9. Alpine land is composed of mountain groups, or groups of mountains. Each mountain group consists of a number of inequalities, which are demominated moun tain? ; and these are arranged into series, forming. what are called mountain chains, or chains of mountains. A number of these chains, when joined together in a de. terminate manner, lorm a mountain group. Mountain groups arc thus excellently distinguished from the other constituent parts of the high land.
It is of much importance to be able to discriminate a mountain group from others on which it may border, or with which it may be surrounded. For facilitating this discrimination it may he remarked, that mountain groups are highest in the middle, and usually become gradually lower and smaller towards the extremities ; and that each individual group has a patticular direc tion.
Mountain groups are sometimes separated from each other by plains ; sometimes by valleys; frequently by mountainous or hilly country.
10. Mountainous land is composed of mountains that are aggravated into chains; but these chains not con nected together by a central or high mountain chain. Ilence it never forms groups. It is lower 'than alpine land ; and is often intersected, which is never the case with alpine land.
11. The third constituent part of high land, namely, Billy land, is much lower than mountainous country, and consists of gentle, rounded, waved•like elevations. These are sometimes arranged into groups.
We have thus a complete series, from the most elevat ed and complete alpine country, through the mountain ous and hilly, to the low country.
12. In regard to the local situation of the constituent parts of high land, we may remark, that alpine land forms its principal constituent part, and that this is dis tinguished from mountainous and hilly land, not only by its frequency, but also by height, and its usually oc cupying the middle part of the high land ; whereas its borders are marked by mountainous and hilly land ; thus forming the transition to what is termed low land.
13. The fourth constituent part of high land is plains. These are small, isolated, and rare ; and hence form but unimportant features in the general aspect of high land.
14. Low land differs very much in its characters from high land. It is principally composed of plains, and sometimes contains flat hilly country, chiefly where it borders on alpine country. Very rarely small mountain groups occur in it, and these always occupy its middle part.
15. As alpine land is very varied in its aspect, it will be proper now to consider its different parts somewhat in detail. When a mountain group, or alpine land, is viewed as a whole, it appears highest in the middle ; and this highest part extends through the whole group without being intersected. This elevated portion is by geographers denominated the high mountain chain, and is one of the most remarkable parts of a mountain group, because nature appears to have arranged all the other parts as subordinate to it. It is the water-shed for the whole of the neighbouring country, and is parallel to the length of the group ; hence the extremities of the group are at the same time that of the high mountain chain.
All the other parts of the mountain group decline on both sides from the high mountain chain ; and the in clined plains thus formed, are denominated the fall or acclivity of die group. The inclination of the acclivity varies from 2° to 6° ; that of the northern declivity of the Pyrenees is lrom 3° to ; that of the southern ac clivity of the Alps, from the line formed by the colos sal summits of Mont Blanc, Mont Cerven, and Mont Rose, and of v hich the general height is 3500 metres, to the plains of Piedmont and Lombardy, is 3/°. But as such a genet al inclination is made up of many parti cular inclinations, on account of the inequalities of the acclivity, we must, before we reach the summit, ascend and descend acclivities much more considerable than j that we have just mentioned. The two acclivities of a group have rarely the same degree of inclination ; on the contrary, one is generally shorter and steeper than the other. Thus, the northern acclivity of the Erzge berge is long and gentle, while the southern is rapid and short. In the Pyrenees, the northern acclivity is more extensive and steeper than the southern. The western acclivity of the Andes is shorter and steeper than the eastern. The lower part of the group, that which bor ders on mountainous and hilly country, is denominated its foot. When a portion of it extends far into low country, it is denominated a mountain arm. We some times meet with small plains on the acclivity of the group, and these may be denominated mountain plains, to distinguish them from those observed in low country. Very considerable heights sometimes occur in the ac clivity of the group, but these seldom reach the height of the high mountain chain.