Date of the Glacial and the Upper Miocene Period. 383 



materials resulting from the destruction of the rocks. These 

 materials accumulate and form a thick soil over the surface of 

 the rocks, which protects them, to a great extent, from the 

 weathering effects of atmospheric agents. So long as the 

 amount of rock disintegrated exceeds that which is being re- 

 moved by the river-systems, the soil will continue to accumulate 

 till the amount of rock destroyed per annum is brought to equal 

 that which is being removed. It therefore follows from this 

 principle that the carrying -power of our river-systems is the true 

 measure of denudation. But during the glacial epoch the thick- 

 ness of the soil would have but little effect in diminishing the 

 waste of the rocks ; for at that period the rocks were not decom- 

 posed by atmospheric agency, but were ground down by the 

 mechanical friction of the ice. But the presence of a thick soil 

 at this period, instead of retarding the rate of denudation, would 

 tend to increase it tenfold, for the soil would then be used as 

 grinding-material for the ice- sheet. In places where the ice 

 was, say, 2000 feet in thickness, the soil would be forced along- 

 over the rocky face of the country, exerting a pressure on the 

 rocks equal to 50 tons on the square foot. 



It is true that the rate at which many kinds of rocks decom- 

 pose and disintegrate is far less than what has been concluded 

 is the mean rate of denudation of the whole country. This is 

 evident from the fact which has been adduced by some writers, 

 that inscriptions on stones which have been exposed to atmo- 

 spheric agency for a period of 2000 years, or so, have not 

 been obliterated. But in most cases epitaphs on monuments 

 and tombstones, and inscriptions on the walls of buildings, 

 200 years old, can hardly be read. And this is not all : the 

 stone on which the letters were cut has during that time been 

 rotted in probably to the depth of several inches ; and during 

 the course of a few centuries more the whole mass will crumble 

 into dust. 



The facts which we have been considering show also how tri- 

 fling is the amount of denudation effected by the sea in compa- 

 rison with that by subaerial agents. The entire sea-coast of the 

 globe, according to Mr. Keith Johnston, is 116,531 miles. Sup- 

 pose we take the average height of the coast-line at 25 feet, and 

 take also the rate at which the sea is advancing on the land at 

 1 foot in 100 years, then this gives 15,382,500,000 cubic feet 

 of rock as the total amount removed in 100 years by the action 

 of the sea. The total amount of land is 57,600,000 square miles, 

 or 1,605,750,000,000,000 square feet; and if 1 foot is removed 

 off the surface in 6000 years, then 26,763,000,000,000 cubic 

 feet is removed by subaerial agency in 100 years, or about 1740 

 times as much as that removed by the sea. Before the sea could 



