues 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 decreas- 

 ing load, and maintaining an equilibrium. Local examples of 

 thi» 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, maturit}^ 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 differ- 

 ent 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 



