Popular Science Monthly/Volume 65/May 1904/The Geology and Geo-Botany of Asia
|THE GEOLOGY AND GEO-BOTANY OF ASIA.|
THE time has not yet come when the geological history of Asia can be written in full. It appears, however, that, with the exception of a marginal zone in the south, which belongs to the Himalayan upheavals, the great plateaus of east Asia are built up of crystalline unstratified rocks, granites, granitites, syenites and diorites, as well as of gneisses, talc and mica-schists, clay-slates and limestones, which all belong to the Archean formation (Huronian, Laurentian, Silurian and partly Devonian). They thus can not have been submerged by the sea since the Devonian epoch. The higher terrace of the plateau of Pamir and the plateaus of the Selenga and the Vitim are formed only of Huronian and Laurentian azoic schists; even Silurian deposits—widely spread on the plains—are doubtful on the plateaus. Their upheaval must date accordingly from an earlier age, and they must have been a continent during the Devonian epoch, while parts at least of the lower terrace were under the sea at that period. During the Jurassic and Tertiary periods immense fresh-water basins covered the surface of those plateaus; they have left their traces in Jurassic coal-beds, and in Tertiary sands and conglomerates, these latter appearing in mighty layers on the borders and slopes of the plateaus. The chains of mountains which fringe the plateaus along their northwestern and southeastern borders are of the same ancient geological origin. They rose above the Carboniferous, Triassic, Chalk and Jurassic seas which covered what are now the lowlands and lower terraces of Asia; the upheaval of these chains has, however, continued throughout these epochs, so that in the outer chains of Asia we see Carboniferous and younger deposits, up to Tertiary, lifted up to great heights. The same is true of the border-ranges along the southwestern border of the great plateau of east Asia, namely, the Himalayas, which were lifted during the Tertiary age, while at their northern foot, on the surface of what is now the surface of the plateau, traces of Triassic deposits seem to have been found near Lhasa. Carboniferous deposits are met with in Turkestan, India and western Asia; while in eastern Asia the numerous coal-beds of Manchuria, China and the archipelagoes are all Jurassic. As to the age of the plateaus of western Asia, it remains unknown at the present time. What arc now the lowlands of Asia must have been widely submerged by the seas of the Tertiary period, as also those of the Quaternary (Post-Pliocene) period. During this last period, the whole of the lowlands of northwest Siberia were under the sea, as far as the fiftieth degree of latitude, a broad gulf of the Arctic Ocean penetrating at the eastern foot of the Urals as far as the watershed which now separates the basin of the Obi from the Aral-Caspian Sea. At the same time there are no traces of that sea on the high plains of east Siberia, which were only intersected by several narrow elongated gulfs of the ocean. The moistness that thus ensued permitted glaciers (which are wanting now throughout the middle parts of east Siberia and Mongolia) freely to accumulate, so that the whole of the upper plateau and its border-ridges were under a mighty ice-cap. Immense glaciers, like those of the Alps and Jura, covered also the alpine regions. How far glaciation extended over the plateau of Tibet and in China still remains unsettled. In Turkestan and Siberia immense accumulations of loess fringe the alpine regions; while in China they cover immense tracts, and are the most fertile regions of Asia.
Many important changes in the distribution of land and water have been going on in Asia since the Glacial period, and even during historical times. Since the Aral-Caspian Sea became isolated from the ocean, its desiccation, as well as that of the numberless lakes which dotted the surface of Asia during the Lacustrine (Post-Glacial) period, has proceeded with a rapidity which may be guessed from the very rapid rate at which the process has been observed to go on in Siberia during the last hundred years. All Asia bears unmistakable traces of having been covered during the Lacustrine period with numberless large and small lakes, which have now disappeared, not in consequence of the action of man, but in consequence of some general causes affecting the earth's surface since the last Glacial period. The process is still more accelerated by the rapid upheaval of the continent—the whole of the Arctic coast, as also most of that washed by the Pacific, the Mediterranean, the Red Sea and parts of the Indian Ocean, being in a state of gradual elevation, while the few areas where traces of subsidence have been noticed are very limited. The influence of the desiccation of Asia has been felt even during historical times, and the migrations of the Ural-Altaians, Turks and Mongols will probably be best explained if this change in the condition of central Asia be taken into account; while the same circumstance explains the present nearly desert state of those regions which were the cradle of European civilization.
Volcanoes play an important part in Asia's geology; no less than 122 active volcanoes are already known in Asia, chiefly in the islands of southeast Asia, the Philippines, Japan, the Kuriles and Kamchatka, as also in a few islands of the Bay of Bengal and Arabia, and of western Asia. Numerous extinct volcanoes are found, not only in the same regions, but also in the Caucasus, on the plateau of Armenia, in east Tian-shan, in the northwestern border-ridges of the high Siberian plateau, and in the southwest of Aigun, in Manchuria (the Uyun Holdontsi region). An immense zone of land was covered with basaltic lavas during the early Tertiary period (or Cretaceous?) along the northwestern border of the high plateau of east Asia, and vents through which scoriæ were ejected, forming small cones of ejection, are still seen on the Mongolian and Vitim plateaus, as well as in the Sayans. Earthquakes are frequent, especially in Armenia, Turkestan and around Lake Baikal.
With the rich botanic materials which we are already in possession of, it would be extremely interesting to make a picture of the distribution of the different floras of Asia upon the surfaces of its high and lower plateaus, their border-ranges, the alpine zones, the high plains, and the lowlands. It would then appear how much these orographical divisions can help us to find out the true distribution of floras, which botanists have hitherto tried to bring into accordance with zones that were traced either along the degrees of latitude, or according to the basins of the different rivers, without taking into account the great orographical divisions and the differences of altitude, which mostly run in diagonal directions. Thus, to take only one example, the Great Khingan is the most important botanical boundary which is found all over Siberia and Manchuria. When one crosses this border-range and goes down its steep slope towards the east, one sees that in one hour, or maybe in half an hour, quite a new flora—Manchurian—takes the place of the Siberian flora; and one notices the appearance of trees, which strike even the most ignorant in botany, because these trees have not been seen before, while the traveler crossed Siberia over a distance of several thousand miles. One sees also how the Manchurian flora endeavors to spread westwards along the valleys of the Upper Amur and the Argun. Another important botanic boundary is the escarpment of the upper plateau, that is, the border-range of the Yablonovoi. This escarpment separates the Daurian flora from the Siberian, properly speaking, as sharply as the Great Khingan separates the Manchurian flora from the Daurian.
The Little Khingan will, I am inclined to think, also appear some day as another interesting botanic boundary between the Manchurian flora and the flora of the Pacific littoral. It is also quite certain that in central Asia, in the Gobi, in India and in western Asia, one could arrive at most interesting botanical generalizations by establishing the connection between the orographical and the botanical data; it is sufficient to mention here, as an instance, the delimitation of botanic regions in the Caucasus lately made by Russian botanists, who have shown that the Pontic range (that is, the border-range of the Armenian plateau) is an extremely important botanic boundary, or the remarks of M. Novitskiy concerning the flora of the Karakoram plateau.
The immense region which is usually represented on geo-botanic maps under the name of Eur-Asian boreal region, and which is bounded on the south by a line traced from the Black Sea to Lake Baikal and thence to the Upper Amur and to the Sea of Okhotsk—this region has not the uniformity which one would be inclined to attribute to it by considering the map only. While it is quite certain that plants easily spread from west to east—from Russia to the lowlands of western Siberia—they spread also with the same facility from the southwest to the northeast along the high plains, the alpine zone, the border-ranges and the plateau itself. Consequently, the flora of Siberia itself can already be subdivided quite naturally into a number of distinct regions running southwest to northeast, and not west to east. Thus we see, for instance, the cedar-tree (Pinus Cembra) spreading along the highest parts of the northwestern border-range of the high plateau, from the Altai to the Lena. We find again the same vegetation on the high plateau in northwestern Mongolia, round Lake Kosogol and the Upper Vitim. The vegetation of the high plains of the Altai offers again a great analogy with the vegetation of the high plains in the west of Lake Baikal (the Minusinsk flora being an intermediate link between the two), and the Transbaikalian flora in the east of the Yablonovoi has very much in common with the flora of the Gobi.
As soon as the Amur emerges from the high plateau, in which it has excavated a deep valley, we find on its banks representatives of the Chinese and Japanese flora under the very same latitudes where we find the Siberian flora further west. And it appears from recent explorations that even round Lake Balkhash, and at the foot of the Tianshan, vestiges of the European-Siberian flora have maintained themselves on the best-watered slopes. The lines of propagation of plants along the degrees of latitude are thus completed by lines of propagation having an oblique direction, from southwest to northeast.
The next zone which is marked on our botanical maps is the zone of the Steppes, which spreads from the prairies of south Russia through the Aral-Caspian depression and the middle parts of the high plateaus.
Western and eastern Asia, including various separate desert regions (the Han-hai, the Gobi, the dry parts of Inner Arabia, of Persia and of northwest India). However, this immense region ought to be subdivided, for central Asia alone, into at least four distinct regions, namely, the Aral-Caspian flora, the Tian-shan, the Tibet and the Mongolian flora.
The flora of the regions situated on the east of the high plateau, including China, Manchuria and Japan, must be considered as a counterpart of the Mediterranean flora. The oak reappears as soon as we cross the eastern border-ridge of the plateau, which is the Great Khingan; so also the walnut tree, the lime tree, the hazel and the myrtle, while several new species of poplar, willow, acacias and many others make their appearance. The forests, consisting of a very mixed vegetation, where southern species meet with northern ones, become really beautiful; while in Japan a variety of species of pine and the reappearance of the beech add to their beauty. A rich underwood of lianas, ivies, wild vines, roses and so on renders the forests quite impassable, especially in the littoral region, which is submitted to the influence of the monsoons. In the lower parts rich prairies cover immense spaces, the grass vegetation becomes luxuriant, and in the virgin prairies of the Amur man and horse are easily concealed by the grasses of gigantic size. Eice and cotton are cultivated in the southern part of the region. However, this region is too vast, and has too many different aspects, to be considered as one single region, and ought to be subdivided into three sections—the Chinese flora, that of Manchuria and the Okhotsk littoral.
A striking change in the character of the vegetation is also seen, as we now learn from recent exploration, at the southeastern extremity of the high plateau of Asia. The traveler who comes from Mongolia and has crossed the Kam region of eastern Tibet finds, as soon as he has penetrated into the region of the tributaries of the Blue River and Mekong, quite a new world, both vegetable and animal. The very fact that the 12,000-feet-high plateau is deeply excavated by the valleys and canyons of the great rivers, and that this part of the plateau is submitted to the influence of the monsoons blowing from the southwest, is sufficient to give quite a new character to the vegetation, which may be described as a mixture of the Chinese and Indo-Chinese floras.
In the Caucasus the rich vegetation of the wet portions of the country, situated between the main chains of the Caucasus and the Pontic chain, belongs to the Mediterranean flora of western Europe. As to the flora of Asia Minor, it combines the species of southern Europe with those of north Africa, as we find there evergreen oaks, the laurel-olive tree, the myrtle, the oleander, the pistachio tree and a great variety of bulbous plants.
These few remarks indicate how much the task of the geo-botanist would be facilitated if he took into consideration the great features of the orography of Asia and the orientation of the main orographic divisions. The same must be said with regard to the fauna of Asia. It would be impossible to understand its distribution if we did not take into consideration, as has been indicated by Syevertsoff, the spreading of many species from the southwest to the northeast along the plateaus 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.
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.
- From an article in the March number of The Geographical Journal.