Hill. — Denudatio7i as a Factor of Geological Time. 667 



owing to the more rapid motion wliich would have taken 

 place ; whilst Professor Tait, agreeing generally with the views 

 put forward by Helmholtz and Newmaii, says that "the 

 amount of heat produced by the impact of masses from space 

 which are assumed to have given origin to our sun could not 

 have supplied the earth even with its present quantity of 

 heat for a much longer period than twenty millions of years. 

 Nature, 3rd January, 1895, contains a criticism by Mr. John 

 Perry of Sir William Thompson's (now Lord Kelvin) esti- 

 mate, in which it is stated that "if at the beginning of time 

 there was an increase of 1'^ Centigrade in 45ft. (towards the 

 earth's interior), and now there is an increase of 1° Centigrade 

 in 90ft., the lapse of time is 28,930,000,000 years, or 290 times 

 Lord Kelvin's estimate, and the core has cooled from 8,000 to 

 4,000 degrees." 



It will thus be seen how diverse are the views of the physi- 

 cists who deal with the age of the earth from a mathematical 

 standpoint, and it seems very doubtful whether such estimates 

 as those given above have any scientific value whatever. Were 

 it to be announced that a bank-note for a large amount had 

 been lost either between Napier and Duuedin, or between 

 those places and the Cape of Good Hope, the announcement 

 would be just as general and unsatisfactory as that made by 

 Sir William Thompson and others as to the age of the earth. 

 It is true that such estimates often form the basis of what are 

 termed " working theories" ; but as standards of scientific value 

 they are of small importance, and it is well to keep this in 

 mind when considering the question of earth-changes such as 

 we now see in daily progress. But it is curious how geologists, 

 having received a "time-period theory" from the mathema- 

 ticians, have endeavoured to bring that theory into the full 

 light of day by striving to show that the organic and surface 

 changes on the earth must have required an allowance of time 

 equal to what the mathematicians say must have gone by since 

 the earth became a solid mass and assumed its present form. 

 The late Dr. Croll, for example, suggested a period of not less 

 than sixty millions of years as being needed to bring about 

 the changes or the conditions such as we now have ; whilst 

 Dr. Houghton, in the second of his able lectures on " Physical 

 Geography," pp. 94-95, estimates " the whole duration ol geo- 

 logical time down to the Miocene-Tertiary epoch at 152,675,000 

 years, and for the whole duration of geological time a mini- 

 mum of two hundred millions of years." Now, Croll and 

 Houghton have based their estimates upon the maximum 

 thickness of the known stratified rocks, and with this they 

 have taken the rates of denudation of certain rivers as deter- 

 mined by the amount of sediment which such rivers annually 

 carry to the sea. Thus, Dr. Houghton savs that the total 



