1 88 HUTTONIAN THEORY 



where the rainfall is heavy, where the daily range of 

 temperature is large, and where frosts are severe. 

 Hence they are obviously much more effective in 

 mountainous regions than on plains ; and their results 

 must constantly vary, not only in different basins of 

 drainage, but even, and sometimes widely, within the 

 same basin. Actual measurement of the proportion 

 of sediment in river water shows that while in some 

 cases the lowering of the surface of the land may be 

 as much as rhr of a foot in a year, in others it falls 

 as low as tsW 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 

 6800 years. 



If now we take these results and apply them as 

 measures of the length of time required for the de- 

 position of the various sedimentary masses that form 

 the outer part of the earth's crust, we obtain some 

 indication of the duration of geological history. On a 

 reasonable computation these stratified masses, where 

 most fully developed, attain a united thickness of 

 not less than 100,000 feet. If they were all laid down 

 at the most rapid recorded rate of denudation, they 

 would require a period of seventy-three millions of 

 years for their completion. If they were laid down 

 at the slowest rate they would demand a period of not 

 less than 680 millions. 



But it may be argued that all kinds of terrestrial 

 energy are growing feeble, that the most active denu- 

 dation now in progress is much less vigorous than that 

 of bygone ages, and hence that the stratified part of 



