612 PHYSIOGRAPHY 



selves to considering surface forms not merely as something existent, 

 but as something that has developed, that we may speak of a geo- 

 morphology which forms a scientific branch of study under physical 

 geography. There are two ways of considering the genetic character 

 of surface forms: (1) One may consider the separate forces at work 

 on the earth's surface and trace out the forms resulting from the 

 activities of each; or (2) we may attempt to analyze the rich collec- 

 tion of forms already present. Both methods of consideration bring 

 us into intimate relations with geology, for in either case the forms 

 which are investigated touch upon changes which the earth's crust 

 has suffered or is suffering. In the latter case, however, one enters 

 the proper field of geology. 



Geology early began to investigate more carefully the forces at 

 work upon the earth's surface. Lyell was very instrumental in 

 establishing the principle that the deposits resulting from the activi- 

 ties of these forces give the best key to an understanding of the rocks 

 which make up the earth's crust, and in this way those deposits 

 have been investigated. It would not have been possible to investi- 

 gate these deposits, if their mode of origin had not been taken 

 account of. The wide distribution of the phenomena of erosion was 

 recognized, and notice even was taken of certain typical forms, but 

 the process which resulted in those forms did not obtain further 

 recognition. It remained for the newer science of geomorphology to 

 do this. This science, in studying the development of streams, dis- 

 covered the sequence of forms which accompanies that development. 

 A stream running down an existing slope cuts a furrow which we 

 designate as a consequent form, because it follows the slope already 

 present. Scarcely is this furrow cut than its steep walls begin to 

 assume more gradual slopes, and the surface waters develop new 

 furrows down these new slopes. These new furrows, coming after 

 the original furrow, we call subsequent forms. Their development 

 follows other rules than those controlling the consequent forms. The 

 latter develop upon existing or original slopes, the former take their 

 origin on slopes of later development, whose courses are essentially 

 dependent upon rock-character. Professor W. M. Davis has specially 

 studied these developments, and has shown how the subsequent 

 forms adjust themselves to the character of the earth's crust, and 

 more especially to its structure. He has shown how a gradual adjust- 

 ment is brought about between the watercourses originally con- 

 sequent upon the existent slopes, and the internal mountain struc- 

 ture. The first position of the original surf ace, forming the surface of 

 departure (ausgangsftdche) for later development, was the result 

 of unequal elevations of the earth's crust. The modeling processes 

 of erosion transform this surface into a surface of adjustment, which 

 offers to further denudation greater and greater resistance, until it 



