254 CHAMBERLIN— THE AGE OF THE EARTH. 



distribution of rain, a larger absorption into the soil, and a slower 

 run-off of the remainder. So, too, accidented surfaces are likely to 

 be ineffectively protected by vegetation, for lack of soil, or of ade- 

 quate moisture. These and other incidental influences add appre- 

 ciably to the total effect. It seems clear, therefore, that a large cor- 

 rection is to be made to the present rate of denudation because of the 

 relatively high elevation of the continents. 



4. Correction for Area. — This is to a large degree, but not wholly, 

 an effect of the elevation of the continents, but none the less it 

 deserves separate recognition. When elevation increases the land 

 area, base-leveling and sea-transgression at once set in and combine 

 their forces to reduce the exposed area. The result is very large 

 variations in the areas of the ancient lands. The estimates of 

 Schuchert and others for North America show variations that range 

 from the full surface of the continental platform down to half that 

 surface. As a rule, of course, the lesser surfaces were also low sur- 

 faces, and the two influences were cumulative. At stages of low 

 elevation and slack drainage deep soils were likely to accumulate and 

 these favored thick vegetation which helped to hold the soils. Thus 

 in several ways small area and low elevation united their influence in 

 a cumulative effect which could not have been other than large. 



Partial Summary. — Summarizing at this point, it appears that 

 four important corrections quite certainly must be applied to the 

 present rate of geologic action to reduce it to a mean rate for the 

 whole of geologic time. These corrections are cumulative. There 

 seems to be no way at present to evaluate them rigorously or perhaps 

 even very closely. The weighing of their value is greatly affected by 

 the individual judgment, and that in turn by individual experiences 

 and opportunities of observation. Speaking for myself alone, it does 

 not seem to be overstraining the importance of these corrections to 

 suppose that their cumulative value will be found great enough to 

 bring the old-time estimates up to figures of about the same order as 

 those of the current radio-active estimates. 



5. The Lozver End of the Geologic Column. — Below the base of 

 the Paleozoic series the geologic terranes are much obscured by 

 diastrophism and metamorphism. It remains to inquire what is the 



