Let us now suppose the precipitation to be increased, the winters and summers remaining both in length and temperature what they were before. The amount of snow which a summer of given length and warmth can dispose of is, roughly speaking, a definite quantity. For it depends to a great extent only on its amount of heat. The summer precipitation may be taken as offsetting itself in the two hemispheres alike. If, then, the snowfall in the winter be for any reason increased daily in both, a time will come when the deposition due the longer winter of the one will exceed what its summer can melt relatively to the other, and a permanent glaciation result in the hemisphere so circumstanced. Increased precipitation, then, not eccentricity of orbit, is the real cause of an Ice Age. And this astronomic deduction we owe not to theoretic conclusions, for which we lack the necessary quantitative data, but wholly to study of our neighbor in space. Had any one informed our geologic colleagues that they must look to the sky for definite information about the cause of an Ice Age, they would probably have been surprised. With this Martian information, received some years ago, it is pleasing now to see that Earthly knowledge is gradually catching up. For that increased precipitation could account for it, the evidence of pluvial eras in the equatorial regions, contemporaneous with glacial periods, indicates. But another and probably the chief factor involved was not a generally increased precipitation, potent as that would be, but an increased snow deposit due to temporary elevation of the ground. [Illustration: GLACIAL MAP OF EURASIA—AFTER JAMES GEIKIE.] [Illustration: MAP SHOWING THE GLACIATED AREA OF NORTH AMERICA—THE ARROWS INDICATING THE DIRECTION OF ICE MOVEMENT—CHAMBERLIN AND SALISBURY.]