394 DAVIS — PLAINS OF MARINE AND SUBAERIAL DENUDATION. 



that the relative importance of subaerial erosion iy rated much higher 

 than at the beginning of the century. While the sea works energetically 

 along a line, subaerial forces work gently over a broad surface. Chiefly 

 for this reason Geikie concludes that " before the sea, advancing at a rate 

 of ten feet a centur}^ could pare off more than a mere marginal strip of 

 land between 70 and 80 miles in breadth, the whole land might be waslied 

 into the ocean by atmospheric denudation." ^ 



A shght movement of elevation usually sets the sea back to begin its 

 work anew on the seaward side of its previous shoreline, but such an 

 elevation onl}^ accelerates the work of sul)aerial denudation all over the 

 elevated region. The waves on the seashore shift their line of attack 

 with every slight vertical movement of the coastal region ; l)ut the sub- 

 aerial forces over large continental areas gain no notice of slight move- 

 ments until a considerable time after they have been accomplished, and 

 hence they perform their task only with reference to the average atti- 

 tude of the land. Observers near a shoreline naturally have their attention 

 directed to the unsteadiness of the land, as indicated by marks of many 

 recent changes of land level ; hence they are ]')erhaps indisposed to admit 

 that any land has ever stood still — or oscillated slightl}" above and below 

 an average attitude — long enough to be nearly or quite baseleveled by 

 subaerial agencies. They prefer to think that the sea is, in spite of its 

 many stops and starts, the great leveler of the lands. 



Some have intimated that the insular position of English observers 

 has led them to exaggerate the relative power of the sea. Thus \V. T. 

 Blanford, after much experience in India and elsewhere, as well as at 

 home in England, writes: 



" It is not surprisintj: that the power of rain and rivers should be recognized with 

 difficulty in regions where their effects are comparatively so dwarfed as in the 

 British isles, while the power of marine denudation is at its maximum from the 

 enormous coastline exposed and the small amount of detritus furnished for its 

 protection by rivers of small length and in which floods are of exceptional occur- 

 rence." f 



But even this well practised observer contended only for the subaerial 

 origin of valleys, not of plains also. On the other hand, those whose 

 studies have been directed chiefly to large interior areas seldom have 

 occasion to observe the action of energetic shore waves, and hence are 

 apt to attribute relatively little importance to their work. The small 

 share of attention recentl}^ given by Powell to shore waves and coastal 

 forms in a general discussion of physiographic processes and features 

 is ])erhaps thus explained. X The citation from Dawson, given a])ove, is 



* Text-book, 18S5, p. 432. 



fGeol. and Zool. Abyssinia, 1870, p. 158, note. 



X Nat. Geog. Monograpiis, vol. i, 189.3, no.s. 1 and 2. 



