TRANSACTIONS OF SECTION C. 727 



as a rule beino- much the smaller. Thus the area of that part of North America 

 which drains into the Gulf of jMexico measures 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 than 180,000 square miles ; while Mr. McGee'estimates it 

 at only 100,000 square miles. Usinji: the larger number, the area of deposition is 

 found to measure one tenth the area of denudation ; the averao-e rate of deposition 

 will therefore be ten times as great as the rate of denudation, or tt Jij 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 seawards, then twice the average would give us the 

 maximum rate of deposition : this would be one foot in ll'O 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 tliis 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 sixth, we arrive at arate of one foot of sediment deposited in a century. 



No doubt this value is often exceeded ; thus in the case of the Mississippi 

 River the bar of the south-west pass advanced between the years 1838 and 1874 a 

 distance of over 2 miles, covering an area 2'2 miles in width with a deposit of 

 sediment 80 feet in thickness ; outside the bar, where the sea is 2-50 feet in 

 depth, sediment accumulates, according to Messrs. Humphreys and Abbot, at a 

 rate of 2 feet yearly. It is quite possible, indeed it is very likely, that some of 

 our ancient strata have been formed with corresponding rapidity. No gravel 

 nor coarse sand is deposited over the Mississippi delta ; such material is 

 not carried further seawards than New Orleans. Thus the vast sheets of 

 conglomerate and sandstone which contribute so largely to some of our ancient 

 systems, such as the Cambrian, Old Red Sandstone, Millstone Grit, and Coal 

 Measures, must have accumulated under ver}' dillereut conditions, conditions for 

 which it is not easy to find a parallel; but in any case these deposits afford 

 evidence of very rapid accumulation. 



These considerations will not tempt us, however, to modify our estimate of 

 one foot in a century ; for though in some cases this rate may have been exceeded, 

 in others it may not have been nearly attained. 



Closely connected with the rate of deposition is that of the changing level of 

 land and sea ; in some cases, as in the Wealden delta, subsidence and deposition 

 appear to have proceeded with equal steps, so that we might regard them as 

 transposable terms. It would therefore prove of great assistance if we could 

 determine the average rate at which movements of the ground are proceeding; it 

 might naturally be expected that the accurate records kept by tidal gauges in 

 various parts of the world would afford us some information on this subject; and 

 no doubt they would, were it not for the singular misbehaviour of the sea, which 

 does not maintain a constant level, its fluctuations being due, according to 

 Professor Darwin, to the irregular melting of ice in the polar regions. Of more 

 immediate application are the results of Herr L. ITolmstrcim's observations in 

 Scandinavia, which prove an average rise of the peninsula at the rate of 3 feet 

 in a century to be still in progress ; and Mr. G. K. Gilbert's measurements in the 

 great Lake district of North America, which indicate a tilting of the continent 

 at the rate of 3 inches per hundred miles per century. But while measurements 

 like these may furnish us with some notion of the sort of speed of these changes 



