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Page:Popular Science Monthly Volume 65.djvu/77

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73
GEOLOGY AND GEO-BOTANY OF ASIA.

and the plains. It may thus be said that the high plateau of Asia has its own fauna, so also its lower terrace, and also the lowlands of the deserts and those of the prairies.

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If the theory of Dana is approximately correct, then the gradual growth of the Asiatic continent, as well as its present shape, can be very well explained. During the Primary epoch, Asia consisted only of the high plateau, which had the shape of a South America, directed by its narrow point towards the Behring Strait. (Was not the North Pole in that direction at that time?) On the line of division between what was then the continent and the oceans which surrounded it at that time, in consequence of the oblique thrust of which Dana speaks, the border-ranges must have been formed all along the fringe of the high plateaus, while a succession of parallel mountain ranges, resulting from as many foldings of the strata, were formed round the continent, just as the islands of Formosa, Japan, and so on, are now lifted all round the continent of Asia in a series of curves indicated by Suess. Later on, when the lower Mongolian terrace of the plateau emerged in its turn from the ocean, the formation of mountains was continued along its borders. It was then that, in all probability, the borderranges of the second terrace (Great Khingan, etc.) and the alpine zone which fringes these border-ranges were formed.

A similar formation is also found, but on a much smaller scale, along the fringe of the third terrace—the terrace of the high plains; but the arrest of upheavals was not sufficiently long at this stage, nor was the difference of level between the high plains and the bottom of the ocean sufficiently large to generate the high border-ranges such as we find along the fringes of the plateau. And finally, during the recent periods, a series of littoral chains has been lifted up, and is being lifted up still, all along the present coasts of the Pacific Ocean. That these chains are not generated in straight lines, but have a crescent shape, as has been suggested by Suess, is pretty correct, and it must also be remarked that the border-ranges of the plateau also are not quite straight lines. We see, on the contrary, that straight lines are intermingled with crescents, and when I speak of chains of mountains having a direction from the southwest to the northeast, or from southeast to northwest, I only mean that such is the general direction of the chain, without pretending to say that the chain follows necessarily a straight line. Sometimes the line is nearly straight, or it follows a great circle of our globe; this last case is very frequent, but sometimes it also has the shape of a curve. It thus appears that the theory of Dana, completed by the study of erosions which took place during an extremely long succession of ages on a grand scale, and the generalizations concerning the orography of Asia, which are expounded here, stand sufficiently in accordance for mutually confirming each other.