CONVeRGEK^CE 01^ EVIDENCE 897 



the mean rate of erosion of the Precambrian continents per unit of area 

 was not more than one-twelfth of the present rate. 



Bnt it Seems probable^ firsts, that the Precambrian continents extended 

 beyond the limits of the present continental platforms ; second, that more 

 than one-fifth of the erosion of igneous rocks has been accomplished since 

 the Precambrian ; third, that Precambrian time was somewhat longer 

 than 670,000,000 years. The mean rate of continental denudation of 

 Precambrian time was consequently not more than one-twelfth of the 

 present rate and was perhaps not more than one-twentieth of that for the 

 Pleistocene. This is a lower rate than was derived for the P^opaleozoic 

 by means of the maximum thicknesses of sediments. 



If at times of Precambrian revolution the rate of continental denuda- 

 tion occasionally reached its present amount and an area of three-fifths 

 of the present continents consisted then of exposed igneous rocks, the 

 annual destruction of igneous rock would have amounted at such times 

 to about seven-tenths of a cubic mile. The total igneous rock destroyed 

 in Precambrian times has been taken as 67,000,000 cubic miles. If the 

 assumed maximum rate had continued, this total erosion would have been 

 accomplished in 100,000,000 years, leaving nothing for all the remainder 

 of the Precambrian ages. It seems clear, therefore, that the times of 

 general continental rejuvenating uplift and denudation were very limited, 

 occupying not more than a tenth of the time. Between these must have 

 stretched long periods of quiescence, during which peneplains lay near 

 baselevel and erosion subsided to a negligible minimum. 



A low average rate of denudation for a contment may exist simultane- 

 ously with a far higher rate for the areas supplying the waste. Such a 

 very low continental rate must have existed in the Eopaleozoic, since 

 obviously when seas spread over the land the erosion rate for the flooded 

 areas became even negative and must have been very small for the slightly 

 emerged portions. For the Precambrian, however, the result is unex- 

 pected, since erosion proceeded over the continents at one time or an- 

 other deeply enough to expose nearly everywhere a crystallized rock-floor. 

 The low mean rate for these earlier periods brings into startling contrast 

 two opposite aspects of Precambrian paleogeography — the almost inter- 

 minable periods of quiet and peneplanation ; the profound revolutions 

 which separated them. 



Such a period of revolution connected with broadly uplifted lands, 

 diversified by mountain ranges and geosynclines, coursed by rapid rivers 

 in a wide-spread network, occurred in the late Precambrian. Its forma- 

 tions are the continental deposits of the Keweenawan, the Torridonian, 

 the Beltian, the Grand Canyon, and other systems — river deposits of semi- 



