Romance of Modern Geology

<h2 class="nobreak" id="CHAPTER_XII">CHAPTER XII</h2> <h3>THE HARDENING OF ROCKS</h3></div> <p>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 <span class="pagenum" id="Page_129">-129-</span> 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.</p> <p>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 Arch&aelig;an 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 <span class="pagenum" id="Page_130">-130-</span> 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 Arch&aelig;an 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."</p> <div class="figcenter"> <ANTIMG class="w100" src="images/fpage130.png" width-obs="423" height-obs="665" alt="" /> <div class="txtlf"><i>Stereo Copyright, Underwood &amp; U.</i></div> <div class="txtrt"><i>London and New York</i></div> <div class="figcaption" style="clear: both; padding-top:1em;"> <p class="tdc"><span class="smcap">The Pinnacled Castle-like Peaks of the Ramshorn Mountains Of Wyoming</span></p> <p>The successive strata of sandstone are clearly evident in the peaks.</p> </div> </div> <p><span class="pagenum" id="Page_131">-131-</span></p> <p>For the present, however, we may concern ourselves with the condition of the earth and its rocks in Proterozoic times, observing merely that the remains of animals which we find there are of an order (Crustacea) which shows that life had progressed a good deal from its earliest beginnings in the age when the rocks containing these crustacea were laid down. After these rocks had been deposited they were subjected to many influences of which we have only dim conceptions. In a previous chapter we have compared the layers of the undermost and harder rocks of the earth to the lines on a page of this book as they would appear if the pages were crumpled up into a ball. Sometimes the beds of solid rock have been so distorted that they look like waves of the sea; sometimes they have been completely overturned; hardly ever have they been suffered to lie down flat. More than that has happened to them. Their very nature has been changed. <span class="pagenum" id="Page_132">-132-</span> This was done partly by heat, partly by pressure, partly by shock.</p> <p>Let us consider the heat first. When a mass of erupted molten rock forces its way through the earth's crust, it produces effects which are quite easily recognised on the rocks it penetrates. Limestone becomes hard and crystalline. Rocks with silica in them become glassy and like quartz or other hard rocks which are sometimes polished to make ornaments. Clayey strata become baked into hard brick-like rocks. The changes are not altogether due to the heat. The eruption of rocks is accompanied by steam at high pressure and with all sorts of acids in the steam, so that chemical changes are also produced. It has been supposed by Sir William Crookes that diamonds, which are crystals of carbon, were produced by carbon being melted at some enormous heat under great pressure. Given the requisite conditions of heat and pressure, the parts of rocks which by their chemical composition are susceptible to crystallisation will form into enormous crystals&mdash;not unlike the intrusive rocks themselves. They can, however, be readily distinguished from the shapes which the intrusive rocks (like basalt) assume. The basalt rocks which form, for example, the Giant's Causeway in Ireland were volcanic lavas. Sometimes the lavas were masses which had solidified underground and had been thrust up by pressure from below or have been exposed by the weathering of the rocks above. Sometimes they have been lava poured out on the surface. The black compact kinds most often are seen in forms like columns. If these veins of volcanic <span class="pagenum" id="Page_133">-133-</span> rock have been thrust up through a bed of coal, the coal is changed or "metamorphosed" where the basalt has pierced it. Sometimes it becomes hard coal like anthracite; sometimes it is changed into graphite&mdash;the black rock of which pencils are frequently made.</p> <p>Limestone pierced by basalt becomes marble. When sandstone is discovered in contact with ancient volcanic rock it is found to have lost its reddish colour, and to have become white, grey, green, or black. It separates into crystals; it becomes glassy and hard. All these instances are those of rocks which we can perceive to have been altered by coming into contact with great heat. But there is another kind of change of a very much more widespread character which can be perceived among the most ancient of those rocks which we know must have been first quietly laid down as sediments. It is sometimes spoken of as "general metamorphism."</p> <p>This widespreading change may extend over great regions and vast extents of country. The most striking series of such rocks was first described by Sir W. E. Logan, Director of the Canadian Geological Survey; and he estimated the thickness of them at 30,000 feet. They lie beneath all the unaltered rocks, and are (in North America) the rocks which were the base or foundation of the North American continent before the later sedimentary rocks were laid down on them. They are called the Laurentian rocks, because they were first found in the neighbourhood of the St. Lawrence River; but they exist in many places besides Canada and North America; and the foundations of Scandinavia and of the Hebrides are <span class="pagenum" id="Page_134">-134-</span> of the same texture and material. Now this change or metamorphism does not appear to be the same as that produced by the intrusion of hot eruptive rocks. Let us take a simple instance. We have seen that limestone is changed by heat into marble. Sometimes its fossils are preserved; sometimes they completely disappear. Sometimes it is threaded by veins of harder and more crystalline rocks. But in the case of the white marble of Carrara, which was once a bed of coral, the change seems to have taken place less violently, less suddenly, more gradually. The change was due, therefore, not to violent heat suddenly applied, but to the penetrating action of water, probably aided by sustained heat, and certainly aided by pressure. When a rock is subjected to sufficient pressure its very structure will alter; its original constituents may be torn out of it, pressed out of it, filtered out of it, and afterwards rearranged.</p> <p>Once more let us call attention to the astounding effects which great pressures can have. If pressure enough be applied iron can be made to flow like treacle; and the pressure of two or three miles of strata is enough to crumple or shear or tear any rock however hard. Now we have shown that in the earth's long history some regions are always being denuded of materials in order that these materials may be laid down as sediments elsewhere. These movements may be compared to those of a pair of scales, in which we are continually taking weight from the scale pan that is weighted in order to put it into the scale that is empty. The scale that is weighted is the land from which material <span class="pagenum" id="Page_135">-135-</span> is being removed by the rains and the rivers; the scale that is empty is the sea, in which the eroded material is laid down to form beds and strata. These two scales are never quite balanced. But suppose a time comes when we have taken all the material we can from the weighted scale, so as to make the hitherto unweighted one the heavier&mdash;what will happen? The newly weighted scale will inevitably fall, and we shall have to begin to reverse our system of taking from, and adding to, the scales. Similarly there will always come a time when there will be a flow of the earth-mass from the areas which have been receiving great loads of sediments, towards the areas which have been robbed to supply them. Think for a moment how the weight of a mountain set up in a plain might act if we can imagine some giant force piling up the mountain higher and higher. The mere weight of the mountain would tend to make it settle, and begin to press outwards all round its base. If you find a difficulty in seeing how this could be, imagine the mountain to be made of pitch. In such a case we can quite easily realise how it would spread. Similarly mountains, or even great plains and plateaux, of sediment built up for millions of years in the oceans, would tend to spread; and they would <i>spread towards the land which in the first place had supplied them with materials</i>. At first, of course, the stiffness or rigidity of the land would resist this spreading. But the masses thus built up would become so great and so heavy in the course of millions of years that no stiffness of the land could resist their spread. They would begin to roll or slide towards <span class="pagenum" id="Page_136">-136-</span> the land; the heavier parts must always roll towards the lighter. The action would be as resistless as the slow moving onward motion of those masses of ice called glaciers; or as the movement of the great ice plains in Greenland; or of those ice plains which in Antarctic regions are always spreading towards the sea.</p> <p>There are two views of it. There is the outward pressure of the regions where the sediments of rock are being laid down. There is the inward pressure towards the regions which have lost soil. Sometimes these two actions may conspire. A region where great denudation is taking place may send its waste material towards the sea, where it is deposited near the coast and not far from the highlands or mountains or plateaux which founded the soil. The shallows near the shore become a belt which is being loaded; the big mountains near by are a belt which is unloading; and thus there are two strains set up together. It is not hard to see the enormous crumpling effect which this would produce on the lower strata one or two miles beneath the surface of the sea and three or four miles below the topmost crests of the mountains. These are circumstances which may not be common; but the reader will find a quite sufficient explanation of some of the crumpling, and many of the changes in composition and appearance of the deep-sunk rocks, if he remembers the great pressure over them, and the fact that the high regions may be supposed always to have a tendency to slip towards the lowlands.</p> <hr class="chap x-ebookmaker-drop" /> <p><span class="pagenum" id="Page_137">-137-</span></p> <div style="break-after:column;"></div><br />