'296 National Geographic Magazine. 



the rocks have been least energetically acted on, valleys are 

 carved where the action of water has been most effective. 



In order that the process of degradation may go on it is essen- 

 tial that a land mass be somewhat raised above the sea, and, 

 since the process is a never-ceasing one while streams have suffi- 

 cient fall to carry sediment, it follows that, given time enough, 

 every land surface must be degraded to a sloping plain, to what 

 has been called a base level. 



With these ideas of mountain genesis and waste, let us con- 

 sider some phases of degradation in relation to topographic 

 forms ; and in doing so I cannot do better than to use the terms 

 employed by Prof. Wm. M. Davis. 



When a land surface rises from the ocean the stream systems 

 ■which at once develope, are set the task of carrying back to the 

 sea all that stands above it. According to the amount of this 

 alloted work that streams have accomplished, they may be said 

 to be young, mature or aged ; and if, their task once nearly com- 

 pleted, another uplift raise more material to be carried off, they 

 may be said to be revived. These terms apply equally to the 

 land-surface, and each period of development is characterized by 

 'certain topographic forms. 



In youth simple stream systems sunk in steep walled canons 

 are separated by broad areas of surface incompletely drained. 

 In maturity complex stream systems extend branches up to every 

 part of the surface ; steep slopes, sharp divides, pyramidal peaks 

 express the rapidity with which every portion of the surface is 

 attacked. 



In old age the gently rolling surface is traversed by many 

 quiet flowing streams ; the heights are gone, the profiles are 

 rounded, the contours subdued. In the first emergence from the 

 sea the courses of streams are determined by accidents of slope, it 

 may be by folding of the rising surface into troughs and arches. 

 During matui'ity the process of retrogressive erosion, by which 

 a stream cuts back into the watershed of a less powerful opponent 

 stream, adjusts the channels to the outcrops of soft rocks and 

 leaves the harder strata as eminences. In old age this process 

 of differential degradation is complete and only the hardest rocks 

 maintain a slight relief. 



Suppose that an aged surface of this character be revived : the 

 rivers hitherto flowing quietly in broad plains will find their fall 

 increased in their lower courses ; their channels in soft rock will 



