Phillips— Geological Address. 179 
aphelion during our winter, a great accession of snow might arise and 
be continued for ages, and glaciers have a large augmentation ; under 
the contrary circumstances, less snow and shortened glaciers. To 
this latter condition the present state of the north corresponds; and 
by consulting the astronomical tables, it appears that a condition of 
extreme glaciation, dependent on the maximum excentricity of the 
earth’s orbit, cannot have happened within the last 100,000 years. 
This, it will be remembered, corresponds with the conjecture of our 
President regarding the possible antiquity of the fluviatile 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 Tiniére on 
the Lake of Geneva, which he supposes to have followed the latest 
extreme extension of glaciation in the Alps. 
Quite a different conclusion, however, was presented a few years 
since by a German mathematician, Herr Adhemar,* who, reflecting 
on the difference of mean annual temperature of the two hemispheres 
of the earth—dependent on the inequality 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 years, are found by this inquirer to be 
part of the system of nature. 
The opinion, however, has long been growing amoug 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 ex- 
plained, which are admitted to have visited the mountain 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 corresponding 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 sunshine. Withdraw the hot Fohn for a season, the glaciers 
would renew their advance ; let it cease, or lose its specific action for 
a century, 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 reoccupied by ice. 
Now the southerly wind, which so rapidly strips the alpine peaks 
of their snows, draws 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 would 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 physical change, then, is the inevitable antecedent to ex- 
tensive glaciation and abundant dissolution of ice round the moun- 
* Revolution des Meeres, Leipzig, 1843. 
N 2 
