Along this whole
space of the Cordillera true glaciers do not now exist even at much more
considerable heights. Further south, on both sides of the continent, from
latitude 41 degrees to the southernmost extremity, we have the clearest
evidence of former glacial action, in numerous immense boulders transported
far from their parent source.
>From these several facts, namely, from the glacial action having extended
all round the northern and southern hemispheres--from the period having
been in a geological sense recent in both hemispheres--from its having
lasted in both during a great length of time, as may be inferred from the
amount of work effected--and lastly, from glaciers having recently
descended to a low level along the whole line of the Cordillera, it at one
time appeared to me that we could not avoid the conclusion that the
temperature of the whole world had been simultaneously lowered during the
Glacial period. But now, Mr. Croll, in a series of admirable memoirs, has
attempted to show that a glacial condition of climate is the result of
various physical causes, brought into operation by an increase in the
eccentricity of the earth's orbit. All these causes tend towards the same
end; but the most powerful appears to be the indirect influence of the
eccentricity of the orbit upon oceanic currents. According to Mr. Croll,
cold periods regularly recur every ten or fifteen thousand years; and these
at long intervals are extremely severe, owing to certain contingencies, of
which the most important, as Sir C.
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