98 /. CroU — Mean Thickness of the Sedimentary Rocks. 



Unless we take into consideration the quantity of rock removed 

 during past ages by denudation, we cannot — even though we knew 

 the actual mean thickness of the existing sedimentary rocks of the 

 globe and the rate at which they were formed — arrive at an estimate 

 regarding the length of time represented by these rocks. For if we 

 are to determine the age of the stratified rocks from the rate at 

 which they were formed, we must have, not the present quantity of 

 sedimentary rocks, but the present plus the quantity which has been 

 denuded during past ages. In other words, we must have the 

 absolute quantity formed. In many places the missing beds must 

 have been of enormous thickness. The time represented by beds 

 which have disappeared is doubtless, as already remarked, much 

 greater than that represented by the beds which now remain. The 

 greater mass of the sedimentary rocks has been formed out of 

 previously existing sedimentary rocks, and these again out of sedi- 

 mentary rocks still older. As the materials composing our stratified 

 beds may have passed through many cycles of destruction and 

 re-formation, the time required to have deposited at a given rate the 

 present existing mass of sedimentary rocks may be but a fraction of 

 the time required to have deposited at the same rate the total mass 

 that has actually been formed. To measure the age of the sedi- 

 mentary rocks by the present existing rocks, assumed to be formed 

 at some given rate, even supposing the rate to be correct, is a 

 method wholly fallacious. 



"The aggregate of sedimentary strata in the earth's crust," says 

 Sir Charles Lyell, " can never exceed in volume the amount of solid 

 matter which has been ground down and washed away by rivers, 

 waves, and currents. How vast then must be the spaces which this 

 abstraction of matter has left vacant ! How far exceeding in dimen- 

 sions all the valleys, however numerous, and the hollows, however 

 vast, which we can prove to have been cleared out by aqueous 

 erosion!"' 



I presume there are few geologists but would admit that if all the 

 rocks which have in past ages been removed by denudation were 

 restored, the mean thickness of the sedimentary rocks of the globe 

 would be at least equal to their present maximum thickness, which we 

 may take at 72,000 feet. 



There are three elements in the question; if two of them are 

 known, the third is known in terms of the other two. If we have 

 the mean thickness of all the sedimentary rocks which have been 

 formed and the mean rate of formation, then we have the time 

 which elapsed during the formation ; or, having the thickness and 

 the time, we have the rate ; or, having the rate and the time, we 

 have the thickness. 



One of these three, namely, the rate, can, however, be determined 

 with tolerable accuracy if we are simply allowed to assume— what is 

 very probable, as will be shown — that the present rate at which the 

 sedimentary deposits are being formed may be taken as the mean rate 

 for past ages. If we know the rate at which the land is being 

 ' " Principles," vol. i. p. 107 (tenth edition). 



