286 Transactions. — Geology. 



Mr. Fishei- tlien proceeds to attack my illustration of the tlieor}^ from tlic 

 "Weald. But the Weald was not " adduced to give verisimilitude to this 

 theory " as Mr. Fisher supposes, neither did I " pretend " to any precise 

 measurements, as any unprejudiced reader will see, but it was given as an 

 example of the way in which the theory might be tested in the field. 



I have not access to any pi-ecise data as to the thickness of the beds, or the 

 height or breadth of the anticlinal, and exact measurements would have been 

 quite useless unless we also knew exactly the rate of expansion. In geological 

 enquiries mathematical investigation can only be used as a check to our 

 speculations, and as giving us a limit beyond which we cannot go. The 

 average thickness of the cretaceous rocks was taken from Jukes' " Manual " 

 (1862, p. 602), and the height of the hills in the Weald from Lyell's 

 " Elements of Geology." If the true thickness was under-estimated, by so 

 much would my example tell against myself. The rocks below the wealden 

 were not taken into consideration because they were the old surface, and had 

 nothing to do with raising the temperature. Neither did I ever regard the 

 wealden area as an isolated dome-shaped elevation, but the other elevated 

 areas have, by the deposition theory, nothing to do with the amount of 

 elevation of the Weald. 



With regard to the latter part of the paragraph, it is, I believe, uncertain 

 whether the tertiary rocks ever extended over the chalk or not ; at any rate 

 the fresh- water beds, as well as the vegetable remains of the London clay, show 

 that land was then in the neighbourhood, which land must have been elevated 

 since the deposition of the chalk in a deep sea. The depression succeeding the 

 Woolwich beds no doubt took place after the dome of the Weald was formed ; 

 bxit I must leave these questions^to be woi'ked out by those geologists who 

 have an intimate local knowledge of the district. 



Hitherto I 'have confined myself to urging the claims of the deposition 

 theory, but as Mr. Fisher says that he has " not had tlie good fortune to hear 

 of the many arguments which have been urged against " the tlieoiy that he 

 advocates, I will briefly state the reasons that have led me to reject the 

 contraction theory as giving a sufficient explanation of the formation of 

 mountains. I do so with the less reluctance, because nearly all rival theories 

 in natural science must ultimately be weighed by the balance of probabilities, 

 and it is therefore just as important to argue against a theory as to argue in 

 favour of it. (Appendix.) 



My reasons for I'ejecting the contraction theory are : — 1. Contraction of 

 the earth could not produce any tangential pressures except in solid rock, so 

 that the lateral compression must be confined to the rigid crust ; consequently 

 the more rapid contraction of the lower beds could only cause the upper beds 

 to rise into anticlinals by one solid portion slij^ping horizontally over another 



