Earth as an Abode fitted for Life. 69 



u Mr. Darwin, in Ms admirably reasoned book on the origin of species, 

 so full of information and suggestion on all geological subjects, estimates 

 the time required for the denudation of the rocks of the Weald of Kent, 

 or the erosion of spac3 between the ranges of chalk hills, known as the 

 North and South Downs, at three hundred millions of years. The 

 grounds for forming this estimate are of course of the vaguest de- 

 scription. It may be possible, perhaps, that the estimate is a hundred 

 times too great, and that the real time elapsed did not exceed three 

 million years, but, on the other hand, it is just as likely that the time 

 which actually elapsed since the first commencement of the erosion till it 

 was nearly as complete as it now is, was really a hundred times greater 

 than his estimate, or thirty thousand millions of years." 



§ 7. Thus Jukes allowed estimates of anything from 

 3 millions to 30,000 millions as the time which actually 

 passed during the denudation of the Weald. On the other 

 hand Professor Phillips in his Rede lecture to the University 

 of Cambridge (18(30), decidedly prefers one inch per annum 

 to Darwin's one inch per century as the rate of erosion : and 

 says that most observers would consider even the one inch 

 per annum too small for all but the most invincible coasts ! 

 He thus, on purely geological grounds, reduces Darwin's 

 estimate of the time to less than one one-hundredth. And, 

 reckoning the actual thicknesses of all the known geological 

 strata of the earth, he finds 96 million years as a possible 

 estimate for the antiquity of the base of the stratified rocks ; 

 but he gives reasons for supposing that this may be an over- 

 estimate, and he finds that from stratigraphical evidence 

 alone, we may regard the antiquity of life on the earth as 

 possibly between 38 millions and 96 millions of years. 

 Quite lately a very careful estimate of the antiquity of 

 strata containing remains of life on the earth has been 

 given by Professor Sollas, of Oxford, calculated according 

 to stratigraphical principles which had been pointed out by 

 Mr. Alfred Wallace. Here it is * : — " So far as I can at 

 present see, the lapse of time since the beginning of the 

 Cambrian system is probably less than 17,000,000 years, 

 even when computed on an assumption of uniformity, which 

 to me seems contradicted by the most salient facts of geology. 

 Whatever additional time the calculations made on physical 

 data can afford us, may go to the account of pre-Cambrian 

 deposits, of which at present we know too little to serve for 

 an independent estimate." 



§ 8. In one of the evening Conversaziones of the British 

 Association during its meeting at Dundee in 1867 I had a 

 conversation on geological time with the late Sir Andrew 

 Ramsay, almost every word of which remains stamped on 



* "The Age of the Earth," < Nature,' April 4th, 1895. 



