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



cial epoch, during the past three millions of years — two as severe, 

 and two much less severe than the glacial epoch. 



It may be thought that so many as four or five ice-periods in 

 the course of 3 millions of years past is inconsistent with the 

 evidence of geology on that point. This, however, is at least 

 very doubtful. It is quite possible that these three millions of 

 years may embrace the greater part of the Tertiary period. Now 

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

 of the Tertiary period — one about the middle of the Eocene 

 period, another during the Upper Miocene period, and the third 

 and last the well-known glacial epoch* ; and it is quite possible 

 that the evidence of more may yet be found. 



But before discussing the nature of the evidence which geo- 

 logy affords of the existence of former glacial periods, we shall 

 turn our attention briefly to Tables II., III., and IV. These 

 three Tables embrace the three periods of greatest excentricity 

 during the past 3 millions of years. The excentricity, longitude 

 of the perihelion, &c. are given at periods of ten thousand years 

 apart. 



There are still eminent astronomers and physicists who are 

 of opinion that the climate of the globe never could have been 

 seriously affected by changes in the excentricity of its orbit. 

 This opinion results, no doubt, from viewing the question as a 

 purely astronomical one. Viewed from an astronomical stand- 

 point, as has been already remarked, there is actually nothing 

 from which any one could reasonably conclude with certainty 

 whether a change of excentricity would seriously affect climate 

 or not. By means of astronomy we ascertain the extent of the 

 excentricity at any given period, how much the winter may 

 exceed the summer in length (or the reverse), how much the 

 sun's heat is increased or decreased by a decrease or an increase 

 of distance, and so forth ; but we obtain no information what- 

 ever regarding how these will actually affect climate. This 

 must be determined wholly from physical considerations, and it 

 is an exceedingly complicated problem. An astronomer, unless 

 he has given special attention to the physics of the question, 

 is just as apt to come to a wrong conclusion as any one else. 

 The question involves certain astronomical elements ; but when 

 these are determined, everything then connected with the 

 matter is purely physical. Nearly all the astronomical ele- 

 ments of the question are comprehended in the accompanving 

 Tables. 



In Tables II., III. and IV., column I. represents the dates 

 of the periods, column II. the excentricity, column III. the 



* See Lyell's ' Principles, 5 vol. i. chap. x. (tenth edition). 



