To sum the contents of this essay in the most general terms, we find that in the conception of denudation as producing the convection and accumulation of radiothermal energy the surface features of the globe receive a new significance. The heat of the earth is not internal only, but rather a heat-source exists at the surface, which, as we have seen, cannot prevail to the same degree within; and when the conditions become favourable for the aggregation of the energy, the crust, heated both from beneath and from above, assumes properties more akin to those of its earlier stages of development, the secular heat-loss being restored in the radioactive supplies. These causes of local mobility have been in operation, shifting somewhat from place to place, and defined geographically by the continental masses undergoing denudation, since the earliest times. 145 ALPINE STRUCTURE AN intelligent observer of the geological changes progressing in southern Europe in Eocene times would have seen little to inspire him with a premonition of the events then developing. The Nummulitic limestones were being laid down in that enlarged Mediterranean which at this period, save for a few islands, covered most of south Europe. Of these stratified remains, as well as of the great beds of Cretaceous, Jurassic, Triassic, and Permian sediments beneath, our hypothetical observer would probably have been regardless; just as today we observe, with an indifference born of our transitoriness, the deposits rapidly gathering wherever river discharge is distributing the sediments over the sea-floor, or the lime-secreting organisms are actively at work. And yet it took but a few millions of years to uplift the deposits of the ancient Tethys; pile high its sediments in fold upon fold in the Alps, the Carpathians, and the Himalayas; and--exposing them to the rigours of denudation at altitudes where glaciation, landslip, and torrent prevail--inaugurate a new epoch of sedimentation and upheaval. 146 In the case of the Alps, to which we wish now specially to refer, the chief upheaval appears to have been in Oligocene times, although movement continued to the close of the Pliocene. There was thus a period of some millions of years within which the entire phenomena were comprised. Availing ourselves of Sollas' computations,[1] we may sum the maximum depths of sedimentary deposits of the geological periods concerned as follows:-- Pliocene - - - - - 3,950 m. Miocene - - - - - 4,250 m. Oligocene - - - - 3,660 m. Eocene - - - - - - 6,100 m. and assuming that the orogenic forces began their work in the last quarter of the Eocene period, we have a total of 13,400 m. as some measure of the time which elapsed. At the rate of io centimetres in a century these deposits could not have collected in less than 13.4 millions of years. It would appear that not less than some ten millions of years were consumed in the genesis of the Alps before constructive movements finally ceased.