SECTION II.-Changes produced by the Agencies which operate on the Surface of the Earth's Crust. IN N civilized countries and temperate climates, and especially in regions like our own, where volcanoes are unknown, where no lofty mountain ranges exist, and where all natural phenomena, such as rainfall, rivers, hills, and valleys, are on a comparatively small scale, people are apt to underrate the amount of change which is really taking place all over the surface of the earth, but which is much more apparent and rapid in some regions than in others. Moreover, those parts of our own country where natural agencies of change are most active are, as a rule, the most thinly populated; men do not congregate on high mountains or on rocky coasts, but build their towns in the lowlands and fertile districts, where perhaps only an occasional flood or storm serves to remind them that the forces of nature are not always gentle and regular in their action. Lastly, the efforts of the human race are continually directed to the control of these natural forces; ditches are dug to carry off the rain that falls on the land; rivers are embanked and made to keep within definite courses; walls, groynes, and breakwaters are constructed to stop the sea from eating away the coast, and the excessive action of every natural agency is prevented as far as possible. It is not surprising, therefore, that those who live in a lowland country, and who have never travelled far from the homes in which they were born, should find it difficult to realize the work that may be done, and the results that are actually accomplished, by the uncontrolled forces of nature in other parts of the world. They see around them in old age the same fields, the same slopes, and the same watercourses which they knew when they were children, and they are apt to think that things had always been as they had always seen them. In these days, however, people travel very much more than their ancestors did, and many thus become aware that in uncultivated districts, on exposed coasts, on moorlands and mountains, and in regions that are exposed to the extremes of heat or cold, operations are in progress which often cause a large amount of change even within the duration of a human lifetime. In other countries, too, the traveller may view the delta of a large river, he may visit a coral island, he may perchance experience or see the effects of an earthquake shock, or he may find sea-shells of recent species bleaching on slopes that are now several hundred feet above the sea. Still, it does not often fall to the lot of one man to witness all these things, as well as the many other evidences of change that might be mentioned; and many a traveller has returned home without comprehending the part which such changes have played in the economy of nature. There are, moreover, certain processes of change which the physical conditions of man's existence make it difficult for him to study, even when he specially desires to do so. Those who live by the seaside are familiar with some of the alterations effected by waves and currents on its shores, but they cannot see below the surface of the open sea and watch the drifting of sand into banks, or the slow accumulation of mud and ooze in the deeper water. It requires the dredge and sounding apparatus to give us any information about the deposits which are being formed at the bottom of our seas and lakes. Consequently, as Sir Charles Lyell remarks, "It is not surprising that we estimate very imperfectly the result of operations thus invisible to us; and that when analogous results of former epochs are presented to our inspection, we cannot immediately recognize the analogy. He who has observed the quarrying of stone from a rock, and has seen it shipped for some distant port, and then endeavours to conceive what kind of edifice will be raised with the materials, is in the same predicament as a geologist, who, while he is con 1 66 'Principles of Geology," tenth edition, vol. i. p. 99. G fined to the land, sees the decomposition of rocks and the transport of matter by rivers to the sea, and then endeavours to picture to himself the new strata which nature is building beneath the waters." In the following chapters we shall endeavour to assist the reader to form some conception of the effects which are produced by these processes of destruction and construction, by bringing to his notice such examples of their action as have been described by experienced observers in various parts of the world. It is assumed that the student has read some elementary treatise on Physical Geography, and has thereby acquired some knowledge of common atmospheric phenomena, and of the general physical features of the globe. He will then be prepared to follow the more detailed descriptions of the important effects which heat and cold, rain and frost, running water and moving ice, produce upon all surfaces exposed to their action. Some of these agents are chiefly engaged in breaking up and wearing down the rock-masses, others are more concerned in transporting and re-arranging the detritus, but the ultimate result to which all contribute is the re-construction of these materials and their deposition as new formations in the beds of rivers, lakes, and seas. In considering the operations of these agencies, therefore, it will be desirable to arrange our descriptions in two divisions, confining our attention in the first part to their destructive and dispersive effects, and leaving to the second division the consideration of what may be termed their collective and reconstructive effects. The processes of disintegration and erosion with which we are to deal in this and the following chapters may be thus classified :: I. Terrestrial Agencies. II. Fluviatile Agencies. III. Marine Agencies. Agents. Tides and Waves Action. Chemical and Mechanical. Chiefly Mechanical. Mechanical. IT A. Processes of Disintegration. CHAPTER VI. I. TERRESTRIAL AGENCIES. T is common matter of observation that all rock-surfaces which have been exposed to the action of the weather for any length of time show a tendency to decay and crumble away. The agencies concerned in producing this weathering, or disintegration, are several; they act both chemically and mechanically, but they are all so intimately connected in their operations, that any given case of disintegration can generally be shown to be the result of the conjoined action of two or more of them. They may, however, be arranged as follows, and we shall endeavour, as far as possible, to consider their separate effects. The mechanical effects of these natural agents can be easily understood, but to comprehend their chemical action, the student must have some knowledge of the rudiments of chemical science. These are now so frequently taught in schools, that we assume the reader has a sufficient knowledge of chemical elements and their compounds to understand the explanation of the chemical action of water; but if he has not this knowledge, he will find a brief notice of such compounds, and of the principal rock-making minerals, in the second part of this book. Atmospheric Agents. Rain, and what becomes of it.-All the rain which falls upon the surface of the land is disposed of in one of three ways: 1. Some of it is immediately returned again to the atmosphere by evaporation. 2. Some of it trickles down slopes, and collects into rills and streams, which flow over the surface of the land. 3. Some of it sinks into the ground, and percolates through cracks and joints of the rocks. The rain-water which flows over the surface operates as a mechanical agent, and carries off with it particles of the soil or rock over which it passes in mechanical suspension. The rain-water which sinks into the ground acts chemically, it dissolves certain portions of the rocks, through which it percolates, and carries them away in chemical solution. Some of this percolating water is thrown out again to the surface in the form of springs, but the remainder penetrates still further into the substance of the earth, where, with the aid of heat and pressure, it effects great changes in the chemical constitution of the more deeplyseated rocks, and is largely concerned in producing the phenomena of hot springs. The Chemical Action of Rain.-Carbonic and Humus Acids. Before proceeding to describe the chemical effects of rain-water on the earth, a few words of explanation are necessary, in order that the source of its chemical energy may be properly understood. Pure rain-water would have little effect upon any rock-substance except common salt; its rotting and dissolving powers are mainly due to its capacity for taking up and absorbing certain active agents, viz., carbonic acid (CO,), the humus acids, and oxygen. Carbonic acid gas exists everywhere in the atmosphere surrounding the earth, though the quantity contained in the air at different places varies considerably. All animals exhale carbonic acid from their respiratory organs, and all |