The changes of the valley of the main river are but slow, the plain indeed is wasted in one place, but it is repaired in another, and we do not perceive the place from whence that repairing matter had proceeded.  Therefore, that which here appears does not immediately suggest to the spectator what had been the state of things before the valley had been hollowed out, or before that plain, through which the river runs so naturally as being in the lowest place, was made.  But it is otherwise in the valley of the rivulet; no person can examine this subject without seeing that the rivulet carries away matter which cannot be repaired except by wearing away some part of the mountain, or the surface of that place upon which the rain, which forms the stream, is gathered.  In those rivulets, or their little plains, we see the detached parts remaining in the soil, and also the place from whence those detached parts were taken.  Here we need no long chain of reasoning from effect to cause; the whole operation is in a manner before our eyes.  In this case, it requires but little study to replace the removed parts; and thus to see the work of nature, resolving the most hard and solid masses by the continued influences of the sun and atmosphere.  In this state of things, we are easily made to understand how heavy bodies are travelled along the declivity of the earth, by means of water running from the height.

Such is the system of rivers and their valleys; nor is there upon the continent a spot on which some river has not run.  But, in the Alps of Switzerland and Savoy, there is another system of valleys, above that of the rivers, and connected with it.  These are valleys of moving ice, instead of water.  This icy valley is also found branching from a greater to a lesser, until at last it ends upon the summit of a mountain, covered continually with snow.  The motion of things in those icy valleys is commonly exceeding slow, the operation however of protruding bodies, as well as that of fracture and attrition, is extremely powerful.

To illustrate those operations of excavating the valleys of rivers and of thus undermining mountains which fall by their proper weight, I shall transcribe some descriptions of what is to be found among the Alps.  But first I would wish to carry my reader to the summit of that country, to examine the state of that part which nothing can have affected but the immediate influences of the sun and air.  After having thus formed some idea of the summit of this wasting country, we shall next examine the valleys through which the materials of the degraded summit must have travelled.

In order to give a proper idea of this central part of the Alps, which is so interesting a part in the natural history of the earth, M. de Saussure, in the plates of his Voyages dans les Alpes, tom. 2. has given us two views, the one in profile, the other in face, of the Mont-Blanc.  I have caused copy those plates, which are necessary to be consulted in reading the following description of this centre of the Alps.