518 THE ICE AGE IN NORTE AMERICA. 



earth's orbit attained two hundred thousand years ago. 

 Through all the past ages of geology, also, the earth has from 

 time to time passed through similar stages of increased eccen- 

 tricity, sometimes having a still higher maximum,* which we 

 should expect, in accordance with Croll's theory, to find re- 

 corded by deposits of glacial drift intercalated in the Tertiary, 

 Mesozoic, and Palaeozoic strata of circumpolar and temperate 

 regions. But the recentness of the Quaternary glaciation, and 

 the general absence of earlier drift formations,! excepting 

 within the Carboniferous and Permian periods, seem to demon- 

 strate that eccentricity has not been the primary cause of gla- 

 ciation, either with the concurrence of climatic conditions and 

 changes of the course of winds and of oceanic currents such 

 as might attend its slight modifications of the seasons, while 

 the present arrangement and relative heights of land and sea 

 were unchanged, or, as Wallace suggests, \ with much greater 

 elevation of the areas glaciated, which he thinks to have been 

 necessary, seconding the effects of eccentricity, for the accumu- 

 lation of ice-sheets. Indeed, we may well doubt that eccen- 

 tricity has exerted any determining influence in producing 

 unusual severity of cold either during the Quaternary or any 

 former period. 



Elevation of broad areas, as half of North America and half 

 of Europe, either synchronously or in alternation, to such 

 heights that their precipitation of moisture throughout the 

 year was nearly all snow, forming gradually ice-sheets of great 

 thickness, seems consistent with the conditions of the earth's 

 crust and interior, which are indicated by the changes in the 

 levels of glaciated countries. A molten magma beneath the 

 solid crust appears, in connection with contraction of the earth 



* Croll's "Climate and Time," chap, xix, with plate iv, representing the 

 variations in the eccentricity of the earth's orbit for three millions of years before 

 a. d. 1800, and one million of years after it. 



f Nordenslriold reports that, in sections observed by him in Spitzbergen and 

 Greenland, including all formations from the Silurian to the Tertiary, and occu- 

 pying in the aggregate, as he estimates, not less extent than a thousand English 

 miles, he has never observed erratic blocks nor any evidence of glacial action. — 

 "Geological Magazine," II, vol. ii, 1875, pp. 525-532, and vol. iii, 1876, p. 266. 



\ "Island Life," chaps, viii, ix, and xxiv. 



