362 Mr. J. Croll on Geological Time, and the probable 



out, for the experiments which speak in its favour are Dot more 

 numerous or more accurate. 



Moreover the result which these heatings of the poles give is 

 nothing more than a confirmation of the conclusions which I 

 have drawn from my earlier investigation on the voltaic arc. 

 No one will doubt that the electromotive force which, from the 

 experiments adduced, is met with in the just-extinguished arc is 

 the same as that which, from my former investigations, occurs in 

 the luminous arc during its existence. But it has been shown* 

 in regard to this force, that it is independent of the intensity of 

 the current. It is only when the intensity approaches the mini- 

 mum with which aluminous arc can be produced, that a decrease 

 in the magnitude of the force begins to be seenf. But the tem- 

 perature of the pole-points is dependent on the intensity of the 

 current, and the same is also obviously the case with their differ- 

 ence in temperature. The force found is thus independent of the 

 heating of the poles, and therefore cannot be of thermoelectric 



XL VII. On Geological Time, and theprobable Date of the Glacial 

 and the Upper Miocene Period. By James Croll, of the 

 Geological Survey of Scotland. 



[Continued from p. 154.] 



AS has already been remarked, if the glacial epoch resulted 

 from a high state of excentricity there must, according to 

 the foregoing Tables, have been at least five ice-periods during the 

 last three millions of years, viz. the glacial epoch, two as severe, 

 and two much less severe. The question which now remains to 

 be discussed is whether this conclusion is consistent with the 

 facts of geology or not. Can it be shown from geological evi- 

 dence that there have not within the past three millions of years 

 been two ice-periods as severe as the glacial period ? In other 

 words, do the testimony of geology and the testimony of physics 

 agree in regard to this point ? 



From what has already been shown respecting the limit to 

 geological time, I presume we may reasonably conclude that three 

 millions of years may probably embrace the greater portion of 

 the Tertiary period. Now, as has been stated, we have good 

 evidence of at least three ice-periods since the beginning of the 

 Tertiary period — one about the middle of the Eocene period, an- 

 other during the Upper Miocene period, and the third and last 

 the well-known glacial epoch. 



* Phil. Mag. S. 4. vol. xxxv. p. 103. 



t (Efoersigt af K. Vet. Akad. Forhandl. December 1867. 



