526 T. C. CHAMBERLIN 



the other way. To see whether the rates of denudation of the 

 coastal border tracts are essentially the same or essentially differ- 

 ent from those of the general continental surfaces, the rates given 

 by Dole and Stabler for the districts of the North Atlantic (i foot 

 in 13,200 years), the South Atlantic (i foot in 8,520 years), the 

 North Pacific (i foot in 19,200 years), and the South Pacific (i foot 

 in 9,320 years), including in each case only the area within the 

 United States, were combined and compared. The rates, aver- 

 aged without weighting, were found to give a common rate of i foot 

 in 12,180 years. If the Laurentian basin (i foot in 19,320 years) 

 is reckoned in, the unweighted mean rate rises to i foot in 13,476 

 years. If the Colorado River basin (i foot in 5,280 years) is also 

 reckoned in, the mean falls back to i foot in 12,110 years. 



The data for some of the sub-districts are very suggestive. 

 For example, the basins of the Penobscot (i foot in 24,000 years), 

 the Kennebec (i foot in 25,200 years); the Androscoggin (i foot in 

 21,600 years), the Presumpscot (i foot in 25,200 years), the Saco 

 (i foot in 25,200 years), the Merrimac (i foot in 32,400 years), and 

 the Connecticut (i foot in 24,000 years) are averaged without 

 weighting, the mean rate is i foot in 25,371 years. These notably 

 low rates are probably due in part to the numerous catchment 

 basins on the morainic surfaces of these New England basins, but 

 in no small part also are they probably due to the fact that large 

 portions of New England once under plow have been permitted 

 in recent years to return to a wooded or grassy state. It is of 

 interest to note here that the drainage area of Lake Superior, so 

 far as it lies in the United States, which has only been brought par- 

 tially under culture, has a denudation rate of i foot in 37,200 years. 



The erosion of the South Atlantic district (i foot in 8,520 

 years) is 2f times as fast as that of the North Atlantic district 

 (i foot in 13,200 years), though the gradient of the former is below 

 rather than above that of the latter. Of like import is the erosion 

 rate of the North Pacific slope (i foot in 19,200 years) when com- 

 pared with that of the South Pacific slope (i foot in 9,320 years) 

 more than twice as fast, though the rainfall of the former is much 

 the higher and its gradients certainly not less steep. One of the 

 most vital factors, probably the most vital factor, in these strong 



