CONDITIONS NEVER UNIFORM. 191 



nation of colder and warmer cycles over the northern hemi- 

 sphere, at least ; and if such has really ^ 

 been the case, we must seek the ex- g / 

 planation not in revolutions and cata- J \ 

 clysms, but in some fixed and contin- N> X N g 

 uously operating law.* Whether the % \5 

 phenomena may depend on causes j % 

 operating on and within the globe ,*'' % 

 itself, so as to change the axis of / 

 rotation, or whether it may not more 5 \ 



* This idea of colder and warmer cycles as N % 



affecting the northern hemisphere was indicated x ^ 



some years ago to the Literary and Philosophi- ' Is 



cal Society of St Andrews. Since then the / o 



author has endeavoured to establish the fact, +'' 



partly by the character and composition of the - / aj 



rocks of the colder periods, and partly by the j ' ' 



nature of their fossil contents. Much, however. ^ / \ 



O ^ \ r~) 



still remains to be done, and he would earnestly # X N X 



solicit the attention of geologists to the subject, S N a 5 



and this altogether apart from the cosmical ^ * "~ *"" 



causes to which the recurrences may be due. ^. J & O 



On this latter aspect of the question some w f S 



discussion took place in the A tkenceum of 1860, g f /' 



on the suggestion of Colonel James of the Ord- / 



nance Survey, that former changes of climute \ 



may be due to changes in the inclination of the ^ \ x 



earth's axis, brought about by alterations in ^ XX N 



the crust that gradually affect the centre of \ ^ 



gravity. Whatever the cause whether it is to 



be sought for on or within the globe itself, or /' g 



in purely astronomical influences this is not /"' 



the place to discuss ; but most unmistakably / 



the gradual uprise of land that is now taking "| { 



place in the arctic regions, the shifting of vol- 55 \ 



canic areas in the northern hemisphere since XX N 



the tertiary period, and the approach and de- NS X J 



parture of the boulder epoch over the same ' 



latitudes, all point to the operation of some J | 



determinate law of secular succession. May it "' 



not be, that in the periodicity of this law we may yet discover the key 



to the expression of geological chronology in years and centuries ? 



