Xliv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



genus Banksia. In places also tree -stems of great magnitude stand 

 erect on the rocks below this great old lava-flow, enter it upwards, 

 and are embraced by it, and partly mineralized within it. In some 

 future day the deep workings may penetrate under the capping of 

 basalt, and disclose new wonders preserved under this rocky wax, 

 which nature freely uses in sealing down the memorials of earlier 

 time. 



Geological Theory. 



Side by side with the numerous — I had almost said the innume- 

 rable — data which have been collected for geological inference, the 

 masters of mechanical, chemical, and physiological science are silently 

 establishing, though in a fragmentary state, those special laws of cau- 

 sation, by which, if at all, we are to combine our data inta general 

 theory. 



If no new force be added to the system of nature, neither has any 

 old force died away. Natural forces may, indeed, be gathered into 

 treasure-houses, and reserved, as in a seed, to germinate another 

 day ; but, like the moral forces which sway mankind, they are not 

 permitted to die, though they may appear to sleep. 



The force which moulded our planet into its spheroidal form still 

 exists to maintain equihbrium in its rotating mass*, the products 

 of chemical affinity are the same to-day as in the earlier ages of the 

 world, under the same circumstances ; but as these circumstances — 

 that is to say, the combinations of natural conditions — alter, so the 

 effects of mechanical and chemical agencies are measured by standards 

 which vary with time. In a still greater degree is this found to be 

 true when we think of the phoenomena of life, which are made to 

 depend on mechanical and chemical quantities, and to vary in some 

 proportion to the changes of these. 



In what direction and to what degree are the general physical or 

 mechanical conditions of the globe subject to variations ? By what 

 results are such variations followed ? 



That the mean dimensions of the globe are not subject to notable 

 changes, seems evident by the experience of 3300 years ; for during 

 that period of time, the relative length of the year and the day, as 

 determined by Hipparchus, has remained sensibly the same, which 

 probably would not have been the case if the unchanging velocity of 

 rotation had been operative on a variable mean diameter f. 



* Sir John Herscliel has indeed suggested, and it may be readily admitted, 

 that the earth, if constructed originally a sphere, and then set to rotate, would, 

 by the action of surface-agencies, change its figure and become spheroidal ; but 

 he has not attempted to show that the actual ellipticity would be so acquired 

 (' Astronomy,' chap. iii.). Professor Hennessy concludes that, on the supposition 

 of the external figure of the earth being due to the abrading action of water at 

 the sm'face, the compression should be ^ix^ ^nd not, as it is, about ^^, this 

 latter fraction agreeing with the hypothesis of primitive fluidity (Mem. Eoy. 

 Irish Acad. ; and Brit. Assoc. Eeport, 1852). 



t It appears that approximate constancy in the length of the day is not incon- 

 sistent with the admission of a real diminution of diameter ; for its effect might 

 be counteracted by a change in the law of density of the fluid nucleus. (Hen- 

 nessy " On the Earth's Internal Structure in relation to the Length of the Day," 

 Phil. Mag., 1856.) 



