PRELIMINARY CONSIDERATIONS. i| 



One of these relates to the absolute duration of the time 

 represented by the geological history of the earth. Such 

 estimates as our present knowledge enables us to form are 

 very indefinite. Whether we seek for astronomical or geo- 

 logical data, we find great uncertainty. To such an extent is 

 this the case, that current estimates of the time necessary to 

 bring the earth from a state of primitive incandescence to its 

 present condition have vari^i from fifteen millions of years 

 to five hundred millions. Of the various modes proposed, 

 perhaps the most satisfactory as well as instructive is that 

 based on the rate of denudation of our present continents, as 

 indicated by the amount of sediment carried down by great 

 rivers. The Mississippi, draining a vast and varied area in 

 temperate latitudes, is washing away the American land at the 

 rate of one foot in 6,000 years. The Ganges, in a tropical 

 climate and draining many mountain valleys, works at the rate 

 of one foot in 2,358 years. The mean of these two great 

 rivers would give one foot in 4,179 years, at which rate our 

 continents would be levelled with the waters in about six 

 millions of years. But the land has been in process of renewal 

 •as well as of waste in geological time ; and a better measure 

 will be afforded by the amount of beds actually deposited. 

 The entire thickness of all the stratified rocks of Great Britain 

 has been calculated by Ramsay at 72,000 feet. Now, if we 

 suppose the waste in all geological time to have been on the 

 average the same as at present, a* \ that this material has been 

 deposited to the thickness of 72,000 feet on a belt of sea 

 margin 100 miles in width, we shall have about 86 millions of 

 years as the time required.i This has the merit of approxi- 

 mating to Sir William Thomson's calculation, based on the rate 

 of cooling of the earth, that a minimum of 100 millions of 

 years may represent the time since a solid crust first began to 

 form. As it is more likely that the rate of denudation has on 



Croll has elaborated this calculation in his work, C/imafe and Time, 



