part 1] THBOTTUII THE ANDES OF PEBF AND BOLIVIA. 55 



times, during the period represented by a gap in the succession 

 between Lower Devonian and Upper Avonian deposits. A resistant 

 horst appears thus to have been formed at a comparatively-early 

 date, bounding the Cordillera on the east, and this seems to have 

 been largely instrumental in confining the area of uplift to a narrow 

 strip of country along the Pacific coast. 



In Lower Devonian times the sea covered a large tract of the 

 country now forming the Bolivian Altaplanicie and the district 

 north and west of Lake Titicaca, fossiliferous deposits of this age 

 having been determined as far west as Taya Taya. 



During Upper Devonian and Lower Carboniferous times the 

 country appears to have been elevated above sea-level, but towards 

 the close of the Avonian period a further great transgression took 

 place, and marine deposits of Upper Carboniferous and Permo- 

 Carboniferous age were laid down over wide areas in the inter- 

 Andean region of Lake Titicaca. In some places they appear to 

 have extended still farther eastwards, and in the Viscachani Valley 

 are seen to lie unconfo nimbly on the older Palaeozoic rocks. 



Indication of the existence of a land-area on the west is afforded 

 by the presence of beds containing Coal-Measure plants on the 

 peninsula of Paracas, south of the port of Pisco. 



Marine Triassic beds are unknown in this area, and the complete 

 absence of deposits of later date than Permo-Carboniferous from 

 the Eastern Cordillera suggests that its elevation into a land-area 

 took place about this period. 



Further direct historical evidence, however, is wanting until 

 Jurassic times. Pocks of this age form the foundation on which 

 are situated the volcanic cones of the Western Cordillera ; but they 

 are largely concealed beneath a later covering of lava and tuff. 

 The Upper Jurassic zone of Northern Chile, extensively developed 

 in the Arica area, has been almost entirely stripped by denudation 

 from the underlying batholitic core, but its continuation has been 

 proved at more than one locality, and farther inland fossiliferous 

 beds of Middle Jurassic age are met with at Lagunillas. On the 

 upturned and steeply-dipping edges of the latter rest horizontal 

 transgressive Cretaceous deposits, giving clear proof of post- 

 Jurassic uplift. 



From evidence obtained farther south, we know that volcanic 

 activity had already broken out in Jurassic times, and further light 

 is thrown on the nature of the igneous rocks of the period, now 

 buried beneath later deposits, by the occurrence of numerous derived 

 pebbles in the basement conglomerate of the Cretaceous Series. 

 Among these are found rocks of plutonic origin which bear a 

 striking resemblance to those forming the core of the Western 

 Cordillera. 



It would appear, then, that the batholitic invasion took place, in 

 part at least, prior to or during the post-Jurassic uplift, and, 

 moreover, that this was sufficiently great to bring the deep-seated 

 core into the zone of active erosion before Cretaceous times. 



There is, in fact, no direct evidence that Cretaceous rocks were 



