ESTIMATES OF TIME 817 



In discussion of Walcott's statements it may be said that, according to 

 his view previously cited, the rate of development of life is controlled by 

 conditions of environment and can not by itself be used very securely as 

 evidence for the shortness or length of time needed for the accumulation 

 of a formation. 



The argument has been stated in Part I that a peneplain is not a favor- 

 able land surface for the promotion of deep chemical denudation, and can 

 hardly be invoked with safety in aid of a postulated rapid accumulation 

 of sediment. The cleanness of a sand formation, on the other hand, speaks 

 of prolonged sorting and is favored by fill and scour, pronounced wave 

 or current action, and, notwithstanding the rapid accumulation of indi- 

 vidual beds, may be accompanied by a slow aggregate rate of accumula- 

 tion. Thus the principles of rhythms are of peculiar force in such a 

 deposit. 



With the conditions of low relief of the land a rate of erosion of one 

 foot in 3,000 years seems more probable than one foot in 200 years. Even 

 this is twice the present rate of erosion for the Mississippi basin, and 

 about three times the rate given by Clarke for the present mean of the 

 continents. It is probable, however, that the retarding effect of land 

 vegetation was not then effective, so that a fairly rapid rate of one foot in 

 3,000 years for the low lands of the early Paleozoic may be provisionally 

 accepted. But even this low rate gives a length of time fifteen times 

 that regarded by Walcott as the more probable. 



On the other side of the question, it may be said that if the mechanical 

 sediments were derived from the unconsolidated deposits of an uplifted 

 coastal plain, the rate of denudation of rock becomes immaterial in the 

 making of the final formation. The unconsolidated deposits may be 

 washed off almost as fast as uplift of the plain occurs, and deposited as 

 fast as subsidence of the sea-floor permits. The time required to develop 

 the original clastic material depended, however, on the rate of rock 

 denudation. The time represented by the final and permanent accumu- 

 lation of 10,000 feet of clastic sediments may consequently be short or 

 long, according to the rate of crust movements, but with the probabilities 

 in favor of millions, or tens of millions, rather than hundreds of thou- 

 sands of years. 



As a basis for estimating the time needed for the deposition of the 

 Paleozoic limestones of the Cordilleran sea, Walicott takes the quantity 

 as amounting to an average thickness of 2,250 feet over an area of 

 400,000 square miles. The calcium carbonate can not be regarded as 

 derived only from the tributary land drainage, since the epeiric sea was 

 in communication with the open ocean. The estimate must rest, then, 



