378 Mr. J. Croll on Geological Time, and the probable 



lions, hundreds of millions, thousands of millions, the mind is 

 then totally unable to follow, and we can only use these numbers 

 as representations of quantities that turn up in calculation. We 

 know, from the way in which they do turn up in our process of 

 calculation, whether they are correct representations of things in 

 actual nature or not ; but we could not, from a mere comparison 

 of these quantities with the thing represented by them, say 

 whether they were actually too small or too great. It is here 

 that some geologists have erred : they have not made the neces- 

 sary calculations, and found by the known rules of arithmetic 

 that 100,000,000 is too small a number to represent in years 

 the probable age of the earth's crust ; but they look first at the 

 phenomena and then at the figures; and as the two produce 

 totally different impressions, they pronounce the figures to be 

 too small to represent the phenomena. 



If the geologist could find a method of ascertaining the actual 

 rate at which these denuding agents do perform their work ; if it 

 could be ascertained at what rate the face of the country is at 

 present being denuded, how much, for example, per annum the 

 general level of the country is being lowered and the valleys 

 deepened, then we should have a means of ascertaining whether 

 or not the agents to which we refer were really capable of pro- 

 ducing the required amount of change in the earth's surface in 

 the allotted time. But mere conjectures in the absence of some 

 positive determinations are worse than useless. 



But happily there is a method of ascertaining, with the most 

 perfect accuracy, the rate at which the face of the globe is being- 

 denuded by subaerial agency. And it is somewhat remarkable 

 that this method has been so long overlooked by geologists. 

 The method to which I allude is that which has already been 

 incidentally referred to, viz. that of determining the amount of 

 solid materials which is being carried down annually by our 

 rivers into the sea. Were it ascertained (and this might be 

 easily done) how much sediment is being carried down by our 

 rivers into the sea, then we should be able to determine exactly 

 the extent to which the area of drainage of those rivers was 

 being lowered annually by subaerial denudation ; for the mate- 

 rial carried down by those rivers must all be derived from the 

 surface of the country drained by them. When I published 

 the result of my calculations, from the amount of sediment car- 

 ried down by the Mississippi, regarding the rate at which the 

 North- American continent is being lowered by denudation, I was 

 not at the time aware that Mr. Alfred Tylor had arrived at some- 

 what similar results by the self-same method nearly fifteen 

 years ago*. His object was to show that the relative level of the 

 * Phil. Mag. S. 4, vol. v. (1853). 



