An important tectonic principle underlies the development of the phenomena we have just been reviewing. The uppermost of the superimposed recumbent folds is more extended in its development than those which lie beneath. Passing downwards from the highest of the folds, they are found to be less and less extended both in the northerly and in the southerly direction, speaking of the special case--the Alps--now before us. This feature might be described somewhat differently. We might say that those folds which had their roots farther 153 to the south were the most drawn-out towards the north: or again we might say that the synclinal or deep-seated part of the fold has lagged behind the anticlinal or what was originally the highest part of the fold, in the advance of the latter to the north. The anticline has advanced relatively to the syncline. To this law one exception only is observed in the Swiss Alps; the sheet of the Breche (_Byecciendecke_) falls short, in its northerly extension, of the underlying fold, which extends to form the Prealpes. Contemplating such a generalised section as Professor Schmidt's, or, indeed, more particular sections, such as those in the Mont Blanc Massif by Marcel Bertrand,[1] of the Dent de Morcles, Diablerets, Wildhorn, and Massif de la Breche by Lugeon,[2] or finally Termier's section of the Pelvoux Massif,[3] one is reminded of the breaking of waves on a sloping beach. The wave, retarded at its base, is carried forward above by its momentum, and finally spreads far up on the strand; and if it could there remain, the succeeding wave must necessarily find itself superimposed upon the first. But no effects of inertia, no kinetic effects, may be called to our aid in explaining the formation of mountains. Some geologists have accordingly supposed that in order to account for [1] Marcel Bertrand, _Cong. Geol. Internat._, 1900, Guide Geol., xiii. a, p. 41. [2] Lugeon, _loc. cit._, p. 773. [3] De Lapparent, _Traite de Geol._, p. 1,773. 154 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.