212 GEOLOGY [CHAP. IX 



Letter 545 almost monstrous that Professor Tait l should say that the 

 duration of the world has not exceeded ten million years. 

 The argument which seems the most weighty in favour of 

 the belief that no great number of millions of years have 

 elapsed since the world was inhabited by living creatures is 

 the rate at which the temperature of the crust increases, and 

 I wish that I could see this argument answered. 



Letter 546 To J. Croll. 



Down, Aug. Qth. 1877. 



I am much obliged for your essay, which I have read with 

 the greatest interest. With respect to the geological part, 

 I have long wished to see the evidence collected on the time 

 required for denudation, and you have done it admirably. 2 I 

 wish some one would in a like spirit compare the thickness 

 of sedimentary rocks with the quickest estimated rate of de- 

 position by a large river, and other such evidence. Your main 

 argument with respect to the sun seems to me very striking. 



My son George desires me to thank you for his copy, and 

 to say how much he has been interested by it. 



VII. GEOLOGICAL ACTION OF EARTHWORMS, 1880-82. 



" My whole soul is absorbed with worms just at present." (From a 

 letter to Sir W. Thistleton-Dyer, Nov. 26th, 1880.) 



Letter 547 To T. H. Farrer (Lord Farrer). 3 



The five following letters, written shortly before and after the pub- 

 lication of The Formation of Vegetable Mould through the Action of 

 Worms, 1 88 1, deal with questions connected with Mr. Darwin's work 

 on the habits and geological action of earthworms. 



Down, Oct. 2Oth, 1880. 



What a man you are to do thoroughly whatever you 

 undertake to do ! The supply of specimens has been 



1 Lecture on Some Recent Advances in Physical Science, by P. G. Tait, 

 London, 1876. 



2 In a paper " On the Tidal Retardation Argument for the Age of the 

 Earth" (Brit. Assoc. Report, 1876, p. 88), Croll reverts to the influence of 

 subaerial denudation in altering the form of the earth as an objection to 

 the argument from tidal retardation. He had previously dealt with this 

 subject in Climate and Time, Chap. XX., London, 1875. 



3 See note, Vol. I., p. 393. 



