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varied from time to time, and occasionally it may even have been sudden and catastrophic. But such evi

dence as we have would lead us to infer that vertical displacements, whether the result of downward or of upward movements, have not been more rapidly effected than horizontal deformations. No doubt a sudden dislocation of the crust of large extent would show directly at the surface. But somewhat similar results would follow if the dislocation, without being quite sudden, were yet to be developed more rapidly than the rate of superficial erosion and denudation. Cases of the kind are well known, and to some of these reference will presently be made. It is with faulted rocks, however, as with folded mountains: when movement has ceased the inequalities caused at the earth's surface tend to be reduced and greatly modified. The epigene forces are untiring in their action, so that in course of time areas of direct subsidence tend to become filled up and the surrounding high-lying tracts to be worn down. To such an ex

tent has this taken place, that in the case of certain great faults of high geological antiquity no inequality at the surface indicates their presence, and it is only by studying the geological structure that we are able to ascertain that such dislocations exist.

Bearing in mind the activity of the denuding agents, we might expect that normal faults of geologically recent date should show most prominently at the surface. And this to a large extent is doubtless true. Nevertheless, as we shall learn by-and-by, there are certain

faults of prodigious antiquity which still cause very marked inequalities at the surface. These often form the boundaries between highlands and lowlands. In such cases, however, the disparity of level is due not so much to vertical displacement, as to the fact that the lowlands are usually composed of less enduring materials than those which enter into the framework of the adjacent highlands. When a fault of great age traverses strata of much the same consistency (say sandstones and shales), the rocks on either side of the dislocation, we find, have been planed down to

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FIG. 61. NORMAL FAULT, WITH HIGH GROUND ON DOWNTHROW Side.

the same level. Thus in the low-lying coal-fields of Scotland the gently undulating surface gives no indication of the presence of the numerous dislocations which have been detected underground. Downthrows of hundreds of feet give rise to no superficial inequalities. It is only when one of these faults has brought relatively hard and soft rocks into juxtaposition that a marked surface-feature results. And in this case the hard rock invariably rises above the level of the soft rock, no matter on which side of the dislocation it happens to lie. Thus in Fig. 61 the hard rock a forms an eminence, although it is on the downthrow side of the fault, simply because it has withstood denud

ation more effectually than the soft rock (b). In Fig. 62, again, it is obvious that the high ground at x owes its origin to the presence of the relatively hard rock (h). To this matter, however, we shall return in the sequel. Meanwhile we must consider, first, the appearances presented in regions where vertical movements of the crust have taken place within relatively recent times. The Colorado Plateau affords some excellent examples of simple folds and normal faults of comparatively recent age. These have often profoundly affected the surface, lines of cliffs and bold escarpments rising along the high side of each dislocation.

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FIG. 62. NORMAL FAULT, WITH HIGH GROUND ON Upcast Side.

The plateau, in short, has been split across by wellmarked normal faults, some of which can be followed for hundreds of miles. Yet the strata on both sides of such dislocations are of much the same character and consistency. Here, then, it might be supposed that the fracturing and displacement had been suddenly effected. There is striking evidence, however, to show that such has not been the case. Although some of the faults referred to have a downthrow of several thousand feet, yet they have had no effect in disturbing the course of the Colorado River, which traverses the faulted region. The same, as we have seen,

holds true with regard to the flexures of that area. It is obvious, in a word, that the process of flexuring and faulting has proceeded so slowly that the river has been able to saw its way across the inequalities as fast as these appeared. But while the rate of river erosion has equalled that of crustal movement, the denudation of the plateau outside of the rivercourses has not. Deformation and dislocation of the plateau have thus given rise to marked surface-features. Yet even in the case of these relatively young faults we find that the features determined by them. have been very considerably modified by denudation. In the following section, for example, we see three

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FIG. 63. FAULTS IN QUEANTOWEEP VALLEY, GRAND CAÑON DISTRICT. (Dutton.)

faults of 1300 feet, 300 feet, and 800 feet displacement respectively traversing the same series of strata, and yet giving rise to marked inequalities at the surface. The dotted lines, however, show to what an extent these features have been modified by denudation. There is an obvious tendency of the escarpments and cliffs to become benched back as they retreat, so that they do not show the abrupt character which they would have possessed had no superficial waste accom

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.

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