“Elevations of the soil,” says Humboldt, “which do not display a marked difference in climate and vegetation from the country around them, are not rightly called plateaus.” His meaning is, that the name does not relate to absolute height measured from the sea, but harmonious climatic relations existing between contiguous districts, one of which is more elevated than the other. Highland and lowland are therefore to him words of unfixed meaning, if they do not stand in the contrast of height, climate, relief, and rates of temperature. Humboldt therefore did not consider the depression of Central Asia, at the Taringol, as a plateau; and table-lands from 200 to 1200 feet in absolute elevation, _i.e._ from the sea level, are passed over by him as not worthy of the same name which he applied to the plains 6000 to 10,000 feet above the sea. Dealing as I do with the elementary features and the physical contrasts of countries which for the most part are now thoroughly explored, I prefer, for the purpose of elucidating the subject of Physical Geography, to consider the plateau as beginning at 500 feet above the level of the sea. By comparing the plateaus of both hemispheres it is not difficult to deal with a variety of features, and to make a number of discriminations which, without an absolute standard, it would be impossible to make. We pass to the consideration of the much more varied and more imposing characteristics of mountains. Mountains and Mountain Lands. Mountain lands cannot, in the strict use of language, be compared with plateaus, except in way of contrast, because they are not uniform, broad, and sharply defined tracts, but extend in a linear direction, having as their chief feature the longitudinal axis of the mountain chain. Groups of mountain ridges may be separated from each other, or may be united in any coherent way which does not make them continuous, and yet, despite the want of continuity, form a perfect whole. Mountains, with their fissures, chasms, abysses, valleys, ravines, clefts, precipices,—in a word, their varied diversities of feature, broken through in every direction, the whole chain rent into fragments by these transverse breaks, are in direct contrast with plateaus. They have quite often a common range of elevation, which, measured from the sea level, is not unfrequently much greater than the districts lying at their base. Yet this relation is only incidental, it is not essential. There is no necessary connection between the height of the outlying plateau and the height of the mountain range. In Switzerland the mountains rise to the altitude of 13,000 or 14,000 feet; the country at the foot of the Alps is but 1000 to 2000 feet above the sea. Here the distance between the summit and the plateau at the base suggests no relation between them.