24 W. M. DAVIS 



with the many second-cycle mountain ranges which are produced 

 by the upKf t and dissection of a peneplain that had been worn down 

 on a mountain mass of disordered structure in an earlier cycle. 



Objection has also been made by German geographers to the 

 use of the organic terms, young, mature, and old, to represent 

 successive phases of an erosion cycle, because they insist on inter- 

 preting them to mean age in time-measure, instead of stage in 

 development. Yet if two geologists, habituated to use young and 

 old as time-measures in relation to geological formations, were 

 walking across country, and one of them said: "See that young 

 oak," and a moment later added: "Look at this old mushroom," 

 neither of them would for a moment imagine the oak to have Kved 

 a shorter time than the mushroom. The young oak might, indeed, 

 be hundreds of times longer-hved in time-measure than the old 

 mushroom. Hence if the organic terms are used in their develop- 

 mental sense, there should be no difficulty in understanding what is 

 meant by them. 



One of the most common grounds for objecting to the scheme 

 of the erosion cycle seems to be a general misunderstanding of its 

 object. Several European geographers have misconceived it as a 

 rigid scheme, to which the varied facts of nature must be forced to 

 conform, instead of as an elastic scheme, readily modified to conform 

 to the varied facts of nature. It has been misunderstood as always 

 demanding a rapid or sudden upheaval, so rapid or sudden that 

 practically no erosion could take place until the upheaval was 

 accompHshed. Yet slow upheaval movement with accompanying 

 erosion is manifestly as easily postulated as rapid upheaval. Others 

 seem to have supposed the scheme to present final and infallable 

 conclusions; and on discovering an error or omission, they feel 

 that the scheme must be discarded in its entirety. In my own case, 

 at least, the scheme has been a growth, and its growth is by 

 no means completed. Moreover, however many modifications, 

 improvements, and extensions the scheme may now or later receive, 

 it should be remembered that they are all based upon the valid 

 principles of the scheme already established, and that but for 

 the previous establishment of those principles the improvements of 

 the scheme could not be made. Such modifications and extensions 



