EVOLUTIONAL GEOLOGY. 311 



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 one twenty-four hundredth foot yearly (according to 

 Professor Penck, one thirty -six, hundredth 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 i-ock 

 one twenty-four hundredth foot thick yearly, i. e., the rates of denu- 

 dation 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 ])eing 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's statements, less than 180,000 

 square miles, while Mr. McGee estimates it at only 100,000 square 

 miles. Using the largest number, the area of deposition is found to 

 measure one-tenth the area of denudation; the average rate of deposi- 

 tion will therefore be ten times as great as the rate of denudation, or 

 one two hundred and fortieth foot may be supposed to be uniforml}^ 

 distrilnited over the area of sedimentation in the course of a year. But 

 the thickness b}' 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 seaward, then twice the average would give 

 us the maximum rate of deposition; this would be 1 foot in one hundred 

 and twenty years. But the sheets of deposited sediment are not merely 

 thicker toward the land, thinner toward the sea, they also increase in 

 thickness toward 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 nearh' represent their form. This form tends to disap- 

 pear 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 graduall}^ let down- 

 ward, so that they come to assume the form of the bowl of a spoon 

 turned upward. Thus a further correction is necessaiy if we are to 

 arrive at a fair estimate of the maximum rate of deposition. Consid- 

 ering 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 alread}' corrected estimate by one-tifth, 

 we arrive at a rate of 1 foot of sediment deposited in a centur3\ 



No doubt this value is often exceeded. Thus in the case of the Missis- 

 sippi River the bar of the Southwest 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, 



