Vol. 70.] THROUGH THE ANDES OF PERU A>~D BOLIVIA. 47 



whence the deposits laid down during these periods were derived, 

 cannot at present be definitely stated, though the general trend 

 of the mountain-ranges may, even at that early date, have been 

 indicated by an elevated chain running parallel to the Pacific coast. 

 During early Devonian times, the sea extended over the site of 

 the present Eastern Cordillera. Deposits with Lower Devonian 

 fossils are known from the Matto Grosso country on the east, 

 and, forming the flanks of the high Bolivian peaks, they continue 

 beneath the alluvial covering of the eastern Altaplanicie as far 

 west as the hamlet of Coniri. Owing to the absence of marine 

 Upper Devonian and Lower Carboniferous rocks, a period of land- 

 elevation appears to have set in at this time, and the present 

 Eastern Cordillera was first raised above sea - level — its rise 

 probably being accompanied by the intrusion of the granitic rock 

 which forms its core. Though doubtless subjected to repeated 

 oscillations, it has probably continued as a land-area down to the 

 present day. The sea once more extended its limits, and deposits 

 of marine Upper Carboniferous and Permo- Carboniferous age were 

 laid down over the Altaplanicie and the Titicaca district. 



With reference to the area under discussion, further historical 

 evidence is wanting until Upper Jurassic times. Marine deposits 

 of this age form the basement of the Western Cordillera, where 

 they are exposed in the deeply cut river- vallej^s and in the Morro 

 de Arica on the coast. Volcanic activity had already broken out, 

 i'or these Upper Jurassic sediments are interbedded with thick 

 sheets of andesite, which appear to have been formed as pillow- 

 lava in a shallow sea not far from a coast-line. 



The great development of so-called 'porphyritic conglomerate,' 

 probably of Cretaceous or Tertiary age, indicates a further con- 

 tinuance of these submarine volcanic conditions. During Cretaceous 

 times a wide transgression took place, and the sea penetrated far 

 into the district now occupied by the western Altaplanicie. 

 The fossiliferous limestones of the north, however, are here 

 represented by shallow-water deposits, consisting of red sand- 

 stones and gypsiferous marls ; and communication with the open 

 sea may have been partly cut off by the accumulation of volcanic 

 material on the west. 



In Middle Tertiary times intense folding and elevation took 

 place, and the mountain-chains of the Andes were raised high 

 above sea-level ; as most of the anticlines have their steepest limbs 

 -on the west, it would appear that the direction of the movement 

 was from the east, or towards the Pacific. Dioritic intrusions 

 broke through the newly-formed Cretaceous rocks along a line 

 running through Corocoro and Comanche, and the Jurassic sedi- 

 ments of the coastal region were penetrated by a great batholithic 

 mass of granodiorite, which now forms the deep-seated core of the 

 Western Cordillera. 



Volcanic activity continued with increasing force, and vast 

 sheets of rhyolitic lavas and tuffs were poured over the western 

 part of the country ; these were succeeded by the trachytic lavas 

 which build up the huge cone of Mount Taapaca, and the andesites 



