78 F. P.GULLIVER — PLANATION AND DISSECTION OF URAL MOUNTAINS 



The cover of unconformable sediments would be dissected by a system 

 of consequent drainage when the land was elevated, and these conse- 

 quent streams would not be adjusted to the structure of the underlying 

 rocks, after the stripping off of the cover. Thus superposed streams 

 would characterize such an area, in contrast to the adjusted drainage 

 normal to a dissected peneplain.* 



Harder rock masses would longer resist sea action, and therefore should 

 be found as projecting tongues of land above the plane of the former wave- 

 base, ideally a very different form from the typical monadnock. 



A more exhaustive analysis of the expectable forms j)roduced by these 

 two planation agents would be helpful in studies of any given locality, 

 such as that of the present pai)er. Either or both combined may have 

 acted to form such surfaces as the summit-level plane of the Urals, the 

 questions being which, or what portion by river and what by sea ? 



PLANATION ACCOMPANIED BY SLOW SINKING OF THE LAND 



Von Richthofenf considers that the subaerial agencies would be un- 

 able to produce broad plains on dissimilar structures, and that the sea 

 acting on a quiet land would in time exhaust its power of land attack. 

 He therefore considers that all planed surfaces are produced by the abra- 

 sion of the waves on a successive series of shorelines in a slowly sinking 

 region. The present writer would class these abrasion surfaces as a 

 special form of sea-planed surfaces, perfectly possible and doubtless 

 actual in certain regions. 



One of the characteristics of this class of planed areas would be the 

 large proportion of coarse conglomerate lying on the abraded surface. 

 There would also be a decided inequality of the abraded surface, accord- 

 ing to the hardness of the rocks, the more resistant kinds standing up 

 out of the water in each ei^icycle of depression and becoming covered 

 with the waste of the succeeding epicycles. The form of these masses 

 would be very different from the subaerially carved monadnocks, and 

 the planation being completed step by step would give a much less con- 

 tinuous jilane than would result from river erosion. 



PLANA TION A T HIGH ELE VA TIONS 



Penck has described % still another method of forming planed surfaces, 

 which depends on the fact that wearing away is more rapid on a very 

 high summit than on its lower neighbor. Thus there would be a tend- 



* W. M. Davis : Bull. Geol. Soc. Am., vol. 7, ISOo-'go, p. ;i97. 



t F. von Richthofen : China, 1882, vol. ii, chap, xiv : Fi'ihrer fiir Fovsehungsrei.sende, Berliu, 1880, 

 pp. .353-301. 

 X Albrecht Penck : Morphologie der Erdoberfliiche, Stuttgart, 1894, vol. ii, pp. 161-1G5. 



