The Romance of Modern Geology / Describing in simple but exact language the making of the earth with some account of prehistoric animal life

by Edwin Sharpe Grew

To do this efficiently the geologist has to learn many things. He has to observe very closely the changes which are going on about him on the world's surface. Only in so far as he makes himself acquainted with these sudden changes can he hope to follow intelligently and successfully the story of earlier phases in the earth's progress. Nor is it sufficient to observe, however closely, inanimate things. If he did not know the peculiarities of fresh-water shells, how would he be able to say the shells in the marl deposit were fresh-water animals (and that therefore a lake once lay there) and not sea shells. If the labour of the geologist were concerned merely with the former changes of the earth's surface—how sea and land have changed places, how rivers have altered their courses, how valleys have been dug out, and how mountains have been carved, how plains have been spread out, and how all these things have been written on the framework of the earth—he would still feel one very great want, the want of living interest. But that also his science gives him, for in these past eras living things dwelt and moved and had their being. And it is one of the most entrancing pursuits of the geologist to trace their lives, their descent and ascent, and the relics of themselves that they left.

CHAPTER III

EFFECTS OF WEATHER ON THE EARTH'S HISTORY

The same causes that produced the layers of peat or sand, or limestone, or clay, which we find by examination of the earth's surface, are acting to-day. Coal is forming now; and so is limestone; and so is sandstone; so even is granite. But these layers or strata form very slowly, so that since man has kept historical records the thickness of new strata laid down could be measured in inches. Consequently we are only able to see the beginnings of the processes. After the materials were laid down by water or the shifting winds, or by the decay of other materials already in position, they underwent various changes. For example, many layers, instead of consisting of loose materials such as gravel, sand, or mud, are now hard stone. Sometimes this consolidation has been the result of pressure. As bed was piled over bed those at the bottom would be more and more compressed by the increasing weight of those laid down upon them; the water would be squeezed out; the particles would stick closer together. Mud, for example, might thus turn into clay; and clay, pressed harder and harder, might be converted into mudstone or shale. But there is another agency at work. We have all seen mortar hardening and binding bricks together; or cement hardening into concrete. Similarly sedimentary deposits are bound together by cements, of which there are many which exist naturally. For example, silica is a natural cement; and so is carbonate of lime; and so is peroxide of iron. All these will bind other particles together. But how do they arrive at the layers of particles? By the same action which lays down the particles themselves. They are rubbed off the places where they exist by the wind or by water. Perhaps they were laid down among the deposited particles of mud or sand. Perhaps they were brought to them by streams or rivers or lakes, and sank with the water into them. In a red sandstone, for example, the quartz grains of the rock may be often observed to be coated with earthy iron peroxide, which serves to bind them together into a rather hard stone. On the other hand, the process is often being reversed. The weather frequently conspires by frost and wind and rain to remove the binding cement, and thereby to allow the stone to return to its original condition of loose sediment.

One of the Colossal Natural Bridges of Utah

This is an instance in which water has hollowed out the lower strata, leaving a harder upper stratum partially intact.

For millions of years the winds have blown over the surface of the earth, the rain has fallen on it, the sun heated it by day, the frost cracked it. Consider the winds that have circled the earth. All movements of the air are due in the first place to the sun which heats the atmosphere and causes it to expand. The sun's rays passing through the air do not heat it at once, or directly, but heat the land and the sea, which absorb some of the rays and reflect others and so warm the air in contact with them. But, as will readily be understood, the land and the sea do not absorb and reflect the heat rays in the same way or to the same extent; nor do the sun's rays fall equally or constantly on all portions of the earth's surface. So that from various causes one part of the earth is always being warmed in a different way from other parts, and the air above the earth is being warmed in an immeasurable number of different ways. Even if the earth's surface were all water or all land, we should expect therefore that there would be movements of the air due to unequal heating. If, however, the earth's surface were quite even and uniform, we should expect that there would be a certain evenness and uniformity about the movements of the air. These movements would be due partly to the regular heating and regular cooling of the surface, and partly due to the fact that the earth is spinning round taking the air with it—but not taking it quite evenly. The air does not fit tightly on to the earth. It is rather like a loose, baggy envelope with a tendency to slip as the earth moves round. Furthermore, a point situated on the Equator has much farther to travel in twenty-four hours as the earth spins round than a point situated in the Arctic Circle, where a tape measure placed along one of the parallels of latitude (let us say the eighty-sixth parallel, where Nansen turned back in his search for the Pole) would show the earth's girth there to be, not twenty-four thousand miles, but only so many hundreds. This also would make a difference in the way the air would be whirled round the earth; but we could take this point into consideration, and should be able, if, as we have said, our earth were quite uniform, to say always and at all times of the year in what direction the prevailing wind should blow.

Even with all the earth's irregularities we do know a good deal with certainty about the earth's prevailing winds: the trades; the anti-trades; the south-west monsoon, which sets in so regularly in India that year by year its advent hardly varies by more than a day; and, in the descending scale of regularity, the east winds that usually sweep England in March, and the prevailing south-westerly to westerly winds which bend most of the young trees of the country a little to the north-east. Besides these regularly or irregularly defined winds, there are certain paths along the earth's surface where the winds always move like a trout stream with eddies in it. These eddies of the air we call cyclones, and they are continually travelling in one direction. No doubt they arise from the air in one place becoming hotter or moister than in the surrounding regions. As the air grows hotter it becomes lighter and ascends, while the heavier air round it pours in. These eddies always travel eastwards and incline in the northern hemisphere towards the north. They usually originate somewhere on the North American continent, and move across the Atlantic about the pace of a slow railway train, winds whirling round them all the time at a much greater pace. Usually the centres of these eddies bear northward past the north coast of Scotland to the north-west of Norway. Sometimes, however, they take a more southerly course, keeping to the south of the British Isles and passing over Central Europe on to Siberia, where they appear to die away.

Such are the cyclones which are in the main part responsible for British weather; and the winds that accompany them vary a great deal in strength. They depend on the size of the eddy. If the eddy is a very big one (and sometimes the eddies are thousands of miles across) the winds will not be so strong as in the smaller ones. It is, therefore, the smaller ones which cause the violent storms. In the tropical regions whirling eddies of a rather different character occur. To quote Mr. J. H. N. Stephenson: "Instead of being measured by some hundreds or even thousands of miles, they are usually only some hundreds of yards across; and as we found that the smaller the cyclone the more violent the wind, we shall not be surprised that the wind in these is more violent than anything we ever experience in this part of the world. They are called by many different names; in the West Indies they are known as hurricanes, in the south-east of Asia as typhoons, and in North America as tornadoes. These hurricanes or tornadoes travel much faster than the larger cyclones, and the winds blowing into them are so violent that everything—trees, houses, bridges—are swept before them, and so strong is the in-draught of air in the centre that strong walls are sucked in just as a piece of paper is in front of a grate when the fire begins to blaze up; and even heavy metal objects are carried upwards. Fortunately these tornadoes do not travel continuously along the ground but bump along it, so to speak, sometimes passing harmlessly overhead, then striking the earth again and causing more havoc. Where they pass over the surface of the sea the water is sometimes sucked in just in the same way, causing what is known as a waterspout. These may do even more damage than a tornado on land, for the water is sometimes carried bodily on to the land, sweeping everything away in a deluge. This happened many years ago in the delta of the Ganges, when thousands of people perished."