/. Or oil — Mean Thickness of the Sedimentary Rocks. 99 



denuded, then we know with perfect accuracy the rate at which the 

 sedimentary deposits are being formed in the ocean.. This is 

 obvious, because all the materials denuded from the land are 

 deposited in the sea; and what is deposited in the sea is just what 

 comes off the land, with the exception of the small proportion of 

 calcareous matter which may not have been derived from the land, 

 and which in our rough estimate may be left out of account. 



But how are we to determine the rate of sub-aerial denudation ? 

 This rate can be determined by a method advanced a few years ago.^ 

 It is this : the rate at which the land is being lowered by denudation 

 is measured by the amount of sediment carried down to the sea by 

 the river systems. The rate, for example, at which the basin of a 

 river is being denuded is determined with perfect accuracy by the 

 quantity of sediment carried into the sea by the river. 



Unfortunately — except in the case of the Mississippi — no very 

 accurate determination has as yet been made of the quantity of 

 sediment carried down to the ocean by rivers. The annual amount 

 conveyed into the ocean by the Mississippi has been accurately 

 measured by Messrs. Humphreys and Abbot. Taking their estimate 

 of the amount of sediment and area of drainage of the Mississippi, 

 it is found, by the method above referred to, that its basin is being 

 lowered at the rate of one foot in 6,000 years. 



Sir Charles Lyell has shown clearly that in regard to the amount 

 of sediment carried down into the sea, there is perhaps no river 

 which may more safely be taken as a fair representative of rivers in 

 general ; and in the mean time we may be warranted in taking one 

 foot in 6,000 years as representing the mean rate at which the land 

 is being abraded. 



Taking the proportion of land to that of water at 576 to 1,390, 

 then one foot taken off the land and spread over the sea-bottom 

 would form a layer five inches thick. Consequently, if one foot in 

 6,000 years represents the mean rate at which the land is being 

 denuded, one foot in 14,400 years represents the mean rate at which 

 the sedimentary rocks are being formed. 



Assuming, as before, that 72,000 feet would represent the mean 

 thickness of all the sedimentary rocks which have ever been formed, 

 this, at the rate of one foot in 14,400 years, gives 1,036,800,000 as 

 the age of the stratified rocks. 



Professor Huxley, in his endeavour to show that 100,000,000 

 years is a period sufficiently long for all the demands of geologists, 

 takes the thickness of the stratified rocks at 100,000 feet, and the 

 rate of deposit at a foot in 1,000 years. One foot of rock per 

 1,000 years gives, it is true, 100,000 feet in 100,000,000 years. But 

 what about the rocks which have disappeared ? If it takes a 

 hundred millions of years to produce a mass of rock equal to that 

 which now exists, how many hundreds of millions of years will it 

 require to produce a mass equal to what has actually been produced ? 



1 Philosophical Magazine for February, 1867, p. 130, and May, 1868, p. 379 ; see 

 also Mr. Geikie's Memoir " On Modern Denudation," Trans, of Glasgow Geol. Soc. 

 for 1868. 



