to Section C {Geology). 461 



It is now time to return to tlie 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. 



Bale of Deposition of Sediment. 



We owe to Sir Archibald Geikie a most instructive method of 

 estimating the existing 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 a^+Vo foot 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 2^voo 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 deposited 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 ai'ea of deposition is 

 found to measure one-tenth the area of denudation ; the average 

 rate of deposition will thei'efore be ten times as great as the rate 

 of denudation, or -^\o- foot may be supposed to be uniformly dis- 

 tributed 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 

 tlierefore 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 

 average would give us the maximum rate of deposition : this would 

 be one foot in 120 years. But the sheets of deposited sediment are 

 not merely thicker towards the land, thinner towards the sea, they 

 also increase 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 upwards. Thus 

 a further correction is necessary if we are to arrive at a fair estimate 

 of the maximum rate of deposition. Considering 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 



^ According to Prof(S5or Penck Tnrrro foot. 



