unquestionably followed every change in the glaciation of the hemispheres from astronomical causes. The winds and the water-currents have always helped to increase the difference in temperature which a considerable eccentricity of the earth's orbit must always have produced between the northern and southern halves of our globe. It matters but little which of the two—the ocean-currents or the astronomical causes—have produced the greater effect, since it is certain that they have ever coöperated in one and the same direction.
On all the tropical seas, between the terminal lines of the two trade-winds, there is what is called the belt of calms, a tract averaging from 300 to 500 miles wide, in which, whatever winds there may be, are exceedingly light and unreliable. It is here, as we have seen, that the air and vapor, heated by the vertical rays of the sun, are continually rising and spreading outward in the upper regions. It is a complete dividing line between the climates of the two hemispheres. One may be frigidly cold, while the other is highly heated; the only difference being that the calm belt would be removed farther into the warmer hemisphere. It now ranges from five to ten degrees of latitude on this side of the equator. In this belt of ascending air-currents is carried up the greater part of the moisture which afterward descends as rain or snow far from the equator. Whatever excess of solar heat there may be on the tropics is here absorbed in evaporating water. To vaporize a pound of water, according to Prof. Tyndall, requires 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 occurred at the same time. Equatorial heat is as necessary to a glacial period as polar cold. The one transforms the waters to vapor and elevates it to the cloud-spheres, while 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 21,000 years or thereabouts. We will take, for example, that one of the Post-tertiary glacial which
