DIASTROPHISM AND THE FORMATIVE PROCESSES 527 



contrasts is with little doubt the different degrees of protection 

 afforded by their vegetal mantles. 



It appears then that a review of the best data that we have 

 relative to the present rate of denudation of the coastal slopes 

 gives a mean rate of about i foot in 12,000 years. It is to be noted 

 that this embraces tracts that reach to the border of the 30° belt 

 where humidity is low, as well as tracts in the fairly humid mid- 

 latitudes, and that the denudation rate of the latter is scarcely 

 a half of that of the former in spite of the theoretical presumption 

 that denudation should rise and fall with the precipitation. The 

 fact that the efficiency of the vegetal mantle as a protection against 

 erosion also rises and falls with precipitation seems to much more 

 than offset the direct effects of increased drainage flow. 



Returning to the broader question, it appears that the best 

 available data relative to the rate of denudation on American 

 coastal slopes at present gives a mean rate of i foot in 12,000 years. 

 This is but half the higher rate usually employed in the past, based 

 on the rate for the Mississippi Valley as a whole, i foot in 6,000 

 years. The rate i foot in 12,000 years still needs to be corrected 

 for (i) the effect of the present conditions in accelerating denuda- 

 tion, (2) the portions of the sediments lost to the abysmal basins, 

 (3) the portions of the solutions that are held permanently in the 

 oceans as part of the saline element of sea water, and (4) the por- 

 tions of the solutions that are precipitated to the ocean depths as 

 the hard parts of pelagic animals and plants. 



But to curtail this discussion, let us leave these corrections in 

 abeyance and proceed on the basis of the present denudation rate, 

 keeping in mind that it must be corrected to give true results. 



Recalling that we had previously fixed upon a belt 200 miles 

 in width as representative of the area directly tributary to the 

 shelf, upon 100,000 miles as its length, and upon 120 miles as the 

 mean width of the shelf, and using the denudation rate i foot in 

 12,000 years, without correction, the formation of the post- 

 Proterozoic terrace would take approximately 108,000,000 years. 

 This is of about the same order of magnitude as the periods usually 

 reached for post-Proterozoic time by employing data based on 

 general denudation in which the present rate is taken as the secular 



