November 23, 1900.] 



SCIENCE. 



793 



offer a comparison of his results with those 

 that follow from our rough estimate of one 

 foot of sediment accumulated in a century. 



TABLE SHOWING THE TIME THAT HAS ELAPSED 

 SINCE THE BEGINNING OF THE SYSTEMS IN" THE 

 FIRST COH'MN, AS RECKONED FROM THICKNESS 

 OF SEDIMENT IN THE SECOND COLUMN, AND BY 

 PROFESSOR BLYTT IN THE THIRD. 



Years. Years. 



Eocene 4,200,000 3,250,000 



Oliogocene 3,000,000 1,810,000 



Miocene 1,800,000 1,160,000 



Pliocene 900,000 700,000 



Pleistocene 400,000 350,000 



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 the sediments have 

 accumulated in places where they attain 

 their maximum thickness is one foot per 

 century. 



KATE OF DEPOSITION OF SEDIMENT. 



We owe to Sir Archibald Geikie a most 

 instructive method of estimating the exist- 

 ing rate at which our continents 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 of an average of 1/2400 

 foot yearlj' (according to Professor Penck 

 1/3600 foot). 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 1/2400 

 foot thick yearly — i. e., the rates of denuda- 

 tion 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 ISTorth America 

 which drains into the G-ulf of Mexico meas- 

 ures 1,800,000 square miles; the area over 

 which its sediments are deposited is, so far 

 as I can gather from Professor Agassiz's 

 statements, less then 180,000 square miles; 

 while Mr. McG-ee estimates it at only 100,- 



000 square miles. Using the largest num- 

 ber, 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 denuda- 

 tion, or 1/240 foot may be supposed to be 

 uniformly distributed over the area of sedi- 

 mentation 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 seawards, then twice the aver- 

 age would give us the maximum rate of de- 

 position ; this would be one foot in 120 

 years. But the sheets of deposited sedi- 

 ment are not merely thicker towards the 

 land, thinner towards the sea, they also in- 

 crease in thickness towards the rivers in 

 which they have their source, so that a very 

 obtuse-angled cone, or, better, the down- 

 turned bowl of a spoon, would more nearly 

 represent their form. This form tends to 

 disappear under the action of waves and 

 currents, but a limit is set to this disturbing 

 influence by the subsidence which marks 

 the region opposite the mouth of a large 

 river. By this the strata are gradually let 

 downwards, so that they come to assume 

 the form of the bowl of a spoon turned up- 

 wards. Thus a further correction is neces- 

 sary if we are to arrive at a fair estimate 

 of the maximum rate of deposition. Con- 

 sidering the very rapid rate at which our 

 ancient systems diminish in thickness when 

 traced in all directions from the localities 

 where they attain their maximum, it would 

 appear that this correction must be a large 

 one. If we reduce our already corrected 

 estimate by one-fifth, we arrive at a rate of 

 one foot of sediment deposited in a century. 

 No doubt this value is often exceeded ; 

 thus in the case of the Mississippi Eiver 



