286 Transactions.— Geology. 
Mr. Fisher then proceeds to attack my illustration of the theory from the 
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 precise 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 ; 
but I must leave these questions to be worked 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 the good fortune to hear 
of the many arguments which have been urged against" the theory 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.) r 
My reasons for rejecting 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 onl y cause the upper beds 
to rise into anticlinals by one solid portion slipping horizontally over another 
