CLIMATE. 2 1 



inference, that in the course of geological ages the climate of 

 the British area has changed again and again, so that this 

 limited tract has probably witnessed as many variations of 

 climate, and scene, as are now distributed over the face of 

 the globe. 



Dr. J. Croll regards changes of climate as due in the 

 main to cosmical causes, in other words, to gradual variations 

 in the excentricity of the earth's orbit. By such means 

 physical agencies that affect climate are brought into opera- 

 tion, and among these, the most important would be the 

 deflection of Ocean currents.^ Of course, when we realize 

 that the orbit of our earth does vary, and that the Sun itself 

 is travelling around some far-distant and at present undis- 

 covered centre of attraction, we can understand that almost 

 endless combinations might take place which would render 

 the earth's surface warmer or cooler. 



Geological changes themselves must have had considerable 

 influence upon climate. The uplifting of mountain masses, 

 the subsidence of large tracts, the denudation of a country, 

 and in fact any changes in the distribution of land and water, 

 must have had considerable effect on the climate. It has 

 been suggested, too, that some displacement of the earth's 

 axis of rotation may have been caused by the elevation of 

 mountains, or the excessive accumulation of ice at particular 

 periods. 



Dr. C. W. Siemens, moreover, has advanced the theory that 

 the heat of the sun is maintained by the combustion of gases 

 diffused in the medium through which it moves, and which 

 are drawn in at the polar, and after combustion, returned by 

 centrifugal force from the equatorial parts of the sun into 

 space. Remarking on this theory, Mr. S. V. Wood, jun., 

 observes that it would furnish an explanation of the variation 

 in the Sun's heat, since the quantity of diffused gases may 

 vary in different parts of the medium through which the Sun 

 moves.2 Hence many causes may have helped directly or 

 indirectly to modify climate. 



In regard to Geological time as compared with Historical 

 time, but little can be said with certainty, except that it is 

 ' inconceivably great.' 



The total thickness of the known sedimentary strata of the 

 British Isles may be estimated at upwards of 80,000 feet ; but 

 we must not unite the thicknesses of the strata at their maxi- 



^ Climate and Time in their Geological Relations, 1875. 



- Q. J. xxxviii. 736 J see also J. Evans, Address to Geol. Soc. 1876. 



