The concavities, or hollows, in a mountain group, arc denominated valleys. The valleys shoot from the high mountain chain, intersect the acclivity of the mountain group, and terminate at its foot. The valleys divide the mountain chains from each other ; and by this division nature has formed all the varieties of structure observed in mountain groups.
The lateral chains shoot on both sides from the high mountain chain towards the foot of the group. Of these lateral chains some are more considerable than others : the most considerable, that which rises from the middle of the high mountain chain, and reaches to the foot of the group, is denominated the principal mountain chain ; the other chains either shoot from the foot of the group towards the principal mountain chain, but never reach the high mountain chain ; or they pro ceed from the high mountain chain, but never reach the foot of the group. The principal mountain chain of one group cannot join with the principal mountain chain of another, nor can the high mountain chain of one group join with that of another. The summit of a mountain chain is denominated its ridge, and that of the highest mountain chain the highest mountain ridge.
We have nearly the same circumstances to attend to in regard to valleys as to mountain chains ; only there is here nothing analogous to the high or middle mountain chain. There are two chief kinds of valleys, namely, the principal, and lateral or subordinate. The principal valleys stretch uninterrupted from the high mountain chain to the foot of the mountain group, and those only are the principal valleys that reach this point ; all the others are subordinate or lateral valleys. These either extend from a mountain chain to a principal val ley, or begin from a lateral chain, or the principal mountain chain, and terminate at the foot of the moun tain group. Mountain chains are divided into moun tains by small valleys or ravines. In the valleys of mountain groups, we have always rivers or streams, but in those of mountain chains, there are either none, or small rivulets. The bottom of such valleys has usually a very rapid ascent ; they are not deep, and are neither so wide nor so extensive as true valleys, or those formed of ranges of mountains.
16. The different parts of individual mountains must also be described. Almost every mountain has a foot, acclivity, and summit, By the first, we understand the lowest and flattest part of a mountain. It sometimes
extends to a considerable distance, and then rises under an angle of 8° or 10° ; when it is less extensive, or has a smaller base, it rises under a somewhat greater angle, but never greatly exceeds 10°. The mountains in wide valleys have generally a considerable foot, but those in narrow valleys are less in extent. Sometimes, as in mountains having a mural ascent, there is no foot.
The Acclivity or Ascent.--By this we understand the space contained between the foot and the summit of a mountain. It is usually the steepest and most consi derable part of it. Its inclination is less or more than 30°, and on this depends the greater or less covering of soil. Upon an acclivity of 30° and upwards, we find a good cover of soil ; at 45°, however, the acclivity is too great to admit the growth of trees. Sometimes the ac clivity is perpendicular, forming mural precipices; and it is either mural on one, two, or all sides, or in single spots. Granite, porphyry, and sandstone, afford exam ples of such acclivities. Humboldt remarks, in regard to acclivities in general, that they are to be reckoned considerable when their angle is 7° or 8°, which is the maximum for carriages ; that they are very rapid when 15°, which is the maximum for loaded beasts of burden ; that an inclination of 35° is so great, that we cannot as cend it without cutting steps in the rock ; and that, even with the aid of steps, an inclination of more than 44° is very difficult of ascent.
The Summit.—This is usually the smallest part of a mountain, and its inclination is generally less consider able than that of the acclivity. There occur, however, exceptions to this : thus there are summits that rise more rapidly than the acclivity ; and these are usually very high, almost of equal length with the acclivity, and completely naked. Such lofty and precipitous summits are, in Switzerland, denominated Peaks. The summit varies considerably in its shape : it is tabular, or round-backed, or obtuse, or acute, or short conical. Generally the shape of the mountain is characteristic of the rock of which it is composed. Thus gneiss and transition rocks form flat or round-backed summits ; clay-slate, conical summits ; and basalt and some other rocks, shot t and obtuse conical summits : granite and limestone often present extremely sharp pointed summits or peaks.