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.Lyell has shown, is the relative position of the land and water.

Mr.Croll believes that the last great glacial period occurred about 240,000 years ago, and endured, with slight alterations of climate, for about 160,000 years.

With respect to more ancient glacial periods, several geologists are convinced, from direct evidence, that such occurred during the miocene and eocene formations, not to mention still more ancient formations.