70 AN INTRODUCTION TO SCIENCE 



the Rocky Mountains, and chalk is a deposit which could be 

 formed but very slowly at any period of the earth's history, 

 seeing that it is made up of the shells of microscopic animals 

 which obtain the carbonate of lime for the manufacture of 

 their testae from the sea. But, neglecting all details, and 

 looking at the matter from the broadest point of view, Geikie 

 endeavours to ascertain the rate at which the materials which 

 form rocks are deposited ; and since all these materials are 

 borne down to the sea by rivers, we can calculate, if we know 

 the amount which any river carries down in a year, the 

 depth to which it will cover a given area of the bottom of 

 the sea. Measurements which have been made show that 

 rivers deposit from T ^ to ^Vcr f a f ot * n a vear > over an area 

 equal to the area from which they obtained the mud, sand 

 and gravel which they wash into the sea. The limits between 

 which the deposit varies are necessarily wide, because the 

 activity of the process of denudation of the land varies so 

 greatly. Mountains are worn down more rapidly than plains, 

 and where the rainfall is heavy, or the splitting action of 

 frost comes into play, denudation is much more rapid than 

 in dry, warm places. Supposing the area of sea to have 

 been always equal to the area of land, and the rivers to be 

 the only carriers of deposits, it is clear that it would take 

 from 70 to 700 millions of years to lay down strata 100,000 

 feet in thickness. These figures are interesting as guiding 

 lines of thought, but it is obvious that corrections must be 

 made for the carrying power of the wind, which robs the 

 rivers of much of the dust and sand which would otherwise 

 find their way into their streams, and for the eroding action 

 of the sea itself. Nor is it certain that the aggregate thickness 

 of the strata would amount to 100,000 feet if it could be 

 measured in any one given place. Sir Archibald Geikie says 

 that "on a reasonable computation these stratified masses, 

 where most fully developed, attain a united thickness of not 

 less than 100,000 feet." But it is unlikely that all could have 

 been fully developed in any one place, since at no time was 



