With the opening of the Quaternary Period, we come to what is undoubtedly the most remarkable era in all geological time. From a climate which had been, heretofore, uniformly, warmly temperate, with but few exceptions, we come to a period known as the Glacial, in which, by a depression in the temperature, all vegetation and animals in high latitudes were killed; _viz._: in the central west--almost to the Ohio River; in Europe--to the northern part of Italy--while the addition of vast quantities of ice to the oceans, destroyed all life in them to about the latitude of the northern portion of the Gulf of Mexico. Nor was this period of cold confined to the northern hemisphere, as the southern part of South America and Africa show. Concerning the cause of the Glacial Period, but little is positively known. Of the theories which have been advanced, it seems very plausible that perhaps two more clearly account for the conditions which must have then existed, if we consider them together, than all the rest. The geological record teaches us that in the so-called Glacial Period, at least two distinct epochs of low temperature, and the consequential accumulation of ice, are to be definitely discerned. Still further back, we see evidence of glacial action in the Permian Strata, and possibly as far back as the Cambrian formations, although these eras of cold are not comparable with the period at the beginning of the Quaternary time. Croll, the Scottish physicist, first called attention to the fact that at certain regular intervals of time, the precession of the equinoxes, and the eccentricity of the earth's orbit, would so act in conjunction as to render favorable a great many conditions which would certainly all point toward a period of extreme cold. He calculated that the earth was traveling around the sun in an ellipse of maximum eccentricity, and that winter was occurring in the northern hemisphere when the earth was furthest from the sun, for the last time some quarter of a million years ago. About eighty thousand years after this date, the coincidence of the two phenomena reached a maximum effect, and about eighty thousand years later, climatic conditions were again about as we have them to-day. Upon this hypothesis, another period of extreme cold must have existed some one-half million years earlier, as calculations upon the same premises as were used in the last computation will show. It is likewise true that, according to this theory, there must have been at least one other such period further back in geological time, and it is now to be seen whether our records, as shown by the strata, establish these facts.