c* 



272 



EXCENTEICITY CAUSING EXTREME COLD. 



[Ch. XIII. 



dissipate during the course of the year. Hence he concludes 

 that such climates as we have described at page 224, as pre- 

 vailing in the Carboniferous epoch, when there was great 

 warmth and moisture in the air throughout temperate and 

 arctic latitudes, would be experienced during that phase of 

 precession (the excentricity being very large), when one 

 hemisphere — the northern, for example — had its winter in 

 perihelion, there being then an equality of seasons, or that 

 perpetual spring alluded to by Herschel. Mr. Croll has also 

 endeavoured to show that the vast accumulation of ice which 



would alternately take place at each pole during those 



thousands of years for which winter would occur when the 

 planet was farthest from the sun, would so derange the 

 earth's centre of gravity as to draw the ocean towards that 

 pole and cause the submergence of part of the land. M. 

 Adhemar, in 1843, had already endeavoured to account for 

 certain geological phenomena by a coincidence of the winter 

 solstice with aphelion, but without connecting them, as Mr. 

 Croll has done, with that greater excentricity of the earth's 

 orbit, which must occasionally in the course of ages vastly 

 exaggerate the effects alluded to. 



Although the deficiency of our data is such that we cannot 

 yet decide to what extent this excess of ice at certain periods 

 would act as a disturbing cause, yet as there can be no doubt 

 that it must have given rise at some points to a sensible dif- 

 ference in the ocean's level, we are greatly indebted to those 

 scientific writers who have called attention to a vera causa 

 hitherto neglected. To this subject I shall again allude in 

 the sequel. 



Mr. Croll, in the memoir alluded to, ascribes little or no 

 influence to abnormal geographical conditions, such as, ac- 

 cording to the principles explained in the last chapter, I 

 should consider indispensable in combination with a large 

 excentricity for the production of great cold. I also differ 

 from him in the opinion that a large excentricity, even as- 

 suming that it might give rise, as he suggests, to a storing 

 up of ice in high latitudes, or to a glacial period, would also 

 bring about, in the same hemisphere in the opposite phases 

 of precession, a period of perpetual spring. On the contrary, 



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