642 HISTORICAL GEOLOGY 



although many characteristic ancient forms, such as the armadillo, 

 are numerous. 



Not only did the North American mammals invade South America, 

 but a similar invasion in the opposite direction was going on at the 

 same time. These immigrants, however, failed to establish them- 

 selves permanently, although they probably lived on in their new 

 home for some time, as the presence of their remains as far north as 

 Oregon shows. Their extinction before the Pleistocene was due 

 either to the competition with the higher forms or to the cold of the 

 Glacial Period, but probably to the former. 



Duration of the Tertiary. — Because of the fact that the Tertiary 

 rocks are, with the exception of the Pleistocene, the last deposited, 

 some evidence of the duration of the period is at hand which is not 

 available in estimating the length of former periods. The most im- 

 portant means which can be employed are the following : 



(1) Biologists are generally agreed that the time necessary for the 

 evolution of modern mammals from the generalized ancestors of the 

 early Eocene was very long. For example, the highly specialized 

 modern horse could not have been evolved from the little Eohippus 

 with his four-toed foot, simple teeth, and carnivorous-like body, in 

 tens of thousands, or hundreds of thousands of years ; but a much 

 greater length of time must have been required. 



(2) Again, as in other periods, we can gain some idea of the vast- 

 ness of Tertiary time by a consideration of the mountain ranges which 

 had their birth and principal growth during the period. At the 

 beginning of the Tertiary, Switzerland was probably a compara- 

 tively flat plain where the lofty peaks of the Alps now stand ; and 

 the grandest mountains of the world, the Himalayas, were not 

 raised until about the middle of the Miocene. It is impossible to 

 state how long, in years, this great deformation required, but it is 

 evident that an almost inconceivable length of time was necessary. 



(3) Since the close of the Eocene the Grand Canyon region has 

 been elevated 11,000 feet; and the Colorado River has been able to 

 carve its way through limestone, sandstone, and granite to a depth 

 of 6500 feet. 



(4) The most approved method of estimating geological time is 

 by the maximum thickness of the sediments deposited during the 

 period (p. 417). Upon this basis the duration of the period has been 

 variously estimated at from 3,000,000 to 4,000,000 years, with the 

 former estimate more generally accepted than the latter. 



