THE GEOLOGICAL CYCLE The fundamental principle of modern physical geology is that processes on the broad scale work in cycles. The simplest observa tions show that the land surface as a whole is gradually being worn away by the action of rain, snow, wind, etc. The material loosened by these agents eventually finds its way to the sea, where it is finally deposited to form new rock-masses. Hence the land is being worn away and the sea filled up. But geology shows that in many areas there has been an alternation of land and sea, often many times repeated ; hence, part of the sea must have been uplifted to form land to be worn away in its turn. It can also be shown that the periods of uplift are also periods of folding of the earth's crust to form mountain ranges, and are usually accom panied by great outbursts of volcanic activity. In the history of the British Isles and of North America there is evidence of four or five such cycles, the phases of quiet or disturbance being nearly or quite contemporaneous in the two areas. In the southern hemi sphere there were also similar revolutions, tending to alternate with those of the north.
In the study of such a continuous series of events it is obviously immaterial where we begin. For most purposes it is convenient to start with a consideration of the stage that is most clearly dis played before our eyes, namely the degradation of the land, and then to follow the material in its course to the sea, with an investigation of the nature of the accumulations that it there forms. These two stages are known to the geologist as denudation and deposition. After these come in natural sequence earth-move ments and vulcanicity, leading to the establishment of new earth structures and land-building, whence we arrive at the starting point of another cycle.
The Atmosphere.--The general character of the atmosphere is described in separate articles (see especially ATMOSPHERE and METEOROLOGY) , and only a few of its properties have here to be considered. Air consists, roughly, of four-fifths nitrogen, one-fifth oxygen, a small percentage of carbon dioxide and a varying amount of water vapour, with small traces of other gases, and a little fine dust in suspension, the last being more abundant in towns and in very dry regions. The oxygen, the water vapour and the carbon dioxide are the geologically active agents, bringing about numerous chemical and physical changes in rocks. The water vapour also is of primary importance as the source of rain, snow and dew.
The Hydrosphere.—This includes all the water on the surface of the globe : not only the sea, but also the rivers and the lakes of the land areas, both salt and fresh. It is the custom to speak of salt and fresh water as if they were essentially different, but in reality the difference is one of degree only. All natural waters contain dissolved salts, but when there is not enough salt to taste the water is called fresh. The water of the open sea is very salt, containing about 3.5 per cent of salts, mainly sodium chloride, but also including chlorides and sulphates of potassium and magnesium, a little calcium carbonate and traces of bromine and iodine. Many salt lakes contain far more salt than this and some are saturated so that they actually deposit solid salts in crystals. The calcium carbonate in sea-water, though small in amount, is of great importance, both in forming the basis of the shells of molluscs, crustacea, etc., and as providing the material with which certain rock-building animals—the corals, for instance—and plants do their work. Vast quantities of carbonate are poured into the sea by rivers running through limestone countries, but it is quickly annexed by these shell- and rock-making organisms.
The Lithosphere.—Under this name is comprised the solid earth, of which the outer part at any rate is composed of rocks (Gk. MM9os, a stone). The outer part is obviously solid, but we do not know what is the condition of the interior (see EARTH). Strictly the geologist is only concerned with the part he can see or whose condition he can infer by analogy, as will be explained later. Here it need only be said that the core of the earth is certainly made of heavier material than the outside rocky part, which is usually spoken of rather vaguely as the crust. This term may well be adopted for convenience, although it must not be understood as necessarily implying a sudden change of material or state at any particular depth. There is reason to believe that concentric depth-zones do exist in the earth, but we do not yet know for certain what they are made of. At any rate the earth as a whole is a very rigid body, as strong as solid steel at least; it is impossible to believe in a liquid interior of any considerable size, although the phenomena of vulcanicity do seem to indicate the existence of local fluid patches, and it is also certain from similar reasoning that the interior is intensely hot. The actual composition of the accessible part of the lithosphere and the mode of origin of its component rocks come within the province of petrology (q.v.). Another point of first-rate significance in modern theories of earth-physics and earth-movements is the undoubted fact that the distribution of density in even the outer crust is not uniform : it is clear that on the whole the continents and mountain ranges consist of light rocks, while those underlying the great oceans are heavier. This discovery has far-reaching consequences, for a discussion of which see ISOSTASY.
The internal heat of the earth, together with the circulation of water from the hydrosphere to the atmosphere and back again to the lithosphere, which is mainly due to the energy of the sun, constitute the main motive power of the geological engine.
Weathering.—This consists of three stages, weathering, trans port and corrasion. The first consists of the loosening of the material of the rocks by various chemical and physical agents, mostly of a meteorological character, rain, snow, ice, the action of atmospheric gases and so on. The meaning of the second term is almost self-evident : to see examples it is only necessary to look at a muddy river during a flood, to examine the material piled up at the foot of a glacier, or to watch the clouds of dust on a windy day. The term corrasion needs a little more explanation : it means the work done by the weathered material during trans port ; the wearing away of the bed of a stream by the sand held in suspension, the scooping out of its bed by a glacier, or the fretting of a rock surface in the desert by wind-blown sand. The solution of limestone by water, which often leads to the formation of caverns, and the wearing away of cliffs by the sea are also further examples. Corrasion at some particular point is often followed by collapse of adjoining rock masses, as when a river undermines its banks. The formation of the screes so well known in mountain regions is due to the breaking up of the rocks by frost.
Purely chemical processes also have an important effect in weathering of rocks. Compounds of iron are very abundant in nature and these readily undergo a process exactly like the rusting of steel under the influence of water and oxygen. Nearly all rocks show a rusty crust very unlike the appearance of a freshly broken surface. In the tropics this chemical action is so powerful that normally hard rocks like granite become so soft to a depth of scores or even hundreds of feet that they can be dug with a spade or washed away by hydraulic power in the mining of the valuable minerals that they contain, such as ores of tin. The natural denudation of such rocks has also set free great quantities of tin ore, gold and platinum and valuable gems, which are carried down to lower levels and form rich alluvial deposits.
Certain sulphides of iron and other metals, which are very abundant in nature, easily undergo oxidation, giving rise among other products to sulphuric acid, which is a most powerful cor rosive, and brings about a pronounced decomposition of many minerals and rocks.
By the action of these and similar processes the rocks are either removed altogether or reduced to a crumbly condition, so that they may easily be removed.
Transport.—The next stage of denudation consists of the re moval of the material thus prepared by weathering. Some constit uents may simply be carried away dissolved in water, but more commonly it is transported in a solid form, the chief agents con cerned being water, ice, wind and gravity.
The transporting power of streams is a commonplace hardly needing elaboration: it is certainly a fact that most streams do nearly all their work in flood-time. The amount of mud and silt brought down by great rivers like the Nile and the Mississippi is proverbial, while steeper streams and mountain torrents bring down vast quantities of gravel and great boulders of rock, often to the plains below. There are instances in some of the drier regions of the world, as in parts of Central Asia, where rivers flow into great depressions among mountain ranges and there dry up, owing to great evaporation. In such cases there may evi dently be terrestrial deposits of very long duration and of very great thickness, but this must be regarded as an exceptional type, although it is believed to have occurred fairly often in the past.
The second great agent of transport is moving ice, but of course this is only effective in cold regions, either in the Arctic and Antarctic, or in high mountains in more temperate latitudes, where glaciers are formed. They carry down vast quantities of rock-waste derived from the sides and floor of the valleys, and deposit it at lower levels. The glaciers of high latitudes, which reach the sea, deposit their material there direct.
It is only in desert regions that wind is the chief agent of transport, where it forms sand-dunes, but even in temperate climates a lot of fine dust is blown from place to place in dry weather. In East Anglia a lot of material is shifted by wind, and the sandhills of the coasts of western England and Wales and the east of Scotland may also be instanced. Such sandhills are com mon where an on-shore wind prevails and blows the beach sand inland. This is really a process of deposition rather than denuda tion, but is mentioned here as an instance of the geological power of the wind.
In a similar way the transporting power of waves, tides and currents, belongs largely to deposition. The wearing away of coasts by the sea will be considered later, as it is so inextricably mixed up with the formation of marine deposits.
The Ideal River-system.—The classical description of the de velopment of an ideal river-system was worked out by G. K. Gilbert in his Geology of the Henry Mountains (1877). Gilbert begins with the conception of the rise of a new continent from the sea, in the form of a simple arch, known technically to geolo gists as an anticline, or anticlinal uplift. Rain soaks into the ground to some extent, to issue eventually as springs a little below the summit line on either side, thus giving two rows, which are usually supposed to alternate, and not to lie opposite to one another in pairs. From these springs streams run down the conti nental slopes in either direction, naturally choosing the steepest slope, which is also the shortest line to the sea. Such rivers are called consequent streams, because they are the direct result of the uplift. These rivers naturally increase in volume downwards, by collecting the rainfall from the lower slopes, which gives rise to tributaries. In Gilbert's original theoretical exposition it is argued that these tributaries should run into the main stream at right angles, for the reason that most stratified rocks consist of layers of varying hardness, of which the harder naturally resist denuda tion more than the softer; hence when the surface of the continent has been lowered somewhat by general denudation there will be strips of hard and soft rock, or of high and low ground, running parallel to the original main line of uplift, and therefore at right angles to the consequent streams. In the slight hollows formed by the softer strata the principal tributaries settle down and are called subsequent streams. In reality they always incline a bit downhill and enter the main stream at oblique angles. These subsequent streams themselves develop tributaries which may in practice run at all sorts of angles into the subsequents.
Some of the rivers of north-eastern England, such as the Tyne, Wear and Tees, conform more or less to this ideal arrangement, though there are complications which cannot be described here in detail. The rivers of the west and south of England are even more complicated, since in the course of their history some of them have interfered with others and spoiled their symmetry.
Diversions of Drainage.—The next thing to be considered is the manner in which some of the more simple of these departures from the ideal development have been brought about. One of the easiest of them depends again on the principle just mentioned that soft rocks wear away more quickly than hard ones. The general lowering of a valley, by steepening the slopes, makes denudation more rapid.
Now let us suppose that for some reason, for example a higher rainfall, denudation is more rapid in the valleys of one subsequent than in another running in exactly the opposite direction along the same line of soft rocks. It may then happen that the first named, by working backwards at its head more rapidly, may capture some of the minor streams that originally fed the other. Or it might even cut back so far as to intercept the next conse quent stream and divert all its water.
Any one who has looked intelligently at the scenery of a moun tain region will have no difficulty in grasping what is meant by the idea of a stream working back at its head. The steep slopes and piles of detritus at the top of any mountain valley are elo quent witnesses of the process. The pass always found between two peaks shows the cutting-down in operation, and indeed the forms of most mountains are the direct result of the encroach ment of valleys on their mass in this way. The eastward-flowing rivers of Yorkshire and the northern Midlands which combine to form the Ouse and Trent, and eventually the Humber, afford a magnificent example of this process of capture and diversion.
Sometimes the original uplift, instead of being an elongated ridge, was a rounded dome. Then the drainage system will be radial like the spokes of a wheel, as is seen in the English Lake District. Along the courses of most of these radial rivers lakes have been formed, for reasons to be discussed in a later section.
Sometimes, again, instead of being a simple arch, it takes the form of two or more parallel ridges. Then it is evident that the consequent streams just described must run as tributaries into a river flowing along the trough between the ridges, which is called a syncline. This trough will probably be tilted one way or the other, so as to determine the direction of this stream, which is also a kind of consequent, since its course is directly determined by the earth-movement. The lower part of the Thames below Reading is a river of this kind, flowing in the trough between the North Downs and the Chiltern Hills.
Meanders.-Rivers do not as a rule keep a straight course throughout the whole of their history, but eventually develop curves, called meanders, from the name of a singularly winding river in Asia Minor. This depends largely on the fact that there is a limit to the deepening of a valley, which cannot be cut below sea-level, and eventually assumes a very gradual slope from its source to its mouth. This limiting level of deepening is called the base-line of erosion. When this is reached, the river has to use up its energy somehow, and does so by cutting sideways. Any original slight variation from a straight course tends to get accen tuated, and this is a cumulative process. Cases are known where the curves have become so sharp that the river in a flood cuts across the narrow neck between two bends and shortens its course, leaving part of its bed as a sort of crescent-shaped lake, known as oxbows in parts of the United States.
Rejuvenation.—If at some time during the existence of a river a further uplift of the land takes place, the river will again begin to deepen its valley backwards from the mouth, thus producing a step somewhere in its course; this is one way in which "hanging valleys" (see Ice and Snow as Geological Agents below) can be formed. If the river was meandering on a wide plain when the sec ond uplift occurred the result may be a deep meandering gorge, like the course of the Wear at and below Durham. The second uplift causes the river to deepen its bed, but it cannot lower the whole plain.
Inconsequent Drainage.—The types of drainage system hitherto discussed are those in which the arrangement of the rivers is the direct consequence of the structure of the underlying rocks, but some examples are known in which it appears to be quite inde pendent of such structures. In certain instances it is observed that rivers, rising at one side of a great mountain chain, cut right through the range and continue their course on the opposite side. Some of the best instances are the Indus and the Brahmaputra, both of which rise on the northern side of the Himalayas. Again, the Danube twice cuts through the great mountain chain of southern Europe, once at Vienna and again at the Iron Gates (see Earth Movements, below). In such cases the only possible ex planation seems to be that the river is actually older than the mountains, and kept its course open through them as they rose. Such a relation is called Inconsequent Drainage.