DENUDATION, OH GENERAL EROSION. ;i55 



Plateau region, an area of not less than 200,000 square 

 miles, an average of 0,000 to 8,000 feet and an extreme 

 of 12,000 feet, has been removed by erosion, and all since 

 the Middle Tertiary. From these examples it is impos- 

 sible to resist the conclusion that the average erosion 

 over all land-surfaces has been at the very least several 

 thousand feet. 



There is another way of making the estimate of the 

 amount of general erosion. Evidently the correlative and 

 measure of erosion is sedimentation. The debris of ero- 

 sion have been accumulated as stratified rock. Now, the 

 average thickness of strata can not be less than several 

 thousand feet. Taking it only as 2,000 feet (it is certainly 

 very much more), since the area of ocean is, and 

 probably always was, three times the area of the land, 

 this would require at least 6,000 feet erosion of all land- 

 surfaces. We may therefore say, with the utmost confi- 

 dence, that over all land-surfaces more than 6,000 feet 

 thickness has been removed by erosion. 



Time. Now, we have seen (page 19) that the rate of 

 rain and river erosion is about one foot in 5,000 years. 

 At this rate it would take 30, 000, 000 years to do the work 

 which we actually find has been done. The time was 

 probably much greater. Exceptions may be taken to 

 some points of our calculation, but, we are sure, not to 

 the result. But this, be it more or less, represents only 

 recorded history. Beyond this, again, is the infinite abyss 

 of the unrecorded. 



