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panied and succeeded the crustal movements. (See Fig. 63.)

In the Great Basin that extends between the bold escarpment of the Sierra Nevada, on the one hand, and the Wahsatch Mountains on the other, we encounter another series of large faults, which have determined the leading features of the region. It would appear that the area of the Great Basin formerly attained a considerably greater elevation than at present. Towards the close of Tertiary times the whole of this area, including the adjacent Sierra Nevada and the Wahsatch Mountains, was upheaved in the form of a broad arch. The crust thus subject to tension yielded by cracking across, and a system of long parallel north and south fissures was formed. In other words, the broad arch was split into a series of oblong blocks many miles in extent. When the movement of elevation ceased and subsidence ensued, the shattered crust settled down unequally between the Sierra Nevada in the west and the Wahsatch Mountains in the east. The amount of displacement along the margins of the Great Basin is very great; the fault at the base of the Sierra, for example, is estimated to be not less than 15,000 feet, while that which severs the Basin from the Wahsatch Mountains is also very great. The numerous parallel ranges that diversify the surface of the Great Basin itself are simply oblong crust-blocks, brought into position by normal faults. Being of so recent an age, they have suffered comparatively little modification.

Nevertheless, they do not fail to show the tool-marks of epigene action-everywhere escarpments are retreating, and one can see that already vast masses of rock have been removed from the surface. The accompanying diagram (Fig. 64) will serve to give a general idea of the geological structure of the Basin ranges. There is no reason to believe that the crustal movements above referred to were sudden or catastrophic in character. Probably they were no more rapid than those which have affected the plateau of the Colorado.

We are not without evidence of similar recent dislocations in the Old World, and there as elsewhere they give rise to more or less pronounced surface-features. One of the most interesting examples is seen in the great depression that extends northwards from the Gulf of Akabah by the Wâdy el Arabah, the Dead Sea, the valley of the Jordan, and Lake Tiberias. This long hollow would appear to

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FIG. 64. RANGES OF THE GREAT BASIN. (Hinman, after Gilbert.) (Length of section, 120 miles.)

Q, Quaternary deposits; xx, granite, schists, and bedded rocks of various age; ff, faults. Each mountain is a "uniclinal orographic block," tilted in the direction indicated by the close lines; the latter, therefore, do not represent bedding. The diagram shows merely the general disposition of the disrupted rock-masses, and not the geological structure of each individual fault-block.

have come into existence at or about the close of Tertiary times. It is everywhere bounded by normal faults or by steep monoclinal folds, the one kind of structure passing into the other. Before this depression came into existence the region it now traverses appears to have been a broad continuous plateau, built up of ancient crystalline and Palæozoic rocks below, and approximately horizontal strata of Mesozoic age above. At what particular date this plateau of accumulation first appeared, and how long it remained undisturbed, we cannot tell. Possibly the

movement of subsidence to which the Dead Sea owes its origin may have coincided with the upheaval that resulted in the formation of the plateau. However that may have been, the latter was eventually traversed by a series of monoclinal folds and parallel faults, and between these the great depression of the Jordan came into existence. The Mesozoic strata of the plateau retain their approximately horizontal position close up to the depression along its eastern margin, while the descent from the west is much less abrupt. But this is only broadly true. When the region is more closely investigated, the relatively gentle dip of the strata along the west side of the depression is found to be interrupted again and again by more or less sharp monoclinal folds and by normal faults, the presence of which is betrayed at the surface by corresponding sudden changes in the form of the ground. In other words, the descent from the plateau on the west is often by a series of broader

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and narrower terraces and escarpments, running parallel with the trend of the great hollow. The western margin of the Dead Sea, for example, is determined by a vertical displacement, similar in character to, but not so extensive as, that which bounds it on the east. The section (Fig. 65) will serve to illustrate the geological structures referred to.

The flexures and faults of this interesting region do not date beyond the close of the Tertiary period, and consequently there has not been sufficient time to allow of a complete modification of the surface by epigene action. The most conspicuous features of the district are determined by folds and fractures-underground structure and surfaceconfiguration to a large extent coincide. But everywhere also we observe the evidence of erosion and denudation. Great sheets of rock have been gradually removed from the surface, which is seamed and scarred by innumerable ravines and water

[blocks in formation]

FIG. 65. SECTION FROM THE MEDITERRANEAN ACROSS THE MOUNTAINS OF PALESTINE TO THE MOUNTAINS OF MOAB. (After M. Blanckenhorn.) 1, schistose and ancient igneous rocks; 2, Carboniferous or Permian; 3, 4, 5, Cretaceous strata;

6, Tertiary; ƒƒ, faults,

courses, many of these being now dry and deserted. According to Professor Suess, the Jordan depression continues north between the Lebanon and the Anti-Lebanon, through the valley of the Nahr el Asi (the Orontes) to near Antioch. The same geologist is further of opinion that the great trough of the Red Sea and most of the lacustrine hollows of East Africa are in like manner due to direct subsidence of the crust, the probability being that they and the Jordan depression all belong to one and the same system of crustal deformation. It is noteworthy that the depressed areas of Africa lie in zones or belts having an approximately meridional direction, that they are not margined or surrounded by mountain-ranges, but are sunk in broad plateaux, and, moreover, are accompanied by abundant evidence of volcanic action. The troughs are mostly broad, and vary much and constantly in height above the sea, so that they are obviously not the result of erosion. In many places they are flanked on both sides by abrupt declivities comparable in character to those that overlook the Dead Sea. In some cases, however, steep bluffs and cliffs are confined to one side of a depression only. In short, we have in East Africa the same phenomena which confront us in Palestine. The earth's crust in all those regions has evidently yielded to strain or tension by snapping across and subsiding. In place of one simple normal fault, however, we see a complex system of parallel dislocations and flexures, the folded and shattered rocks having settled down un

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