34 THE POLAR GLACIERS. 



the equator. In this belt of ascending air-currents is carried up 

 the greater part of the moisture which afterwards descends as rain 

 or snow far from the equator. Whatever excess of solar heat 

 there may be in the tropics is here absorbed in evaporating water. 

 To vaporize a pound of water, according to Prof. Tyndall, re- 

 quires as much heat as to raise fifty-five pounds of ice-water to 

 the boiling-point. It is manifest, therefore, that there must have 

 been, during the glacial periods, an enormous amount of sun- 

 power somewhere on the face of the earth to have supplied the 

 vapor that buried one zone and half of another beneath a solid 

 ocean of ice. 



These facts effectually do away with all the theories, except 

 the astronomical, which have been advanced by physicists to 

 account for glacial phenomena ; one, that our solar system has, 

 during certain ages, passed through a colder region of space ; 

 another, that the sun in glacial times for some cause failed to 

 supply his usual quantity of heat; and, as a consequence of 

 either, that the glaciation of both hemispheres occured at the 

 the same time. Equatorial heat is as necessary to a glacial period 

 as polar cold. The one transforms the waters to vapor, and ele- 

 vates it to the cloud-spheres, wdiile the other sends in the cold 

 winds beneath, which compel the vapors to come over to the 

 frozen side and build up the glacier. 



The system of the stratified rocks has been called the great 

 geological book, with its uncounted leaves overlying each other. 

 Now as it is a part of the glacial theory that each of these leaves 

 or strata, at least in greater part, was the work of a glacial period, 

 it is important for us to examine closely and particularly the 

 course and effect of one of these great cycles of about 21,000 

 years. We will take for example, that one of the Post-tertiary 

 glacial which was of the greatest extent and severity. Ten 

 cycles back, about 210,000 years ago, one of the periods of 

 maximum eccentricity had just commenced, the highest since 

 four times that number of years. The perigee, or nearest ap- 

 proach to the sun, happened then as now, a few days after the 



winter solstice of our half of the world. It was the great summer 



o 



of the northern hemisphere. But over the southern hemisphere 



