Let us begin our study of erosion by a general survey of the striking features of the earth's surface at the present day. We know that at various parts of the surface there rise lofty mountain chains, whose summits are often permanently snow-clad, and which, from the sharpness of their forms and from the masses of rock rubbish which are accumulating round them, have obviously only been exposed for a geologically short period to the action of the atmosphere and of running water. When examined such mountain chains are all found to have the same peculiarities of internal form, the rocks composing them being elaborately folded and fractured. Careful investigation has convinced geologists that all the existing great chains owe their origin to a series of earth movements which occurred in the period called Tertiary, that is, in the third of the great geological periods, the one immediately preceding that in which we live. These lofty mountain chains of Tertiary origin are most familiar in the great series of folds which appear at the surface to form the Pyrenees, the Alps, the Caucasus and the Himalayas, but the Atlas Mountains belong to the same series, as does also that great mountain chain which, under various names, runs down the western coast of the American continent. [Illustration: Fig. 1.--The main points in regard to the structure of Europe. The shaded areas (1) are regions of ancient rocks, much folded and crumpled, which once formed mountain regions but are now mostly worn down to uplands. The lines (2) show the regions affected by Tertiary folding, largely occupied by mountain chains. The unshaded areas are mostly plains and basins, only slightly affected by folding, and made up of rocks which are often almost horizontal.] As already indicated, these areas are recognised not only by the fact that there appear at the surface a great number of peaks forming a mountain chain, but also by the internal structure, the characteristically complex folding of the rocks. Now outside of these recently elevated areas in, for example, the continent of Europe, we find two conditions. On the one hand, there are regions of upland type but with rounded and smoothed forms, which are sometimes almost reduced to the condition of a plain. Such regions occur in Ireland, in the west of Great Britain generally, in Brittany, in the central plateau of France, in the Ardennes, in Bohemia, in the central plateau of Spain, in Scandinavia, and so forth. Between these relatively elevated areas we have plains and low-lying river basins, such as the London basin, the Paris basin, and so on. When the rocks are examined in both cases it is found that in the basins and plains the rocks, as a general rule, are only slightly inclined, while in the uplands and plateaux there are obvious remnants of folding, and the rocks are of ancient types, not relatively modern like those of the Alps, Himalayas, etc. (see fig. 1).