RATE OF DEPOSITION 37 



It is now time to return to the task, too long postponed, 

 of discussing the data from which we have been led to 

 conclude that a probable rate at which sediments have 

 accumulated in places where they attain their maximum 

 thickness is one foot per century. 



We owe to Sir Archibald Geikie a most instructive 

 method of estimating the existing rate at which our con- 

 tinents and islands are being washed into the sea by the 

 action of rain and rivers : by this we find that the present 

 land surface is being reduced in height to the extent on 

 an average of 2^00 ft yearly." If the material removed 

 from the land were uniformly distributed over an area 

 equal to that from which it had been derived, it would 

 form a layer of rock 24 J o foot thick yearly, i.e., the rates 

 of denudation and deposition would be identical. But the 

 two areas, that of denudation and that of deposition, are 

 seldom or never equal, the latter as a rule being much the 

 smaller. Thus the area of that part of North America 

 which drains into the Gulf of Mexico measures 1,800,000 

 square miles ; the area over which its sediments are de- 

 posited is, so far as I can gather from Professor Agassiz' 

 statements, less than 180,000 square miles ; while Mr. 

 McGee estimates it at only 100,000 square miles. Using 

 the larger number, the area of deposition is found to 

 measure one-tenth the area of denudation ; the average 

 rate of deposition will therefore be ten times as great as 

 the rate of denudation, or ^ J- - foot may be supposed to be 

 uniformly distributed over the area of sedimentation in 

 the course of a year. But the thickness by which we 

 have measured the strata of our geological systems is not 

 an average but a maximum thickness; we have, therefore, 

 to obtain an estimate of the maximum rate of deposition. 

 If we assume the deposited sediments to be arranged 

 somewhat after the fashion of a wedge with the thin end 

 * According to Professor Penck gVoo foot. 



