276 DENUDATION, OR GENERAL EROSION. 



six miles thickness of strata (King). Over the whole Colorado Plateau 

 region the succession of cliffs, separated by broad tables (Fig. 245), 

 shows enormous erosion. The average erosion over the whole region 

 has been estimated by Powell and Dutton as 6,500 feet ; and the ex- 

 treme general erosion, not including the canon-cutting, as 11,000 feet. 

 The whole of this immense mass has been removed, too, since the mid- 

 dle Tertiary. 



It seems impossible to avoid the conclusion, therefore, that the 

 average erosion over all present land-surfaces has been at least several 

 thousand feet. 



There is another mode of estimating the average erosion, viz., by 

 the average thickness of stratified rocks. The debris of erosion is car- 

 ried down into seas and lakes, and forms strata, and the amount of 

 stratified rocks becomes thus the measure of the erosion ; the average 

 thickness of sediments, if they had been spread over an equal area, 

 would be an accurate measure of the average thickness removed by 

 erosion. Now, the stratified rocks are in some localities 10,000 feet, 

 20,000 feet, and sometimes 40,000 and 50,000 feet thick. They are 

 scarcely ever found less than 2,000 or 3,000 feet. It is certain, there- 

 fore, that the average thickness of strata over the whole known surface 

 of the earth is not less than several thousand, feet. Let us take it at 

 only 2,000 feet. But the area of sedimentation, the sea-bottom, is now, 

 and has probably always been, at least three times the area of erosion, 

 the land-surface. Thus an average of 2,000 feet of strata would require 

 an average erosion of 6,000 feet. 



Estimate of Geological Times. — There are many facts connected 

 with geology, especially the facts of evolution, which can not be under- 

 stood without the admission of inconceivable lapse of time. For this 

 reason it is important that the mind should become familiarized with 

 this idea. It will not be out of place, therefore, to make a rough esti- 

 mate of time based upon the amount of erosion. 



We have already seen (p. 11) that, taking the Mississippi as an aver- 

 age river in erosive power (it is probably much more than an average), 

 the rate of continental erosion is noiv about one foot in 5,000 years. At 

 this rate, to remove an average thickness of 6,000 feet would require 

 30,000,000 years* 



* The above estimate takes the average thickness of strata, and supposes it spread 

 evenly over the whole sea-bottom. This is strictly admissible only if we suppose, with 

 Lyell, that land and ocean have often changed places, so that every portion of earth-sur- 

 face has received sediments. But if, as is now most generally believed, the ocean-basins 

 have remained substantially unchanged, and sediments have accumulated almost wholly 

 on their margins, then we must, it is true, make our measuring rod, i. e., the rate of sedi- 

 mentation, much greater, but we must also take the sum of the extreme thickness of strata 

 in different localities, as the thing to be measured. Wc, therefore, make another esti- 



