12 W. M. DAVIS 



Embodied in the first principle of the cycle — that the erosion of 

 the earth's surface has been accomplished in successive intervals of 

 time marked off by movements of upheaval — is a closely associated 

 second principle to the effect that, during the epochs of rest between 

 the movements, the destructive action of the sea waves on the shore 

 line, or of weather and streams on the entire extent of an upHfted 

 region, may reduce it to a low and nearly featureless surface; a 

 plain of marine abrasion in the one case, a plain of subaerial degrada- 

 tion in the other. An extension of this second principle allows it 

 to include, besides the two just named, other kinds of destructive 

 processes : weather and glaciers, weather and wind (with occasional 

 rain), weather and solution; the latter process finds appHcation 

 particularly in limestone regions and has been worked out chiefly 

 by European observers in the Karst region east of the Adriatic. 

 It may be noted in passing that the reduction of a forest-covered 

 highland to a lowland by the ordinary agencies of subaerial degrada- 

 tion is chiefly accomplished, after the streams have cut their 

 valleys down to grade and after the valley sides have become 

 covered with a sheet of locally weathered soil, by the very slow 

 process of soil creep, attention to which was first directed in this 

 country, as far as I have read, by Lesley in his admirable little 

 book on ''Coal and its Topography" (1856). 



The most striking illustrations of these two principles are found 

 in regions which, during an earlier cycle of erosion, had reached 

 peneplanation or extensive abrasion, and which are now, after a 

 later uphft of simple character, in so early a stage of renewed 

 dissection as to preserve in their unconsumed uplands little modified 

 remnants of their former lowlands of small reHef . Many examples 

 of this kind are now known; indeed a good number of the greater 

 mountain ranges of the world seem to be in a later cycle of erosion 

 than the one introduced by their deformation, and to have suffered 

 far advanced erosion in the cycle preceding the one now current. 

 On the other hand, many other regions show that the movement 

 or deformation by which a later cycle is introduced may interrupt 

 an earlier cycle at any stage of its advance; and also that the 

 interrupting deformation may be of any kind, small or great, simple 

 or complex, slow or rapid, brief or long-enduring. Finally, it 



