244 DAVIS — SYSTEMATIC GEOGRAPHY. [Aprils, 



logics between water streams and waste streams, and from this 

 arises a simple terminology for waste forms by which the power 

 that words have of suggesting things is greatly increased. 



It is still further necessary to distinguish between the several 

 kinds of agencies that are chiefly responsible for erosion, as de- 

 termined by climatic conditions. Thus far, a normal climate has 

 been assumed, of sufficient rainfall to fill all depressions to over- 

 flowing and of insufficient snowfall to form glaciers. On one side 

 of this norm there is the arid climate, where rainfall is small and 

 vegetation scanty, and where the wind therefore takes a significant 

 part in the work of shaping the land surface ; here the whole surface 

 swept by the wind corresponds to the bed of a water stream. On 

 the other side is the glacial climate, where precipitation is chiefly 

 in the form of snow and where drainage is chiefly in the form of 

 glaciers ; here the slender and nimble water streaais of the normal 

 climate are replaced by clumsy and sluggish ice streams, with the 

 result of greatly increasing the proportion of drainage channel to 

 drainage area. 



Finally the border of the lands where they dip under the sea is 

 attacked by waves and currents and appropriately carved ; the cycle 

 of shore erosion being just as systematic and helpful as the cycle of 

 rain-and-river erosion. Each kind of land form, as determined by 

 its rocky structure, exhibits forms peculiar to itself and appropriate 

 to their stage of littoral erosion. Here, as in the normal and 

 special cycles of subaerial erosion, such terms as young, mature and 

 old are highly suggestive because of the systematic correlations of 

 various elemental forms that they imply. 



This system of classification is at present by no means fully- de- 

 veloped, for it has been directly applied to but a relatively small 

 part of the lands ; yet it is so efficient where it has been applied 

 that there is every reason to expect that it will be all the more 

 efficient when it shall have been more widely applied and more fully 

 developed. Some of its essential features may now be given fuller 

 exposition. 



8. Physiographic Classification involves Explanation. — Ex- 

 planation of origin is regarded as essential to a complete descrip- 

 tion in this evolutionary method of physiographical classification. 

 Not only must forms of simple and manifest origin, such as sand 

 dunes and stream gorges, be explained ; but all forms, difficult and 

 obscure as their origin may be, must if possible be brought under 



