48 REPORT — 1864. 



happened witliin tlie last 100,000 years. This, it -will be remembered, corresponds 

 with the conjectm-e of oiu- President regarding the possible antiquity of the liuvi- 

 atile gravel-beds with flint implements at St. Acheul ; and with the computation 

 of M. Morlot, of the age of the oldest gravel-cone of La Tiniere on the Lake of 

 Geneva, which he supposes to have followed the latest extreme extension of glaci- 

 ation in the Alps. 



Quite a different conclusion, however, was presented a few years since by a 

 German mathematician, Ilerr Adhemar *, who, reflecting on the difference of mean 

 annual temperature of the two hemispheres of the earth — dependent on the in- 

 equality of the half-yearly periods, our hemisphere having now the advantage of 

 position — finds that within each half ' tropical ' period (about 10,500 years) snows 

 would gather and glaciers thicken round one pole, to be afterwards melted 

 while glaciation was spreading round the other. Thus, periodical deluges, at 

 intervals of 10,500 jears, are found by this inquirer to be part of the system of 

 nature. 



The opinion, however, has long been growing among geologists, that it is rather 

 by rising and falling of the land, and displacement of the sea, that the alternations 

 of snows and floods must be explained, which are admitted to have visited the 

 moimtain regions of the north. In Switzerland two great extensions of ice in 

 former times have been traced by Escher and the eminent geologists of that country 

 — the latter one con-esponding perhaps to the age of our glacial drift. 



The melting of snow and ice in the valleys of the Alps is far more rapid under 

 the influence of certain winds than by the direct effect of simshine. Withdraw 

 the hot Fohn for a season, the glaciers would renew their advance ; let it cease, or 

 lose its specific action for a centm-y, the progress of the ice would be considerable. 

 In many centuries the Rhone glacier might reach again to Sion, Villeneuve, and 

 Lausanne ; in many thousands of years, all the valleys, and lakes, and borders of 

 the Alps might be reoecupied by ice. 



Now the southerly wind, which so rapidly strips the alpine peaks of their snows, 

 di'aws its melting power from the hot northern tracts of Africa. Were these tracts 

 again covered, as once they were, with an expansion of the Mediterranean, the 

 wind woidd lose its excessive dissolving power, — snows would gather above, and 

 glaciers extend below to levels and distances now quite unattainable without some 

 great physical change. 



Great phi/sical change, then, is the inevitable antecedent to extensive glaciation 

 and abimdant dissolution of ice round the mountains of the north. Astronomical 

 vicissitudes returning in cycles of long duration, changes of level of the land, ex- 

 pansions and contractions of the sea, deviations of the currents of the ocean, 

 alterations in the prevalent direction and quality of the winds — whichever of these 

 causes we assume, and however we combine them, it is e%ident that we are ap- 

 pealing from the existing order of nature and the present measures of effect in 

 time, to some other combination of natural agencies, some other standard of 

 physical energy. The conclusion is obvious. Inductive geology refuses to accept 

 definite periods for phenomena produced imder conditions not yet really determined. 



I will not, by any further observations, discourage you from exploring this 

 attractive field of research, or restrain the freedom with which you will desire to 

 discuss it. Only let me add, that to one fresh from the Alps — from the old 

 Pfahlbauten of the lakes, and much older monuments of overspreading snow and 

 gliding ice, the later ages of geology and the earlier ages of mankind seem to be 

 fairly united in one large field of inquiiy. That it must be trodden with heedful 

 steps, and demands all possible care in the scrutiny of facts, in the estimation of 

 natural agencies, and in the choice of right measm-es of time, before the Pleistocene, 

 Quaternary, or Hmuan period can be said to be accurate!}- known by natural phe- 

 nomena, even in this the best-examined part of the world, is obvious. 



But the same remark applies to every one of the many perplexing questions 

 which have been considered by geologists. By following the same good processes 

 of strict inquiry and cautious interpretation which have settled those difficulties, 

 we \i\AY hope to settle this. " Let every one join in the effort, and bring selected 



* Eevolution des Meei'es. Leipzig, 1843. 



