GROWTH OF GEOLOGY. 243 



the decay of continents, it has been a recognised 

 fact that there is a universal degradation of the 

 dry land. The span of the longest human life is but 

 a tick of the geological clock, and so we speak of the 

 eternal hills. But there is no doubt in the mind of 

 any observer that even the hills are slowly melting 

 and crumbling away. " The hills are shadows, and 

 they flow from form to form, and nothing stands." 

 Rain and frost, lichens and burrowing animals, run- 

 ning water and whistling wind, and other agencies 

 contribute to the unceasing weathering and denuda- 

 tion. There are, indeed, conservative agencies, but the 

 wasting goes on steadily. The present land surface 

 is being reduced in height, on an average of ^rW 

 to ^Vrr foot per annum. But what is lost here is 

 gained somewhere else, denudation and deposition 

 must be almost equivalent in amount (though not in 

 area, the latter being usually much smaller), and 

 thus we can arrive at some estimate of the amount 

 of wasting by measuring the amount of sediment 

 deposited. " Actual measurement of the proportion 

 of sediment in river water shows that while in some 

 cases the lowering of the surface may be as much as 

 TTJT of a foot in a year, in others it falls as low as 

 TSVV- In other words, the rate of deposition of new 

 sedimentary formations, over an area of sea-floor 

 equivalent to that which has yielded the sediment, 

 may vary from one foot in 730 years to one foot in 

 6,800 years." * 



Now, a considerable part of the outer crust of the 

 earth is made up of sedimentary rocks ; among these 

 it is possible with considerable accuracy to distin- 

 guish the deposits which were laid down at different 



* Sir Archibald Geikie, Pres. Address, Report Brit. Ass. 

 for 1892, p. 21. 



