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succession of undulations more or less symmetrical.  As the orogenic force continues and develops, these undulations give place to folds, the limbs of which are approximately vertical, and the synclinal parts of which become ever more and more depressed into the deeper, and necessarily hotter, underlying materials; the anticlines being probably correspondingly elevated.  These events are slowly developed, and the temperature beneath is steadily rising in consequence of the conducted interior heat, and the steady accumulation of radioactive energy in the sedimentary rocks and in the buried radioactive layer of the Earth.  The work expended on the crushed and sheared rock also contributes to the developing temperature.  Thus the geotherms must move upwards, and the viscous conditions extend from below; continually diminishing the downward range of the translatory movements progressing in the higher parts.  While above the folded sediments are being carried northward, beneath they are becoming anchored in the growing viscosity of the medium.  The anticlines will bend over, and the most southerly of the folds will gradually become pushed or bent over those lying to the north.  Finally, the whole upper part of the sheaf will become horizontally recumbent; and as the uppermost folds will be those experiencing the greatest effects of the continued displacement, the deferlement or overlap must necessarily arise.

We may follow these stages of mountain evolution

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in a diagram (Fig. 9) in which we eliminate intermediate conditions, and regard the early and final stages of development only.  In the upper sketch we suppose the lateral compression much developed and the upward movement of the geotherms in progress.  The dotted line may be assumed to be a geotherm having a temperature of viscosity.  If the conditions here shown persist

{Fig. 9}

indefinitely, there is no doubt that the only further developments possible are the continued crushing of the sediments and the bodily displacement of the whole mass to the north.  The second figure is intended to show in what manner these results are evaded.  The geotherm of viscosity has risen.  All above it is affected mechanically by the continuing stress, and borne northwards in varying

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degree depending upon the rigidity.  The folds have been overthrown and drawn out; those which lay originally most to the south have become the uppermost; and, experiencing the maximum amount of displacement, overlap those lying beneath.  There has also been a certain amount of upthrow owing to the hydrostatic pressure.  This last-mentioned element of the phenomena is of highly indeterminate character, for we know not the limits to which the hydrostatic pressure may be transmitted, and where it may most readily find relief.  While, according to some of the published sections, the uplifting force would seem to have influenced the final results of the orogenic movements, a discussion of its effects would not be profitable.