Vol. 70.] THEOrCrH THE A>~DES OF PEEL" ASH BOLIVIA. 43 



I am not prepared to deny the possibility of a slight recent 

 elevation on these grounds, though climatal changes and the former 

 presence of the lake might have completely altered the appearance 

 of the landscape. The amount of depression required to reproduce 

 suitable conditions for the existence of the city would not appear 

 to be very great, if comparison is made with the present city 

 of La Paz. It is. however, necessary to suppose a considerable 

 elevation since Pleistocene times, to account for the presence of 

 a large mammalian fauna at an altitude now almost devoid of" 

 vegetation. That this elevation amounted to over 13,000 feet, 

 as suggested by Sundt, appears to be entirely unsupported by 

 facts. The determination of the maximum altitude at which such 

 life could have flourished must be left in the hands of biologists* 

 to whom we must also appeal for further light on the possibility 

 of recent migration of marine life to the fresh waters of Lake 

 Titicaca. 



If the lake once formed an arm of the sea, its origin, according 

 to my observations, must date at least as far back as Miocene 

 times, during which the greater part of the elevation of the Andes- 

 probably took place. It would then have formed a lagoon between 

 the mainland and the rising Western Cordillera, on which alreadv 

 a terrestrial fauna and flora were in existence. The movement 

 appears to have been continued since Pleistocene times, and possibly 

 within the period of human existence, though it must be pointed 

 out that it is hard to reconcile any theory of recent elevation with 

 the geological evidence of the former extension of the glaciers 

 of the Cordillera in this latitude. 1 



(iii) The Eastern Cordillera and the Amazon Slopes. 



(a) The Palaeozoic Sedimentary Rocks. 



The Bolivian Altaplanicie is bounded on the east by the great 

 mountain-range usually known as the ' Cordillera Real." Its snow- 

 capped peaks (Illimani. Mururata, Caca-Aca or Huaina Potosi, 

 Illampu. etc.) rise to still greater heights than those of the Western 

 Cordillera, and are built up entirely by Palaeozoic sediments with 

 a granitic core. All traces of recent volcanic activity are com- 

 pletely absent. 



Our line of section, as stated above (p. 5), was continued 

 from La Paz eastwards across the Cordillera Keal down to the 

 hacienda of Mururata. A remarkably wet season at the time of 

 our visit to this locality, with practically continuous tropical rain 

 on the eastern slopes, made detailed work almost impossible. The 

 description of this portion of the section is therefore necessarily 

 somewhat incomplete, and I do not attempt to give more than 

 a rough outline of its geological features. 



1 At the present day, the glaciers of Mount Illimani extend down to about 

 15,750 feet. 



