Turn now to the land. We note at once the two characters of marked irregularity of surface, and of changeableness. The changeableness is due to the forces of erosion which act upon the surface, and of these forces the most important to the geographer is running water. It is running water, aided by other agents, which carves the land into hill and valley, which produces gorge and lake, only ultimately to fill up the lake and plane away the gorge. It is running water which spreads out on the lower ground the waste of the higher, and thus prepares the way for the operations of man. The result of the long-continued action of the varied forces of erosion must necessarily be to reduce the surface to an almost level condition. The denuding agents first produce irregularities and then finally remove these, until the whole surface is once again almost level. The whole globe would thus be reduced to the condition of a plain were it not for the intervention of the internal forces which raise up the surface anew into folds, or which produce volcanoes and outbursts of molten rock. This constantly repeated series of changes may be said to be chiefly the concern of the geologist, especially as it is a series which has repeated itself in all time. But it is to be noted that at various parts of the surface of the globe at the present time every stage in the process occurs, and everywhere the question whether a particular land area has been exposed for a relatively long or for a relatively short period to the forces of erosion, has a profound influence upon life. It is therefore important for the geographer to be able to recognise the different stages. This he cannot hope to do without some detailed knowledge of the effects of erosion. Theoretically every land surface elevated above sea-level should pass through what has been called a cycle of erosion. There should be a period when the active forces are working upon a surface as yet but little modified; this is the period called by analogy youth. At a later stage the drainage has been well established, and the rivers run in broad valleys, from which lakes and waterfalls have largely disappeared. To this condition the term mature has been applied. At a still later stage the land surface has been so worn by the eroding forces that the whole process of erosion is slackened, and an uplift must occur before the erosive forces regain their lost strength. This is the so-called "cycle of normal erosion," but it is constantly liable to variations due to local crust movements, to changes in climate, and to local conditions, though at the same time the distinction of the various stages has value for the geographer because of their varying effects upon human life. It is necessary for us, therefore, to consider how the different stages may be recognized, and how the forces of erosion act.