AGE OF MAMMALS. 275 



tures. The Rocky Mountains, which began to be raised 

 at the close of the Cretaceous age, did not reach their 

 full height till late in the Tertiary period. The Pyrenees 

 and Carpathians were lifted up in the Eocene part of the 

 Tertiary. It was during the Tertiary age that the Alps, 

 Appenines, the Caucasian range, and the Himalayas at- 

 tained, very nearly at least, to their present altitude. On 

 the completion of this period the mountains of the earth 

 were very generally raised to their full height. 



Observe that these mountains, which were raised thus 

 late, comparatively, in the course of the formation of the 

 continents, are many of them of very great height. Com- 

 pare them with the Alleghanies, that were raised, as is 

 supposed, at the close of the Carboniferous age. The 

 contrast is still greater if you compare them with the 

 Laureutian Hills, so called, in Canada, which were the 

 mountains in the Azoic age on that long island, the germ 

 of the North American continent, spoken of in 267. 

 These first mountains of the earth are nowhere more 

 than 1500 or 2000 feet above the level of the sea. The 

 reason of this difference between the older and more re- 

 cently formed mountains is thus given by Agassiz in 

 speaking of the Laurentian Hills. " Their low stature, 

 as compared with that of more lofty mountain ranges, is 

 in accordance with an invariable rule, by which the rela- 

 tive age of mountains may be estimated. The oldest 

 mountains are the lowest, while the younger and more 

 recent ones tower above their elders, and are usually 

 more torn and dislocated also. This is easily understood 

 when we remember that all mountains and mountain 

 chains are the result of upheavals, and that the violence 

 of the outbreak must have been in proportion to the 

 strength of the resistance. When the crust of the earth 

 was so thin that the heated masses within easily broke 

 through it, they were not thrown to so great a height, 

 and formed comparatively low elevations, such as the 

 Canadian hills, or the mountains of Bretagne and Wales, 



