6i 8 THE POPULAR SCIENCE MONTHLY. 



than or as fast as it accumulated ; if the mean temperature is low, 

 the snowfall may be light and yet the glaciers accumulate, as the 

 heat would be insufficient to melt that which did fall ; but if the 

 mean temperature is so high as to prevent the accumulation of 

 snow, or so low as to prevent the formation of aqueous vapor, 

 there can be no glaciers formed, the last conclusion being subject 

 to the qualification that a vapor-laden atmosphere may be carried 

 by prevailing winds from a warm climate to a cold one, and the 

 vapor there condensed and precipitated. 



One hypothesis is that the whole solar system passes at times 

 through stretches of space of different temperatures, and that 

 the Glacial period coincides with a time when the solar system 

 was passing through a low-temperature area ; another is that the 

 heat of the sun varies, such variations being the result mainly of 

 contact with meteorites, as the impact of bodies generates heat, 

 the idea being that the Glacial epoch coincides with a time when 



FIG. 2. SECTION THROUGH CAVE. 



few meteorites were colliding with the sun, the heat emanating 

 therefrom being therefore decreased. These two hypotheses agree 

 in one particular they can neither be proved nor disproved, con- 

 sequently their only value is speculative. 



Again, it is supposed that the earth's axis has shifted that 

 during the Glacial period the north pole was in Greenland. This 

 seems to be negatived by the slight observed shifting of the pole, 

 and the fact that Tertiary fossil flora, immediately preceding the 

 Glacial period, of both Greenland and the present arctic regions, 

 indicate a temperate climate. 



Adhe'mar the astronomer advanced the hypothesis, also ad- 

 vocated by the late Dr. James Cjroll and Prof. James Geikie, 

 that, as the earth's orbit is elliptical and as the sun is not central 

 to this orbit but some three million miles nearer one end than the 

 other, this fact in connection with the precession of the equinoxes 

 may explain the Glacial epoch ; it being held that during that 



