1874.] Climate of the Glacial Period, 451 
mountains of Syria, the Atlas Chain, the mountains of New 
Zealand, of California and Central America, and many 
others, show distinct traces of glaciers having descended 
either on ranges where snow now never accumulates or even 
falls, or else thousands of feet below the present snow-line. 
It has by some been considered a simple explanation of these 
facts, to suppose that each mountain chain was elevated a 
few thousand feet in the glacial period, and has since sank 
down. Here the land went up and here it went down, they 
say, and think they have found a solution, without explaining 
why it should or how it could have done so. I shall have 
some more remarks to make on this assumed elasticity of 
the earth’s surface, but now pass on to remark how a general 
lowering of the sea-level would cause the snow-line to 
descend on every mountain chain. Mr. H. W. Bates has 
pointed out to me, what seems perfe¢tly obvious when once 
noticed, but what had certainly not occurred to me when I 
first wrote on this subject, namely, that a lowering of the 
sea-level would produce the same effect upon the climate of 
any place as arise of the land to about the same amount 
as the atmosphere would sink with the sea. I find that 
Humboldt, in whose writings are found the germs of many 
later theories, had made the same observation.* I fail to see 
how glaciers could be produced in the tropics on mountain 
chains far below the present snow-line in the Glacial period 
if it was caused by an increase in the eccentricity of the 
orbit ; for that could not affect the mean temperature of the 
tropics where day and night were equal, and the heaping up 
of ice at one pole could not lower the sea-level much; but 
if it was caused by an increase of the obliquity of the 
ecliptic, the mean temperature of the tropics would be 
lowered through the path of the sun being lengthened, the 
snow-line would descend still farther by the lowering of the 
sea, and stili farther from increased precipitation, owing to 
the greater evaporation that would take place when the 
shallowing of the sea shut off cold currents from the polar 
regions. The combination of these factors could not fail to 
lower the snow-line in the tropics thousands of feet, as we 
find it to have been lowered in the Glacial period. 
The examination of the deltas of the great rivers—the 
Mississippi, the Ganges, the Nile, and the Po—have shown 
that there are land-surfaces and freshwater deposits hundreds 
of feet below the level of the sea. All our English rivers run 
in old channels now filled up nearly to low-water line, but 
* Edinburgh Philosophical Journal, vol, iv., p. 267. 
