Especially important are the combinations and groupings of plateaus, as well as their relation to adjacent lowlands.
In Africa the plateau form embraces the larger southern half of the continent. Low plains are, on the contrary, the prevailing form in the north, broken, however, by the Sahara, and the high coast plateaus of the Atlas range, and of Barca.
In Asia there is a vast central plateau with gradual declivities toward the east, toward Yoorkistan and Persia on the west, and toward Lakes Baikal and Zaisan on the north. On the south the descent is abrupt to the Indian lowlands.
In Europe there are, for the most part, scattered and disconnected plateaus of small size and little elevation, often passing by an imperceptible gradation to the other forms. The Spanish plateau is, however, a marked exception, and has the sharply-defined character of the northern African plateaus. In eastern Europe the central situation of the isolated Valdai plateau, whose elevation is very moderate, but 840 to 1080 feet, is remarkable, and is of very great influence in determining the hydrographical character of the great Russian lowlands. And in fact, the hydrographical influence of both mountains and plateaus is so great, that it is worthy of careful and special study.
The combinating and grouping of plateaus in different continents give rise to great contrasts, observable most distinctly in Asia and America.
Asia, with all its great internal depression from Cashgar to Lake Lop, yet displays such immense districts of plateaus, all ranges of elevation, low, moderate, and very great, that the very grandeur and extent of its colossal mountain chains are subordinate in comparison. Asia is the land pre-eminently of plateaus.
America displays, not in its central but on its western coast, the greatest chain of mountains on the globe, flanked by plateaus of great elevation, but of superficial area quite out of proportion to the length of the mountain chain, and to the extent of the lowlands of both the northern and the southern divisions. And while in Africa the regions of depression are in the north, and in Asia around the great central plateau system, in the Americas, both North and South, they are thrown into the eastern portion.
Australia, in perfect contrast again, is, with the exception of its southeastern corner, a vast tract of unbroken lowland. No diversity is possible there, no change in the condition of life, but a ceaseless uniformity of monotonous but prodigal gifts.
Is not the imposing grandeur of these harmonious, provisional arrangements for the use of man calculated to fill the soul with admiring wonder, and to lead us to suspect, above all this display of cause and effect, above all this working out of a manifestly preconceived plan, the existence of a great and active Being, who has planned and executed it all with higher ends and a loftier purpose than to satisfy the mere earthly life of man?