EROSION. 1 69 



maintaining tliat no threat part of the earth can be fluid. The hypotliesis 

 that only a superficial layer of the globe is affected by upheaval and snljsi- 

 dence appears to me to imply that beneath this thin shell the earth is not 

 the highly viscous solid of Sir AYilliam Thomson, bnt a body of absolute, 

 ideal, and impossible rigidity, for only then could it fail to share in the def- 

 ormation of the surface. 



The problem viewed as one of erosion. — The avorage thickness of the sedimentary 

 rocks is in my opinion often greatly exaggerated. It is true that if the 

 greatest thicknesses of the formations are added they form an enormous 

 total ; but we all know that sediments are thickest near shore lines and dis- 

 appear altogether at a distance from the shore. According to those author- 

 ities who maintain that even the igneous rocks are fused sediuients, of course 

 the later sedimentary rocks are composed of the same material which en- 

 tered into earlier strata. That this is to a large extent the case is evident. 

 Clearl}^, however, it must also have been the case to some extent from the 

 date of the first npheaval after oceans formed on the surface of the gk^be. 

 As time went on the exposed areas of the primitive rocks nnist have de- 

 creased while a larger and larger proportion of freshly formed rocks was 

 produced at the expense of the older beds. After a certain time the addi- 

 tions to the total amount of detrital material in a given period, say one 

 thousand years, would be very small, and from that time onward the quan- 

 tity of detrital material would remain nearly constant. Now, if one supposes 

 the average thickness of sedimentary rocks at some past epoch to have been 

 only one mile, it is evident that only a minute proportion of any land area 

 similar to the present continents, or even of much bolder configuration than 

 these, could be occupied by exposed primeval rocks.^ If an average thick- 

 ness of one mile of sedimentary material would I'educe the area of primitive 

 rocks to a very small one, how is it possible to account for the formation 

 of twenty times this quantity of detritus 1 I do not think it can be done. 



Character of the process of degradation. Tlie llVpOtliesis that tllls ahllOSt inCrcdiblo 



(piantity of detrital material exists, as applied by advocates of the sedi- 

 mentary origin of massive rocks, involves the assumption that degradation 



' Gaunelt's estimate of the mean elevation of tlie Uuited States, excluaiug Alaslia, is '2,000 feet, say 

 lialf a mile. LeipoUlt's estimate for Europe is 297 meters, or 97.5 feet, say a sixtli of a mile. 



