244 PROGRESS OF SCIENCE IN THE CENTURY. 



and successive times (as proved in some cases de- 

 cisively by their fossils and in other cases by other 

 facts) ; and " on a reasonable computation, these 

 stratified masses, where most fully developed, attain 

 a thickness of not less than 100,000 feet." * There- 

 fore, if we assume that the present rate of change is 

 at all comparable to the past rate of change, we can 

 form geologically some estimate of the antiquity of 

 our earth. ^' If they were all laid down at the most 

 rapid recorded rate of denudation, they would re- 

 quire 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 six hundred and eighty millions.'' f 



But how much experts may differ is here again 

 illustrated, for Prof. Sollas says : — " The total maxi- 

 mum thickness of the stratified rocks is 265,000 feet, 

 and consequently if they accumulated at the rate of 

 one foot in a century, as evidence seems to suggest, 

 more than twenty-six millions of years must have 

 elapsed during their formation." :}: 



Against this line of argument various objections 



may be raised. It may be said that the rate of 



denudation and therefore of deposition may have 



been much more rapid a few million years ago than 



it now is, and the possibility cannot be denied. But 



some evidence should be forthcoming; and there is 



not much. In ancient sedimentary rocks we see 



ripple marks and sun-cracks and worm or mollusc 



tracks and it may even be the markings of desiccated 



jelly fishes, just as we see them on the beach to-day, 



and this certainly does not point to rapid deposition. 



* A. Geikie, op. cit., p. 21. 

 t A. Geikie, op. cit., p. 21. 



$W. J. Sollas, Address Section C, Rep. Brit. Ass., 1900. 

 Nature, Sept. 13, 1900, p 485. 



