A. Ge'ikie — On Denudation now in Progress. 251 



removed, represents a yearly lowering of the surface of this littlo 

 basin by i^o of a foot. 



Comparing the measm-ements which have been made of the pro- 

 portion of sediment in different streams we shall probably not assume 

 too high an average if we take that of the carefully elaborated Survey 

 of the Mississippi. This gives an annual loss over the area of drain- 

 age equal to gooo of a foot. If then a country is lowered by eooo of a 

 foot in one year, should the existing causes continue to operate undis- 

 turbed as now, it will be lowered 1 foot in 6000 years, 10 feet in 60,000 

 years, 100 feet in 600,000 years, and 1000 feet in 6,000,000 years. 

 The mean height of the Continents, according to Humboldt's calcula- 

 tion,^ is in Europe 671, North America 748, South America 1151, and 

 Asia 1132 English feet. Under such a rate of denudation therefore 

 Europe must disappear in little more than four million of years, 

 North America in about four millions and a half, South America and 

 Asia in less then seven millions. These results do not pretend to be 

 more than approximative, but they are of value inasmuch as they 

 tend to shew that geological phenomena, even those of denudation, 

 which are often appealed to as attesting the enormous duration of 

 geological periods, may have been accomplished in much shorter in- 

 tervals than have been claimed for them. The demands made by 

 geologists for unlimited ages during which the history of the earth 

 has been advancing are opposed by modern physics. It will as- 

 suredly be necessary to revise the whole subject of geological time, 

 and in the end to accept a much lower antiquity for our planet than 

 has been on geological grounds assigned to it. 



The material carried to sea by rivers has been spoken of in the 

 previous part of this paper as having been removed from the general 

 area of drainage, which in consequence is thereby reduced in level. 

 It is of importance to look at the subject from this point of view in 

 order to obtain some adequate idea of the extent of the loss which 

 the land is constantly undergoing before our eyes. But it is obvious 

 that the material so removed does not come equally from the whole 

 area of drainage. Very little may be obtained from the plains and 

 watersheds ; a great deal from the declivities and valleys. It may 

 not be easy to apportion its share of the loss to each part of a 

 district, but the sum total of denudation is not affected thereby. 

 If we allow too little for the loss of the table-lands, we increase the 

 proportion of the loss sustained by the slopes and valleys, and vice 

 versa. There can be no doubt that the erosion of the slopes and 

 water-courses is very much greater than that of the more level 

 grounds. Let it be assumed that the waste is nine times greater in 

 the one case than the other (in all likelihood it is more) ; in other 

 words, that while the plains and table -lands have been having one 

 foot worn off their surface, the declivities and river-courses have 

 lost nine feet. Let it be further assumed that one-tenth part of the 

 surface of a country is occupied by its water-courses and glens, while 

 the remaining nine-tenths are covered by the plains, wide valleys, 

 or flat grounds. Now, according to the foregoing data, the mean 

 ^ Asie Centrale, tome i. 168. 



