Cjur. XI. OF THE NORTH AND SOUTH. 343 



Pncific, where tlic climate is now so difiercnt, as far south as 

 lat 4G''. Erratic bowlders have, also, been noticed on the 

 Rocky Mountains. In the Cordillera of South America, nearly 

 under the equator, glaciers once extended far Ix'low their pres- 

 ent level. In central Chili I examined a vast mound of detri- 

 tus with grciit bowlders, crossing the Portillo valle}', 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 lat, 13° to 30° S., at about the height of 

 12,000 feet, deeply-furrowed rocks, resembling those with 

 which he was familiar in Norway, and likewise great masses 

 of detritus, including grooved pebbles. Along this whole 

 space of the Cordillera true glaciers do not now exist even at 

 nmcli more considerable heights. Farther south on both sides 

 of the continent, from lat. 41° to the southernmost extremity, 

 we have the clearest evidence of former glacial action, in nu- 

 merous immense bowlders transported far from their parent 

 source. 



From these several facts, namely, from the glacial action 

 having extended all round the northern and southern hemis- 

 pheres — from tlic period having been in a geological sense re- 

 cent in both hemispheres — from its having lasted in both dur- 

 ing a great length of time, as may be inferred from the amount 

 of work effected — and lastly from glaciers having recently de- 

 scended to a low level along the whole line of the Cordillera, it 

 formerly appeared to me that we could not avoid the conclusion 

 that the temperature of the whole world had been simultane- 

 ously lowered during the Glacial period. But now Mr. CroU, 

 in a series of admirable memoirs, lias 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 orlnt. All these causes tend toward the same 

 end ; but the most powerful appears to be the influence of the 

 oocentricity of the orbit upon oceanic ciuTcnts. It follows, from 

 Mr. Croll's researches, that cold periods regularly recur every 

 ten or fifteen thousand years; but that at much longer inter- 

 vals the cold, owing to certain contingencies, is extremely se- 

 vere, and lasts for a great length of time. Mr. Cn)ll believes 

 that the last great Glacial period occurred about 2-40,000 years 

 ago, and endured with slight alterations of climate for about 

 160,000 years. With respect to more ancient Glacial periods, 

 several geologists arc convinced from direct evidence that such 

 occurred during the Miocene and Eocene fonnations, not to 



