EARTH CHANGES AND EVOLUTION 189 



at 14,000 feet, and in the Himalayas at 16,000 feet elevation. 

 Innumerable cases of marine fossils at lesser heights are to be 

 found in every part of the world, and 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 exceedingly 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,^ and is 

 generally accepted by geologists as of great value. The sur- 

 plus 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 measurements have been very carefully made for a num- 

 ber 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 1 foot in 6000 years. 



1 See Phil. Mag., April 1853. 



