Slike stranica

the traces of ancient volcanic activity. Still farther to the east, the chain which has girdled the Caspian sinks from the lofty height of the Northern Taurus to 1872 feet; in Meshed, 2628 feet; in Herat, an average elevation of 3400 to 4000 feet. But east of Herat it rises abruptly to the lofty plateaus of Bamian and Cabool, 7000 and 8000 feet high, and in the peaks of Colubeba 16,800 feet. The Hindoo Koosh, at Dsellalabad, rises to a height of 18,984 feet; the table-lands of the Bolor, at the Issikul, are at an elevation of 14,664 feet; while the gigantic Pameer is not yet measured, though its noted Pass is estimated at 18,000 feet above the sea.

At this point we reach almost the 40th degree of N. lat., whence northward the mountain ranges gradually decline in height, after throwing off eastward the great chain of Thian-Shan. From the sharp angle formed by the Hindoo Koosh and the Bolor, where the head-waters of the Gihon rise, that large but commercially unimportant river takes its way westward through the Bokharan table-land, falling so rapidly in its course to Bokhara that at the city its surface is but 1116 feet above the sea, then striking northwesterly to the Aral and Caspian. The course of that stream indicates, therefore, the direction and degree of the mountain slope toward the great depression east of the Caucasus and Armenia, north of the Persian highland, and west of the Hindoo Koosh and the Bolor systems.

The lower course of the Gihon, from Bokhara downward, is through masses of mud, sand, and gravel, and can very easily be conceived to have changed its course in the lapse of centuries, from the Caspian to the Aral, as the course of the Sihon seems also to have changed. The great Bokharan plain is covered in this part with a deposit of dried mud; it is a steppe formed evidently from a now

dry sea-basin, which, no less than the northern shores of the Caspian and the Aral, displays the traces of the oceanic character of entire regions.

Halley, the astronomer, made an attempt to solve the mysterious origin of this great sunken basin, and attributed it to the stroke of a first-class comet! Arago, instead of calling into the scene meteorological forces little known, contented himself, in his theory of its origin, with the forces which we know are active even now on the earth, the plutonic powers which are only half confined by the surface of the globe. No one, he says, will hesitate now to accept the upheaval theory, through which geology is able so clearly to indicate the forces and progress of structure of the soil and rocks. The upheaval of great masses in one place predisposes the depression of districts in their neighborhood, to make good the true relation of highland to lowland. And in this case a compensation may be found, according to Arago, for the great semicircle of mountains which passes around the southern margin of the Caspian basin, in the depression caused by the natural falling in of the adjacent region when the great mountains of western Asia were upheaved.

In longitudinal mountain chains the parallel ranges of valleys have a similar origin; in volcanic chains, which have been thrown up in a circular form, similar depressions have been found in the middle, although, it must be confessed, on a much smaller scale of dimensions than in the Caspian hollow. The same feature is observable in the upheavals, by Von Buch, as observed in the Island of Palma, one of the Canaries, or in the Val di Bove, near Etna. Such depressions would at once fill with water, if connected with the sea, as in the cup-shaped island Santorini, or remain land-locked, if they occur in the interior,

as in the case of Lake Laach, as the half-ring of mountains girding the lower portion of the Caspian seems to consist mainly of trachyte thrown up by volcanic agency: the analogy just drawn does not seem too remote. Yet the process of structure must have had other concomitant conditions to account for the vast reach northward of the Caspian depression. It is clear that any such volcanic convulsion as would throw up those vast mountain ranges at the south, must have affected largely the geological condition of all the adjacent region; the extent vertically of this effect would be best ascertained perhaps by deep boring. Unquestionably there was many a revolution in the upper portions of the earth's crust during the formation of the great Caspian hollow, before it assumed its present condition. From the agencies at work in connection with a great internal ocean, the upper soil, as we have it to-day, was formed.

The Aral and the Caspian Seas remain as the lowest places of that great depression, water being found in them, while elsewhere it has entirely disappeared by evaporation : leaving us broad, low plains, instead of that great ocean which once extended from Persia over all Siberia, and west of the Caspian to the Sea of Azof. A more thorough account, geographical as well as geological, cannot be given till after much more extended investigations have been made into the physical characteristics of this region than as yet have been prosecuted. It may be remarked here, that the waters of the Aral and Caspian are bitter and salt, but not so much as those of the ocean; the bottom is covered with slime and sand. The Aral has a depth ranging from 90 to 222 feet; the Caspian, beginning with its extensive shallows at the north, deepens toward the south, till, reaching the lower third, its depth is over 600 feet;

and thence southward it is no less deep, till it reaches the bold shore of Eusellis. From this lowest point the upheaval begins, which culminates in the great mountains on its southern border.

According to Humboldt's view, the great Caspian hollow embraces not only the basin of the sea, but a vast dry plain, extending northward as far as Saratov and the Obstshei-Syrtis; even Uralsk lies lower than the level of the Black Sea. The same physical feature, though on a less extended scale than here, is found in Holland, China, Lower Egypt, and Palestine. Subsequently to the emergence of the continents, long before the filling in of huge. fissures by mountain chains, and during the continuance of those great convulsions which reach back into the remotest geological periods, the surface of the continents must have been subjected to frequent partial changes of level. The surface undulated probably in that same wave movement which is now observable, though in much less degree, in those earthquakes and upheavals which the whole western part of South America is experiencing

even now.

The depressions which have assumed a permanent form since the convulsions which formed them, have gradually filled with deposited soil, and, were the naturalist able to lay bare the primitive rock, he would discover that it exists in the shape of great concavities, without a trace of that evenness which now characterizes the surface. Eichwald has made it probable, by his personal observations, that the upheaval of Ararat and of the Armenian plateau on which this trachyte mountain rests, has driven the Caspian Sea back east of the flat steppe of Karabagh and Mogan, on the lower Aras, to the neighborhood of Bakoo. The water of that sea formerly extended to the confluence

of the Bargashad, (called also Bergershat and Bergernet,) with the Aras, below Ireben. The traces of volcanic action there are decisively evident: the Caspian reached, before that action occurred, up what is now the valley of the Aras, as far as Ararat; and in many places south of Erivan-at Saliyan, in Shirwan, and elsewhere-salt beds of the most crystalline quality, forming whole mountains and whole belts of salt lakes at the confluence of the Aras and the Koor, demonstrate their formation in a former sea which once covered that region. The very recent upheaval of the Ural chain cannot fail also to have had an influence in contracting the dimensions of the Caspian hollow.

Only two kindred depressions to this remain to be spoken of, which, though of not so great superficial dimensions, are of yet greater depth-the depression of the Jordan Valley and the bitter salt lakes on the Isthmus of Suez. These we must consider before we pass from the contrasts between highlands and lowlands to the transitions between them.

The Depression of the Jordan Valley and of the

Dead Sea.

The nearest relationship to the Caspian hollow, displayed by any similar feature, is found near the heart of the Old World, in the comparatively diminutive and isolated valley of the Jordan, including the Dead Sea, whose absolute depth below the level of the ocean has been determined only within the most recent period. Many former travelers had noticed, in the deep gulf which holds the Dead Sea, and especially at its north end, near Jericho, a much greater degree of heat than elsewhere in Palestine, and the existence of many plants and fruits which

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