By means of such cracks, rivers often pass through lofty mountain-chains, and when we come to the investigation of the glacial phenomena connected with the course of the Rhone, we shall find that river following the longitudinal valley which separates the northern and southern parts of the chain of the Alps till it comes to Martigny, where it takes a sharp turn to the right through a transverse crack, flowing northward between walls fourteen thousand feet high, till it enters the Lake of Geneva, through which it passes, issuing at the other end, where it takes a southern direction. For a long time mountains were supposed to be the limitations of rivers, and old maps represent them always as flowing along the valleys without ever passing through the mountain-chains that divide them; but geology is fast correcting the errors of geography, and a map which represents merely the external features of a country, without reference to their structural relations, is no longer of any scientific value. It is not, however, by rents in mountain-chains alone, or by depressions between them, that valleys are produced; they are often due to the unequal hardness of the beds raised, and to their greater or less liability to be worn away and disintegrated by the action of the rains. This inequality in the hardness of the rocks forming a mountain-range not only adds very much to the picturesqueness of outline, but also renders the landscape more varied through the greater or less fertility of the soil. On the hard rocks, where little soil can gather, there are only pines, or a low, dwarfed growth; but on the rocks of softer materials, more easily acted upon by the rain, a richer soil gathers, and there, in the midst of mountain-scenery, may be found the most fertile growth, the richest pasturage, the brightest flowers. Where such a patch of arable soil has a southern exposure on a mountain-side, we may have a most fertile vegetation at a great height and surrounded by the dark pine-forests. Many of the pastures on the Alps, to which from height to height the shepherds ascend with their flocks in the summer,--seeking the higher ones as the lower become dry and exhausted,--are due to such alternations in the character of the rocks.