238 J, Evans on Geological Changes, &c. 
level, would cause a great disturbance in the equilibrium of the 
crust, sufficient to overcome considerable resistance in its at- 
tempts to regain a state of equilibrium by a movement over its 
fluid nucleus. 
Whether the thickness of the earth’s crust was not in early 
ave here a vera causa such as would account for extreme 
variations from a tropical to an arctic temperature at the same 
spot, in a simpler ca more satisfactory manner than any other 
naposoesia. 
‘The former existence of cold in what are now warm latitudes 
might, and probably did in part, arise from other causes than & 
change in the axis of rotation, but no other hypothesis can well 
account for the existence of traces of an almost tropical vegetation 
within the arctic circle. 
* Lyell, ‘Principles of Geology, 1853, p. 88. 
