x EARTH-CHANGES & EVOLUTION 175 



in rocks of very various geological age. But the causes that 

 have produced these great changes of level are still obscure. 

 It is certain, however, that such changes have been exceed- 

 ingly gradual in their operation, and have in all probability 

 been of the same general nature as those going on at the 

 present day — such as the earthquakes which, at irregular 

 intervals, occur all over the world. 



There is one very instructive mode of ascertaining the 

 rate of certain changes of the earth's surface which was first 

 pointed out by Mr. Alfred Tylor more than half a century 

 ago, 1 and is generally accepted by geologists as of great 

 value. The surplus water of the land is carried into the sea 

 by rivers, each of which has a drainage area which contains 

 a certain number of square miles. By careful measurements, 

 it is possible to ascertain how much water flows away every 

 year, and also how much solid matter is suspended in the 

 water, how much is chemically dissolved in it, and how much 

 is pushed along its bed at the mouth. The sum of these 

 three quantities gives us the cubic yards or cubic miles of 

 solid matter denuded from the surface of each river-basin in 

 a year ; and from this amount we can easily calculate how 

 much the whole surface is lowered each year, while some 

 corresponding area of the adjacent sea-bottom, on which it 

 is deposited, must be proportionally raised. These measure- 

 ments have been very carefully made for a number of large 

 and small rivers in various parts of the world, and the 

 following results have been accepted as fairly accurate by 

 Sir A. Geikie : — 



The Mississippi lowers its basin I foot in 6000 years. 

 Ganges 



We can easily see here that the rapidity of denudation 

 is proportionate to the height and extent of the mountain- 

 ranges in which the river has its sources, combined with the 

 amount of the average rainfall, and the proportion of plains 

 to uplands in its whole basin. The Ganges has a large pro- 



1 See Phil. Mag., April 1853. 



