160 ALTERNATE GLACIAL PEEIODS [Chap. XII. 



detritus with great boulders, 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 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 Cor- 

 dillera true glaciers do not now exist even at much 

 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 gla- 

 cial 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 con- 

 dition 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 



