Date of the Glacial and the Upper Miocene Period. 145 



On looking over Tabic I., it will be seen that there are three 

 principal periods when the execntricity rose to a very high value, 

 with a few subordinate maxima between. It will be perceived 

 also that during each of those periods the excentricity does not 

 remain at the same uniform value, but rises and falls, in one case 

 twice, and in the other two cases three times. About 2,650,000 

 years back we have the excentricity almost at its inferior limit. 

 It then begins to increase, and fifty thousand years afterwards, 

 namely at 2,600,000 years ago, it reaches "0660 ; fifty thousand 

 years after this period it has diminished to *0167, which is about 

 its present value. It then begins to increase, and in another 

 fifty thousand years, viz. at 2,500,000 years ago, it approaches 

 to almost the superior limit, its value being then '0721. It 

 then begins to diminish, and at 2,450,000 years ago it has dimi- 

 nished to *0252. These two maxima, separated by a minimum and 

 extending over a period of 200,000 years, constitute the first 

 great period of high excentricity. We then pass onwards for 

 upwards of a million and a half years, and we come to the second 

 great period. It consists of three maxima separated by two 

 minima. The first maximum occurred at 950,000 years ago, 

 the second or middle one at 850,000 years ago, and the third 

 and last at 750,000 years ago — the whole extending over a pe- 

 riod of nearly 300,000 years. Passing onwards for another 

 million and half years, or to about 800,000 years in the future, 

 we come to the third great period. It also consists of three 

 maxima one hundred thousand years apart. These occur at the 

 periods 800,000, 900,000, and 1,000,000 years to come, respec- 

 tively, separated also by two minima. Those three great periods, 

 two of them in the past and one of them in the future, included 

 in the Table, are therefore separated from each other by an in- 

 terval of upwards of 1,700,000 years. 



In this Table there are three periods when the excentricity 

 approaches to about its inferior limit, and the orbit becomes 

 almost circular. The first is 2,650,000 years ago, when the ex- 

 centricity was -0053 ; the next is at 1,300,000 years ago, when 

 it was only *0022 ; and the next will occur in about 24,000 

 years hence, when its value will be '0033. There is, there- 

 fore, an interval of 1,350,000 years between the minima in 

 the one case, and an interval of 1,324,000 years in the other 

 case. 



The Table shows also four subordinate periods of high excen- 

 tricity, of which the one that I have supposed corresponds to the 

 glacial epoch is the chief. Three are in the past, and one in the 

 future. 



If the glacial epoch resulted from a high state of excentricity, 

 there must have been at least five ice-periods, including the gla- 



Phil. Mag. S. 4. Vol. 36. No. 241. Aug. 1868. L 



