590 HISTORICAL GEOLOGY 



The Himalayas were being eroded during the Pliocene; and 

 thousands of feet of sandstones and conglomerates were deposited 

 at their foot before its close* some of which, however, were laid down 

 in the later Miocene. In South America the coasts of Argentina 

 and Patagonia were submerged, and the last upheaval of the south- 

 ern Andes was accomplished at this time. 



REFERENCES FOR PALEOGEOGRAPHY 



Blackwelder, E., — Regional Geology of the United States of North America, pp. 39-46. 

 Osborn, H. F., — Age of Mammals, pp. 84-93; 182-184; 244-246; 304-306. 

 Schuchert, Chas., — P 'ale v geography of North America: Bull. Geol. Soc. America, 



Vol. 20, 1910, pp. 597-600. 

 Willis and Salisbury, — Outlines of Geologic History, pp. 200-264. 



Life of the Tertiary 



Rise of Mammals. — In the present imperfect state of our knowl- 

 edge of the life at the beginning of the Cenozoic, it is, perhaps, even 

 more difficult to account for the presence of highly developed mam- 

 mals, as soon as the reptiles became extinct, than to account for the 

 disappearance of the latter. 



It is improbable that the mammals on the earth to-day were de- 

 scended from any of the mammals whose remains have been found 

 in the later Mesozoic rocks. Indeed, it is even doubted that the 

 true (Eutheria) mammals were descended from the marsupials 

 (Metatheria) mammals. Two theories are offered to explain 

 their sudden appearance. (1) The first postulates their existence in 

 some isolated country, in the Arctics whose climate was not cold at 

 that time, or elsewhere, for a long period of time during which they 

 had been developing along different lines, but from which they were 

 prevented from spreading because of some barrier to their move- 

 ment, either water or mountains. When this barrier was removed, 

 the mammals deployed over the world, and finding the new conditions 

 favorable for their existence, rapidly took the place in nature formerly 

 occupied by the reptiles. (2) The second theory (p. 549) is based 

 upon the supposed existence of mammals on the uplands of the 

 Mesozoic. Since practically all of our knowledge of the life of that 

 period is obtained from coastal swamp and delta deposits in which 

 almost no forms of life are found except those which frequented 

 marshes, little is known of the life of the higher and more extensive 



