THE GLACIAL PERIOD AND THE EARTH-MOVEMENT HYPOTHESIS. 263 



observation agree in showing that the extent of glaciation depends 

 chiefly on the height of the snow-line so defined. There is a region 

 in Eastern Siberia where the ground, at the depth of a few feet, is 

 frozen all the year round, showing that the mean temperature of 

 the year is below frost ; and yet over that frozen subsoil cattle 

 graze, crops of rye are harvested, and pine forests flourish. It is 

 obvious that if from any cause the extremes of that climate were 

 to disappear, while its mean temperature were to remain unchanged, 

 so that there was a temperature below freezing for every month of 

 the year, all the precipitation would be of snow, which would 

 remain unmelted, and the land would be covered with continual 

 ice like Greenland. 



There is an astronomical cause which must produce such changes. 

 The major axis of the earth's orbit is unchangeable, but the minor 

 axis is subject to slow fluctuating changes of length ; and as the 

 sun is always in one of the foci of the ellipse of the earth's orbit, 

 it follows that the narrower the orbit, the greater is its eccentricity, 

 and the greater the difference between the earth's perihelion and 

 aphelion distances — in other words, its least and its greatest distance 

 from the sun. Wow, when the earth's aphelion occurs in the 

 summer of either hemisphere, there must in that hemisphere be a 

 cold summer ; and a cold summer, as we have seen, produces 

 glaciation, so that the hemisphere having an aphelion summer had 

 a glacial climate. During the winter of the same hemisphere, the 

 earth was at its perihelion, or minimum, distance from the sun, 

 giving the glaciated hemisphere a mild winter, which had no effect 

 whatever on its glaciation ; and the opposite hemisphere had at 

 the same time an intensely hot summer, which promoted evapora- 

 tion, part of which evaporation must have fallen in snow on the 

 glaciated hemisphere. It thus appears that at definite times in the 

 past, the two conditions of maximum glaciation must have been 

 fulfilled in each of the Earth's hemispheres, namely, a cold summer 

 and a snowy winter. 



If this view of the nature and cause of the glacial climate is 

 correct, the northern and southern hemispheres were never glaciated 

 at the same time. But the periods of great eccentricity of the 

 earth's orbit last for a long time, and during their continuance the 

 two hemispheres were glaciated alternately, at the geologically 

 short interval of about 10,500 years ; at the end of which period 

 the perihelion and aphelion have arrived at opposite points in the 

 circle of the year to those which they respectively occupied at its 

 beginning. Either solstice coincides with the perihelion or aphelion 

 once in about 21,000 years; so that if, as is nearly the case at 

 present, the northern mid- winter falls in perihelio and the southern 

 in aphelio, at the end of half this period, or 10,500 years, the 

 positions of perihelion and aphelion, relatively to summer and 

 Avinter, will be reversed. 



This explains the fact of inter-glacial periods : while there was 



