DENUDATION, OR GENERAL EROSION. 277 



Some may object to this estimate, on the ground that geological 

 agencies were once much more active than now. It is probable that this 

 is true of igneous agencies, since these are determined by the interior 

 lieat of the earth, and this has evidently been decreasing. It is probable 

 also that this is true of the chemical agencies of water in disintegrating 

 rocks and forming soils, since chemical effects are also usually increased 

 by heat. But there is good reason to believe that the mechanical agen- 

 cies of water, i. e., its erosive poiver, have been constantly increasing 

 with the course of time, and are greater now than at any previous epoch 

 except the Glacial epoch. 



For observe : The erosive power of water is determined entirely by 

 the rapidity of circulation of air and water, and this is determined by 

 the diversity of temperature in different portions, and this in its turn 

 by the size of continents and the height of mountains. Continents and 

 seas are two poles of a circulating apparatus — at one pole is condensa- 

 tion, at the other evaporation. In proportion to condensation are also 

 evaporation and circulation. Now, there is good reason to believe that, 

 amid many oscillations, there has been throughout all geological times 

 a constant increase in the size of continents and the height of mount- 

 ains. If so, then the circulation of air and water has been becoming 

 swifter and swifter ; the life-pulse of our earth has beaten quicker and 

 quicker, and therefore the waste and supply (erosion and sedimentation) 

 have been greater and greater.* 



We therefore return to our estimate of 30,000,000 years with greater 

 confidence that it is even far within limits of probability. For, 1. We 

 have taken the average thickness of strata at 2,000 feet, while it is 

 probably much more 2. We have taken the Mississippi as an average 

 river, and therefore the present rate of general erosion as one foot in 

 5,000 years : it is probably much less. 3. We have taken the rate of 

 erosion in previous epochs as the same as now, while it is probably 

 much less, for two reasons : 1. The land-surface to be eroded was smaller ; 

 and, 2. The erosive power of water was less. Taking all these things 

 into consideration, the time necessary to produce the structure which 

 Ave actually find is enormously increased. 



mate, on this basis, following Mr. Wallace. Taking the whole land-surface (erosion area) 

 at 57,000,000 square miles, and the sedimentation area as thirty miles wide along a coast- 

 line of 100,000 miles ( = 3,000,000 square miles), then with an erosion-rate of one foot in 

 3,000 years instead of 5,000 years, the sedimentation rate would be nineteen feet in the 

 same time, or one foot in 157 years. But the extreme thickness of strata is at least 

 177,000 feet. This would take 28,000,000 years.— (Wallace, Island Life, p. 210.) 



* It is possible that the erosive effect of tides in earliest geological times, far greater 

 than now, on account of the greater proximity of the moon, is an element which should 

 not be neglected (Nature, vol. xxv, p. 79). But this probably belongs to a time anteced- 

 ent to the recorded history of the earth. 



