1902.] DAVIS — SYSTEMATIC GEOGRAPHY. 243 



ments in response to which many an organic condition has been 

 called forth. 



The lands need subdivision into relatively small areas, for their 

 forms vary greatly from place to place. It has long been habitual 

 with geographers to describe and classify these forms empirically ; 

 but there is to-day a well-defined trend of opinion in favor of 

 rational, evolutionary or explanatory description and classification, 

 even though this more modern method has not yet found general 

 acceptance in practical exploration. An eclectic system of sub- 

 division, based on the suggestions of various writers, may be briefly 

 stated as follows ; 



7. Classification of Land Forms. — Land forms are classed first as 

 to kind, according to their rocky structure ; thus one area may be 

 of horizontal structure; a second may consist of broken and 

 tilted blocks ; a third may have a domed structure ; a fourth may 

 be folded ; a fifth may be of volcanic origin, and so on. 



Each kind of land form is then to be further classified according 

 to its stage in the cycle of erosion, to which it is introduced 

 by initial processes of deformation and (relative) upheaval, and 

 through which it progresses by the action of weathering and wash- 

 ing towards an ultimate goal of obliteration in a featureless plain 

 close to sea level, or in a smooth platform at an undetermined 

 depth beneath sea level. There is to-day abundant warrant for as- 

 serting that the sequence of developmental stages through this 

 destructive cycle of erosion is remarkably systematic, and that very 

 effective description of land forms may be given by characterizing 

 them simply as young, mature or old. This is therefore not a 

 matter of abstract theory, but of practical convenience to the field 

 geographer. 



There is need of distinction between the inert land mass, offered 

 to erosion by the telluric forces of upheaval, and the physiographic 

 agencies by which the erosion is accomplished; the chief of the 

 latter being river systems. There is again need of discriminating 

 the forms assumed by the slow-moving waste of the land on the 

 way to the sea, from the inert land mass on the one hand, and 

 from the more active agencies of erosion on the other hand. With 

 respect to the active water streams, the land waste is relatively 

 inert and passive; but with respect to the inert underlying rock 

 mass, the waste may be treated as part of the superficial river 

 system. The latter treatment brings forth many interesting homo- 



