INORGANIC GEOLOGICAL DYNAMICS. 550 



Sect. 6. Theories of Changes of Climate. 



As we have already stated, Geology offers to us strong evidence that 

 the climate of the ancient periods of the earth's history was hotter than 

 that which now exists in the same countries. This, and other circum- 

 stances, have led geologists to the investigation of the effects of any 

 hypothetical causes of such changes of condition in respect of heat. 



The love of the contemplation of geometrical symmetry, as well as 

 other reasons, suggested the hypothesis that the earth's axis had ori- 

 ginally no obliquity, but was perpendicular to the equator. Such a 

 construction of the world had been thought of before the time of Mil- 

 ton," as what might be supposed to have existed when man was ex- 

 pelled from Paradise ; and Burnet, in his Sacred Theory of the Earth 

 (1690), adopted this notion of the paradisiacal condition of the globe 



The spring 



Perpetual smiled on earth with verdant flowers, 

 Equal in days and nights. 



In modern times, too, some persons have been disposed to adopt 

 this hypothesis, because they have conceived that the present polar 

 distribution of light is inconsistent with the production of the fossil 

 plants which are found in those regions, 12 even if we could, in some 

 other way, account for the change of temperature. But this alteration 

 in the axes of a revolution could not take place without a subversion of 

 the equilibrium of the surface, such as does not appear to have occurred; 

 and the change has of late been generally declared impossible by phy- 

 sical astronomers. 



The effects of other astronomical changes have been calculated by 

 Sir John Herschel. He has examined, for instance, the thermotical 

 consequences of the diminution of the eccentricity of the earth's orbit, 

 which has been going on for ages beyond the records of history. He 

 finds 18 that, on this account, the annual effect of solar radiation would 

 increase as we go back to remoter periods of the past ; but (probably 

 at least) not in a degree sufficient to account for the apparent, past 



11 Some said he bade his angels turn askance 

 The poles of earth twice ten degrees and more 

 From the sun's axle, <tc. Paradise Lost, x. 214. 

 a Lyell, i. 155. Li ulley. Fossil Flora. " Geol. Trans, vol. iii. p. 295. 



