This chapter may be ended by a brief application of some of these considerations to the case of the planet Mars. Next to the Moon Mars is our nearest neighbour, and the erection of great telescopes in America, one of them at Flagstaff Observatory, Arizona, where the air is extraordinarily clear and telescopic vision unusually penetrating, has stimulated the observation of the planet to a very great degree during recent years. Mars has an atmosphere not nearly so dense as that of the earth, but still dense enough in all probability to support some form of organic life. It may, for example, support vegetation. In some other respects Mars resembles the earth. It has arctic circles; it has clouds, though whether these are of vapour or of dust is not quite certain; and it has a less variable temperature by far than that of the Moon. There are, at any rate, some of the conditions to support and perhaps to encourage life; and if we could be certain that the atmosphere in Mars more nearly resembles that of the earth, and that its temperature was such as to be sometimes above that of our Arctic regions, then it would be difficult to deny that life, and probably intelligent beings, existed there. One very able and intelligent astronomer is convinced that life and intelligent beings do exist there. This is Professor Lowell, of Flagstaff Observatory, who has devoted a number of years and a great deal of money to the careful observation of the planet. He has brought forward many cogent arguments to show that Mars might be inhabited, and that great telescopes can discover signs on it, and may discover further signs, which are a reason for supposing it to be so. It is not, however, within the range of this book to examine these reasons in detail; and we need only say that in the first decade of the twentieth century most astronomers, despite the close examination of Mars and its markings, which had been conducted for more than a generation, were still not convinced that life as we know it could exist there.
CHAPTER XII
THE HARDENING OF ROCKS
After the time when the great overflows of lava took place, spreading over continents and sometimes seas, there was an era when the explosions and outbursts began to diminish in violence, and the world slowly settled down to conditions something like those which we see in our own day. The seas were forming; there was rainfall and summer and winter on the earth. The rains and the winds, the summer heats and winter snows were more violent than now, and the volcanic activity of which we have spoken was much more fierce than anything of which mankind has any recollection. In the British Isles the rainfall in a year averages something in the neighbourhood of thirty inches. In some regions of the earth it is as much as four times that amount, and deluges of fifteen inches have fallen in a day. But in the era of which we have spoken deluging rains that were to be measured in feet rather than inches fell incessantly. The air was saturated with moisture, and it no sooner descended on the warm earth than it steamed back to the clouds again. For reasons not unlike these, nor unconnected with them, the great currents of air fed by the constant transference of vapour from the earth to the skies, and the condensation of the vapour to rain, falling again on the earth, were greatly magnified. Thus the rocks of the earth, some of them only cooling and not yet hardened, were subjected to "weathering" of a kind of which it is hard to form any sufficient idea. The key to all geology is that what is going on now on the earth is similar to what always has been happening, (differing in degree rather than in kind), and that consequently the rocks of millions of years ago were washed by rivers down to the lower levels and were deposited as sediment in streams, in lakes, and in the sea. Thus the age of the "sedimentary rocks" began while the earth was still too warm to preserve any vestiges of life.
Earthquakes much more violent than now and volcanic outbursts often upset the steady order of things, but the earth was settling down. During this settling-down process rocks, as we have seen, were being formed by deposits; but they were very liable still to be invaded by bursts of volcanic activity from the inner cauldron of the earth, and they were very apt to be twisted out of their regular shape by great earth movements. They were also liable to be baked by the neighbourhood of the restless, unconfined molten rocks, nearer then to the surface than now. Geologists call the great period of time when all the rocks continually flowed out on to the surface of the earth, and were, in fact, all molten before they solidified, the Archæan Era (from a Greek word signifying the beginning). Next in order to these rocks are those which were laid down in the agitated times when the earth was still warm, and when the climate of the earth might be described as a continual thunderstorm. In this period earthquakes still had a great deal to do with the formation of the rocks, but then, as now, the sea and lakes and oceans laid them down. Geologists call this the Proterozoic Era. There are great masses of these Proterozoic rocks in North America. In Arizona the three periods of rock formation are sometimes visible together, and may, indeed, be perceived in some of our photographs; the Archæan all jumbled together being the lowest; Proterozoic lying crumpled or tilted over them, and the later rocks resting more regularly on these strata. In America, however, these separate ages of the Proterozoic rocks can be identified, and each age is represented by rocks thousands of feet in thickness. Three separate ages of rocks are found in this great era in North America. It is not very important to remember their names, which are merely those of the localities where these great deposits are most marked, but it is important not to forget that each of these depositions of rocks represents a period in the earth's history older than the lifetime of a river or a lake and as old as the lifetime of a continent. The lowest of these divisions consists of rocks that are much altered by the heat of the rocks below. The topmost division is hardly altered at all. In Scotland we have similar rocks. The Torridonian sandstones, 8000 to 10,000 feet thick, are believed to belong to this era. In France also, in Spain, Germany, Finland, Sweden, India, and Brazil, the Proterozoic rocks are found. In the lowermost of them are no signs that living things ever existed, but in the upper ones f life begin to appear. We may see in them to-day the first fossils. A fossil means literally a thing dug up, and was a term applied at first to all kinds of mineral substances taken out of the earth. We use the word now exclusively for the remains of plants and animals embedded in any kind of rock. In later chapters of this volume a good deal will have to be said about fossils, and of the way in which they tell us the kind of life that existed when they were first sunk in the rocks where now they are found, and how also they give us information about the climate and the distribution of land and sea, of lake and of river, in those eras far "in the backward and the dark abyss of time."
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The Pinnacled Castle-like Peaks of the Ramshorn Mountains Of Wyoming
The successive strata of sandstone are clearly evident in the peaks.