The valleys which are situated between the parallel ridges of the malt] chain are termed longitudinal valleys ; their axis, and conse quently the principal watercourse, is nearly parallel to the direction of the chain. The valley of the Ithbne above the lake of Geneva, the valley of the Magdalena in South America, &c., may be taken as examples. Two thinge have been remarked in longitudinal valleys ; first, that there is sometimes so perfect a conformity between the re entering angles on one aide and the salient angles on the other, that if it were possible to bring the two sides into contact, they would perfectly correspond, so as to leave no trace of their having boon separated; and, secondly, it has been observed, that the side of the valley opposite tosthe centre ridge is the steepest. These observations are true as regards many places, but are by no means to be received as universally correct. The other valleys, whose axes form various angles with the direction of the great chain, are the principal valleys of a country, and are usually designated by the names of the chief rivers which flow through them. The valleys of the tributary streams which empty themselves into the main rivers are called lateral valleys. The terms upper and lower valley are sometimes used to denote the parts of a valley as they lie along the higher or lower part of a river's course. Such then are the parts of a regular chain of mountains, but we are not to suppose that all those great elevations to which we give the name of chain are thus regularly formed. Mountains are some times grouped, as we have said, so as to present no appearance of a chain ; sometimes the chains run parallel, but wholly independent of each other ; in some cases they radiate from a common centre or nucleus. Indeed nothing can be well more irregular than the arrange ment of mountains, and they differ as much in height, steepness, and particular appearance. By sonic (Bergmann) it has been imagined that in chains running north and south the western slope is the most abrupt, and that when the chain runs east and west the southern slope is the steepest. But this is far from being always the case, and General Andreossy has laid down as a principle that the steep side of a chain of mountains is that which looks towards the higher part of the general slope on which the chain is set. This opinion is doubtless better founded than that of Bergmann, nevertheless it is not strictly correct; and there seems reason to believe that no general law obtains on this subject.
The appearance presented by chains of mountains is not only very different in different parts, but the very same mountains when seen at a distance no way resemble their aspect when seen nearer. At a dis tance the minute irregularities are lost in the general contour, and the particular shadows are blended into a uniform tint. The forms of rocks generally depend on their nature, and a practised eye can some times pretty correctly divine the latter from the former. These may either present the aspect of needles or sharp pointed masses, or the summits may be dome-shaped, or stretch along like a vertical wall, either entire or bearing a resemblance to ruined battlements and towers ; sometimes the whole mass is piled up into a succession of gigantic steps or terraces. Individual mountains and hills also vary in form ; those which are volcanic are generally conical; others are round, oval, lumpy, saddle-backed, &c.
Mountain-chains are the natural watersheds (the divortia °quorum), but it must not be thence inferred, as has too frequently been the case, that all water-sheds are mountain-chains. This erroneous idea has covered our older maps with mountains where in nature not a hill is to be seen.
Another error is to regard the mountains of the earth as so many connected chains, which, by starting from some particular point, may be traced stretching and branching continuously over the whole surface of the globe. The fallacy of such pretended continuity is evident from the difference in the arrangement as described by different writers. The truth is, that mountains are scattered over the surface of the land in the greatest confusion, here isolated, there in groups or in chains ; tho chains being in some places single and independent, in others connected; in one place running in parallel directions, in another intersecting, crossing, or branching off at different angles ; in some cases completely enclosing a certain extent of country so as to form an entire and perfect basin, in others only partially enclosing a space. In one country the mountains arc set in the centre, or near one of its coasts, and in the direction of the greatest length of the country; in another, they are set across the country. In fact, the chains ru-o observed to be in all possible directions, both as regards the points of the compass and the trending of the coast-lines. Thus with the exception of the Andes and the Rocky Mountable, the Appala chians, the Chants of India, the coast-mountains of Abyssinia, the Scandinavian Alpe, and the Apennines, there is very little conformity between the direction of mountain-chains and the configuration of countries.
Mountains have a very important part to perform n the general economy of the earth ; they arrest the fleeting clouds, whose pre cipitated waters they store up in their interior and exhaustless reser voirs, whence springs are continually issuing, which unite and form those streams that fertilise the plains, or, collected into mighty rivers, favour the transport of commodities and facilitate the intercourse between the ocean and the interior of the continents. The influence of mountains on local climate is all-powerful, and depends upon the direction in which they lie as regards the aim's course, their height, their position on the surface of the globe, their proximity to or remote ness from the sea, the winds they arrest or give passage to, 3.:e. mountains have moreover a climate of their own, or rather a great variety of climates. Thus, in ascending from the sea towards the summit of the Andes, almost every kind of climate is passed through, nearly as completely as if the traveller were to proceed from the equator towards the pole. At an elevation of about 16,000 feet on the Andes under the equator, we reach the lower limit of perpetual congelation ; and mountains in that region which exceed that height have their summits covered with eternal snow. The line of perpetual congelation is, however, far from being parallel with the general surface of the earth. It approaches that surface as it advances towards the poles, but the laws which determine this line are still imperfectly known. [CLIMATE; SNOW, PERENNIAL.] The limits of perpetual congelation, as ascertained from the stability of snow on the mountains, are subject to vary with. the particular aspect of the mountains and other local circumstances. Thus, gene rally speaking, the snow-line is highest on the south side of mountains in the northern hemisphere, and rice verse! ; but this is not always the case; and the line of perpetual snow is much higher on the northern than on the southern side of the Himalaya mountains. Mr. Pentland states that the limit of perpetual snow is at an elevation of 15,800 feet on the mountains of Vilcanota in South America, which are as far south as 14' 33'.