300 Searles V. Wood—The Cause of the Glacial Period. 
way whatever connected with any diversion of the ocean currents ; 
and without such a diversion Croll freely admits that variations in 
the eccentricity of the earth’s orbit, and in the position of the 
aphelion point, could have had no effect on the mean annual tem- 
perature of either hemisphere; because whatever be the amount of 
this eccentricity, and wherever be the position of the aphelion in the 
orbit, equal quantities of heat must, as D’Alembert showed, and as 
has always been conceded, reach both hemispheres alike during 
every revolution of the earth round the sun, the deficiency of one 
part of the year being made up by the excess of the other. 
Since the ocean currents were thus unaltered, and the glaciation 
of Europe and North America follows (as Dana justly observes) the 
present isothermals, it results also that neither geographical changes, 
nor any alteration in the position of the earth’s axis, nor in the obliquity 
of the ecliptic, can have been the cause of the Newer Pliocene re- 
frigeration; but my present concern is to examine how the land ice, 
due to the action of this refrigeration upon the precipitation, is 
consistent with that reduction im the evaporation, and consequent 
precipitation, which is involved in a decrease of the sun’s heat. 
Rink, whose long residence in Greenland, and observation of the 
phenomena there, has been the means of much of our present know- 
ledge of glaciation, estimates the annual precipitation on that region 
(inclusive of what rain falls there in summer) at only 12 inches ; ! 
so that to this small precipitation is due the mass of ice under which 
West and South Greenland is buried—a mass, which, from the 
depth Sutherland? considers it must descend below the sea-line in 
the buried channels (2000 feet), cannot be much, if at all, inferior 
in thickness to that under which the glaciated area of North America 
was buried. Therefore, as this precipitation is but little more than 
a quarter of that which now takes place over the North American 
area, we may, after allowing liberally for any under-estimate in 
Rink’s figures, admit that the land-ice to which the glaciation of 
North America was due arose from a much less evaporation than 
that to which the present heat of the sun gives rise, the ocean cur- 
rents flowing then as they now do. This comparison applies with 
even greater force to Britain, because the present precipitation of 
1 Proc. Royal Geographical Soc. for 1862-3, p. 76; a.so (but I quote this only 
from Brown's Physics of Arctic Ice, in Quart. Journ. Geol. Soc. vol. xxvii. p. 681, 
note) Naturhistorisk Tidsskrift, 3rd series, vol. i. part 2, 1862. Of this 12 inches 
he estimates that only two are represented by the actual ice escape as glaciers to the 
sea, the rest, beyond the insignificant amount represented by evaporation, passing off 
by melting as water through the ice in the form of sub-glacial rivers, which boil up 
in copious springs of freshwater through the sea in front of the glaciers. I think, 
however, that perhaps it may not all pass off, and that the overwhelming of Rein- 
deer pastures by the ice during the centuries of Danish occupation, and the indications 
of subsidence afforded by the position of ancient dwellings, may show that the ice is 
now augmenting and the land sinking under its weight. If so, this probably has been 
going on since the close of the Minor Glaciation, from the change in the places of 
ereatest snow precipitation discussed in the text; and the ancient depression indicated 
by marine shells high above present sea-level in Greenland may possibly have 
been due to a similar augmentation of the ice in the interval between the Major 
and Minor Glaciations, to that which is thus now only in progress. 
* Quart. Journ. Geol. Soc. vol. ix. p. 301. 
