In speaking of the solar heat and man’s dependence upon it in a constant definite quantity, as one of the conditions of his existence, perhaps it will give us some just appreciation of his place in nature when we consider that the earth receives somewhat less than one two-billionth part of the heat radiated away by the sun, and while this expression makes the quantity which we receive seem rather small, it is, nevertheless, large enough annually to melt a layer of ice one hundred and seventy-five feet thick—all over the surface of the earth, and is a little more than one six-thousandth part of the quantity of heat which would be generated by the burning of a mass of coal as large as the sun.
The researches of Halley and Adams have shown that from some cause, probably the result of gravity acting in conjunction with the varying eccentricity of the earth’s orbit, the motion of the moon has been slightly accelerated as time went on, while the diurnal motion of the earth has been reduced by the action of the tides, and that the amount of this loss, in time, is equal to about one second in the length of our day, in 168,000 years. Now, this retardation in the earth’s motion has not taken place at a uniform rate if caused by the reaction of the tides, as the nearer to the earth the moon was, the greater would be the tides, and, consequently, the greater would be the reaction; i. e., the retardation. But assuming that this retardation took place, on the whole, at twice the rate now prevailing, we would still have a period of six million years since the moon was thrown off by the earth, when our days were but three hours long.
Turning from the theories of astronomy, which are obviously more or less inaccurate, owing to their very nature and the character and duration of the observations upon which they are based, we come to the nearer and more certain deductions of geology. Here we have the phenomena of denudation and deposition with which to deal, and inasmuch as these are measurable at many places, and under many conditions upon the earth to-day, it is safe to assume that computations made from these measurements cannot be far from the truth. We know that practically all of the great formations of the earth were depositions of material from water which contained them, and that, in many cases, heat caused these strata to be metamorphosed or crystallized ages after they were deposited, and that in this crystallization many of the fossils remaining imbedded in the deposited matter were destroyed. Concerning this deposition we know that it is going on to-day in the Atlantic and Pacific Oceans, where, in the deeper portions the Globigerina ooze is filling in these depressions with a deposit, resembling chalk, at the rate of perhaps an inch per century. We know that the Gulf of Mexico and several other ocean areas are being filled in with silt at the rate of as high as three inches per century. This silt is brought down in the tributary rivers and emptied into the gulfs. We also know that large areas in the Indian Ocean are being covered with coral and the débris from the coral reefs. We are absolutely certain that every geological period has had its characteristic fauna and flora, and that, in both the animal and vegetable kingdoms, some persistent types have connected it with both the past and the future, so that the fossils have become the “open sesame” to the geological records. We further know that the strata composing the earth’s surface are subject to elevation and subsidence, such as is now going on in the delta of the Nile, on the coast of the Netherlands, and in many other places, and that such movement is a measurable quantity, given only the necessary time.
The total thickness of known strata measures but about one-three hundred and twentieth part of the earth’s diameter, or, in round numbers, twenty-five miles. Thirty thousand feet of this is quite readily identified as belonging to the old Archaic or Laurentian period, and constitutes the oldest stratified deposit known. Even in this, we find the remains of the Eozoon Canadense, which is now universally acknowledged to be the petrifaction of a foraminiferous living organism with a chambered shell. This means that, at this time, the earth’s atmosphere must have been very similar to what it is at the present, and that the temperature of the sea was somewhere between the boiling and the freezing points of water. What time had elapsed since the earth was thrown off by the sun in an incandescent state can only be faintly imagined. At the rate of deposition given for the deepest of ocean deposits, this Archaic period would have taken perhaps thirty-six million years; but inasmuch as the water may have been far warmer then than now, and the rainfall more abundant, and the forces of denudation in all respects more active, this figure may be excessive. The next eighteen thousand feet of strata are easily identified as Lower Silurian, by the Diatoms which occur imbedded in them, and these formations include some of the largest deposits of limestone known. At our rate of calculation, this deposit would require no less than nine and one-half million years, and, in assuming this figure, no account is made of the intervals of time during which no deposit took place, although such periods of inactivity must necessarily have been. The Upper Silurian strata consists of twenty thousand feet, the fossils of which are the lower fishes, and for which we must assign a period of time equal to no less than twenty-five million years, inasmuch as these deposits are limestones and sandstones, or the remains of water-living animals and plants.
Coming now to the Devonian and Carboniferous periods, the strata of the former, which is filled with fossils of the dipnoi, and the latter with those of the amphibia; we have deposits aggregating about forty thousand feet, and inasmuch as long intervals of time must have existed during the subsidence and elevation, and vice versa, of the land, 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.