PHYSICAL GEOGRAPHY IN THE UNIVERSITY. 79 



foresight regarding the difficulties that these courses may involve 

 in the process of valley-trenching. The load that they have to 

 carry is relatively light ; being only the waste that creeps and 

 washes down the broad constructional slopes, under the guidance 

 of the unconcentrated drainage. 



In youth and adolescence, the drainage lines are increased in 

 number and greatly improved in their ability to gather and dis- 

 charge the rainfall quickly. Numerous little trenches are incised 

 in the broad constructional surface, and the distance that the land 

 waste washes and creeps under the guidance of unconcentrated 

 drainage is much lessened ; delay in lakes is decreased ; the 

 steep lateral slopes of the young consequent valleys furnish an 

 increasing amount of load to the streams, although they still as 

 a rule have carrying power to spare in their impetuous currents. 

 A good beginning is made in the search for the best location of 

 subsequent streams. As the subsequent streams are better devel- 

 oped in later adolesence, the original broad constructional forms 

 are minutely carved, many subsequent divides are established, 

 the discharge of rainfall is very prompt, and the load of waste 

 that the streams have to carry is notably increased. 



In maturity the relief retains much of the intensity of adoles- 

 cence, and adds thereto a great variety of features. The valley 

 lines are closely adjusted to the structure of the region, this con- 

 dition having been gained by a delicate and thorough process of 

 natural selection, in which the most suitable drainage lines sur- 

 vive, and the less suitable ones are shortened or extinguished. 

 The impetuosity of youth has disappeared ; all the larger streams 

 have developed grades on which their ability to do work is nicely 

 adjusted to the work that they have to do ; the lower courses 

 already show signs of age, while the' upper twig-like branches 

 are relatively youthful. The whole drainage system is earnestly 

 at work in its task of baseleveling the region, and the forms 

 that the region has assumed bear witness to the close search 

 made by the streams for every available line of effective work. 



From this time onward, there is a general fading away of 

 strength and variety, both of forms and a-ctivities. The deepen- 



