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Clarke, it appears also that there are traces of former glacial 

 action on the mountains of the south-eastern corner of Australia. 



Looking to America: in the northern half, ice-borne fragments 

 of rock have been observed on the eastern side of the continent, 

 as far south as latitude thirty-six and thirty-seven degrees, and 

 on the shores of the Pacific, where the climate is now so different, 

 as far south as latitude forty-six degrees. Erratic bowlders have, 

 also, been noticed on the Rocky Mountains. In the Cordillera of 

 South America, nearly under the equator, glaciers once extended 

 far below their present level. In Central Chili I examined a vast 

 mound of detritus with great bowlders, crossing the Portillo 

 Valley, which, there can hardly be a doubt, once formed a huge 

 moraine; and Mr. D. Forbes informs me that he found in various 

 parts of the Cordillera, from latitude thirteen to thirty degrees 

 south, at about the height of 12,000 feet, deeply-furrowed rocks, 

 resembhng those with which he was familiar in Norway, and like- 

 wise great masses of detritus, including grooved pebbles. 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 forty-one degrees to the 

 southern -most extremity, we have the clearest evidence of former 

 glacial action, in numerous immense bowlders transported far from 

 their parent source. 



From these several facts, namely, from the glacial action hav- 

 ing 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 mem- 

 oirs, 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 toward 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 in- 

 tervals 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 



