WAEKEN UPHAM, ESQ., ON CAUSES OP THE ICE AGE. 211 



dnring a cycle of about 31,000 years varies 12° in its inclina- 

 tion to the plane of the ecliptic or path of the earth around 

 the sun. In this long cycle the axis and poles of the earth 

 are thought to describe a circle in the heavens with its 

 centre 6° from the pole of the ecliptic. At present the 

 obliquity of the ecliptic or angle between its plane and that 

 of the earth's equator is about 2H^°, which therefore is the 

 distance of the arctic and antarctic (urcles from the poles ; and 

 this, according to General Drayson's computations, is nearly 

 their minimum distance. He claims that this obliquity of 

 the ecliptic, which gives the distance of the arctic circles 

 from the poles and of the tropics from the equator, about 

 5,000 years ago was some 2° more than now ; that 7,500 

 years ago it was increased 6-^" more than at present ; that 

 its maximum, nearly 12^^ more than at present, was about 

 13,500 B.C.; and that the beginning of this latest cycle of 

 variation in the widths of the intertropical and polar zones 

 was about 31,000 years ago. During the middle portion of 

 the cycle. General Drayson affirms that the Arctic circle 

 reached approximately to 54° north latitude, and that the 

 resulting climatic changes caused the Ice age. 

 20 It is true that the obliquity of the ecliptic varies slightly 

 and is at present decreasing about an eightieth part of a 

 degree in a hundred years. Sir John Herschel computed, 

 however, that its limit of variation during the last 100,000 

 years has not exceeded 1° 21' from its mean, although for a 

 longer time in the past, as millions of years, it may range 

 three or four degrees on each side of the mean. The 

 portion of the present cycle of variation which is used as 

 tlie basis of this theory seems insufficient to establish its 

 conclusion of a wide range of obliquity ; but. even if this 

 Were true, the same arguments forbid its application to 

 account for the Glacial period as are urged by Gilliert, Chani- 

 berlin, and Le Conte, in their dissent from Croll's theory.* 

 These objections consist in the absence of evidence of 

 glaciation during the long history of the earth previous to 

 the Ice age, excepting near the end of Paleozoic time, and 

 the unsymmetric geogiaphic areas of the ice-sheets, northern 

 Asia and Alaska having not been ice-enveloped. Accord- 

 ing to General Drayson's astronomic conditions capable of 

 ]>roducing an ice age have recurred every 31,000 years; but 

 geologists have recognized no other time of glaciation of 



* See Wright's Ice Age hi Nortk America, pp. 439, 440. 



