372 ALTERNATE GLACIAL PERIODS 



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 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 

 having extended all round the northern and southern hemi- 

 spheres — 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 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 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 

 ocourred about 240,000 years ago, and endured, with slight 

 alterations of climate, for about 160,000 years. With re- 

 spect 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. But the most important result for us, 

 arrived at by Mr. Croll, is that whenever the northern hemi- 

 sphere passes through a cold period the temperature of the 

 southern hemisphere is actually raised, with the winters 

 rendered much milder, chiefly through changes in the direc- 

 tion of the ocean currents. So conversely it will be with the 

 northern hemisphere, while the southern passes through a 

 glacial period. This conclusion throws so much light on 



