[2] Lugeon, loc. cit., p. 773.

[3] De Lapparent, Traite de Geol., p. 1,773.

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the recumbent folds and the peculiar phenomena of increasing overlap, or deferlement, an obstacle, fixed and deep-seated, must have arrested the roots or synclines of the folds, and held them against translational motion, while a movement of the upper crust drew out and carried forward the anticlines.  Others have contented themselves by recording the facts without advancing any explanatory hypothesis beyond that embodied in the incontestable statement that such phenomena must be referred to the effects of tangential forces acting in the Earth’s crust.

It would appear that the explanation of the phenomena of recumbent folds and their deferlement is to be obtained directly from the temperature conditions prevailing throughout the stressed pile of rocks; and here the subject of mountain tectonics touches that with which we were elsewhere specially concerned—­the geological influence of accumulated radioactive energy.

As already shown[1], a rise of temperature due to this source of several hundred degrees might be added to such temperatures as would arise from the mere blanketing of the Earth, and the consequent upward movement of the geotherms.  The time element is here the most important consideration.  The whole sequence of events from the first orogenic movements to the final upheaval in Pliocene times must probably have occupied not less than ten million years.

[1] Mountain Genesis, p. 129, et seq.

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Unfortunately the full investigation of the distribution of temperature after any given time is beset with difficulties; the conditions being extremely complex.  If the radioactive heating was strictly adiabatic—­that is, if all the heat was conserved and none entered from without—­the time required for the attainment of the equilibrium radioactive temperature would be just about six million years.  The conditions are not, indeed, adiabatic; but, on the other hand, the rocks upraised by lateral pressure were by no means at 0 deg.  C. to start with.  They must be assumed to have possessed such temperatures as the prior radiothermal effects, and the conducted heat from the Earth’s interior, may have established.

It would from this appear probable that if a duration of ten million years was involved, the equilibrium radioactive temperatures must nearly have been attained.  The effects of heat conducted from the underlying earthcrust have to be added, leading to a further rise in temperature of not less than 500 deg. or 600 deg. .  In such considerations the observed indications of high temperatures in materials now laid bare by denudation, probably find their explanation (P1.  XIX).

The first fact that we infer from the former existence of such a temperature distribution is the improbability, indeed the impossibility, that anything resembling a rigid obstacle, or deep-seated “horst,” can have existed beneath the present surface-level, and opposed the northerly movement of the deep-lying synclines.  For