while the process of coal-forming was going on, it is certain that our rate of deposition as heretofore used, is entirely too high. Dawson and Huxley have estimated, after most careful investigation, that the period of time consumed in laying down the coal measures, could not be less than six million years, and upon this basis it is safe to assume that between seventy-five and eighty million years were consumed in laying down the Devonian and Carboniferous deposits. This makes Paleozoic time occupy about one hundred and fifty million years, which is probably under- rather than over-estimated. The flora of the Carboniferous period was composed of tree ferns of the Sagillaria and Lepidodendron species which have since become extinct; but the Lingula, a shell in the Cambrian and Upper Silurian formations, and the Terbratula, another shell, is found in the Devonian rocks. Both of these are found living to-day, of the same identical genus and species.

In the Silurian rocks, we find the remains of an air-breathing scorpion, very similar to that found to-day, which shows that the atmosphere at that remote period was practically the same as we have at the present time.

In the Mesozoic time, we find deposits aggregating some fifteen thousand feet, and inasmuch as the Triassic sandstones were formations of slow deposition, our heretofore established rate will not answer the conditions. It has been estimated, after the most careful study of the Triassic and Jurassic measures, that probably no less than thirty million years were occupied by these periods, and that the chalk deposits of the Cretaceous must have taken at the present known rate, in like formations, somewhat over six million years of ceaseless activity. This gives to Mesozoic time a period of thirty-six million years, as a minimum, and, from what we know of the rate of biological evolution, this figure is conservative. The first period of the Mesozoic time was characterized by monotremes, the Jurassic by marsupials, and the latter by the first of man’s direct progenitors, the placentals. The flora of this period consisted almost entirely of gymnosperms, or naked seed plants, and, as far as we know, at the close of this second great division of geological time, conditions on the earth were, in all respects, very much as they are to-day.

Concerning the climatic conditions at the beginning of the Cenozoic time, we have every reason to believe that from the commencement of the Lower Silurian epoch, until then, there were no climatic zones upon the earth. Not only have coral formations been found in what are now Arctic waters, when we know that such reefs are formed only in waters where a moderately warm temperature is constantly maintained, but the cephalipods of the genus Ammonitoidea are found in what is now the Antarctic zone, and in the torrid. While, at the present time, we cannot see how the obliquity of the earth’s poles to the plane of the ecliptic could have been changed after the earth began its career as an independent planet, yet the facts above stated show that the climatic zones must have been unknown during the Tertiary period. Our common cypress, which is now so plentiful in Florida and California, had very close relatives living as far north as Spitzbergen, as lately as Miocene time. Magnolias, which are now so abundant in all of the Gulf States, are plentifully found in the Miocene strata of Greenland.

Returning to the length of the Tertiary period, it is well to note that, covering Wyoming and Nebraska, there was an immense lake, at least as large as Lake Superior is to-day, and into which several quite large rivers emptied, whose head waters were in the surrounding mountain ranges. This lake was at one time at least five thousand feet deep, and was completely filled up by the fine mud and silt, as the formation now shows, although at the known rate of filling in of smaller modern lakes, into which rivers, which originate in glaciers, empty, this would have taken the better part of fifty thousand years. This figure is particularly conservative, as during the Eocene period, there could have been neither glaciers nor melting snowfields to assist in the denudation at the head waters of the tributary rivers. During the Miocene period, many of the best geologists hold that America and Europe were connected, and there are certain similarities in their fauna and flora which make this very probable. Supposing that this depression which constitutes the bed of the North Atlantic Ocean, took place at the highest known rate of subsidence, as measured upon the coast of Sweden to-day, it is almost impossible to state the amount of time that necessarily elapsed from the beginning of the sinking of this strip until it finally went below the surface of the water. That such changes in level did take place in the Tertiary period, no one can doubt, as chalk deposits in England, which must have been laid down in the deep oceans, have now an elevation of thousands of feet. The Nummulite limestone of this same period is found in both the Alps and the Himalayas, at an elevation as great as ten thousand feet. The consideration of the fact that the greatest known rate of elevation or subsidence is, perhaps, scarcely more than two feet per century makes the figure of five hundred thousand years, as a minimum for Pliocene time, seem rather conservative.

Toward the close of the Tertiary era the finishing touches were placed upon some of the greatest of the geological works. The folding of the strata, which had been going on for a long period in Eastern New York, was brought to an end by a violent rupture therein, and the out-rushing igneous rock, which was subsequently cooled rapidly by the floods of water flowing over it, gave us the beautiful palisades of the Hudson River. In the west, this folding resulted in the Rocky Mountains and the Coast Range, with their attendant high plateaux. In Europe, the Alps and the Pyrenees Mountains both belong to this period, while the grandest and highest of all mountain chains, the Himalayas, of Asia, were the culminating effect of the gigantic foldings of the earth’s crust.

The deposits of the Tertiary period will aggregate somewhat more than three thousand feet, and, inasmuch as this entire time was one of continued change in level, or the fluctuation between the subsidence of the earth’s strata on the one hand and the elevation on the other (particularly in the Pliocene period), it is very hard to form any conjecture as to the actual amount of time required to do this work. Certainly, from what we know of the rate at which like phenomena are taking place at the present time in Northeastern North America, in Northwestern Europe, and Western Asia, the figure, as sometimes given, of ten million years seems very conservative.

In the brief review which we have just given, of what can be conservatively considered the minimum limits of geological time, we have taken into account generally only periods of activity, and in but a few cases has any estimation been hazarded as to the proportion which this was of the whole time consumed in bringing about the changes which the fossils show so clearly to have taken place during the various epochs. But one thing should be kept clearly in mind, and that is, that no matter how long geological time may seem, it is but an infinitely small fraction of the period which must have elapsed since the world came into existence, as this globe had to cool down to below the boiling point of water before any geological records could be made. When thought of in this way, the Laurentian period becomes as but yesterday, and even man’s dwelling place, which seems relatively so large,