568 MARIUS R. CAMPBELL 



PART I. PRINCIPLES OF DRAINAGE MODIFICATION. 



(l) RELATION OF DRAINAGE FORMS TO LAND FORMS. 



The great advancement in the interpretations of physiographic 

 forms which has marked the last decade has led to a better under- 

 standing of the late geologic history of certain continental areas 

 than has ever been attained from the study of the sediments 

 deposited around their margin. Although such important results 

 have been derived from this study in so short a time, they are 

 but scattering chapters in the complex history of continental 

 development, and much yet remains to be done before a clear 

 insight can be obtained into the various conditions of the past. 

 Our success in reading this history lies in following out all of the 

 lines of corroborative evidence available, in the hope that where 

 the evidence along any one line is weak, that along another may 

 be strong and complete, enabling us to build up a more or less 

 perfect whole from the different classes of facts. 



The process of erosion, according to Gilbert,^ consists of 

 three parts : weathering, transportation, and corrasion ; and is 

 modified by three conditions : declivity, character of rock, and 

 climate. Physiographic forms resulting" from the process of ero- 

 sion are necessarily modified by any change in the above men- 

 tioned causes and conditions. But since streams are the principal 

 agents in transportation and corrasion ; and since transportation 

 and corrasion are dependent upon weathering (which is modified 

 by climate), declivity, and character of rock, it follows that any 

 change in the causes or conditions affecting erosion will modify 

 the action of the streams and any modification of the action of 

 the streams will tend to change their alignment in accordance 

 with the changed conditions. 



This intimate relation between stream alignment and physi- 

 ographic forms suggests the advisability of thorough study of 

 drainage systems, in the hope of finding some record of past con- 

 ditions which will throw additional light on the question of the 

 physiographic history of continental areas. 



^ Geology of the Henry Mountains. 



