280 ME. J. A. DOraLAS 01s GEOLOGICAL SECTIOiN^S [vol. Ixxvii, 



In Southern Peru the succession of marine deposits recommences 

 with rocks of Upper Carboniferous or Permo-Carboniferous age. 

 These are best known from the region of Lake Titicaca, where 

 the}' are well- exposed on the Isthmus of Copacabana and the 

 Island of the Sun. The existence of a land-area on the west is 

 indicated by the occurrence of beds containing a flora characteristic 

 of the Coal Measures, on the peninsula of Paracas, south of the 

 port of Pisco. This is the onh" known record of fossiliferous 

 Palaeozoic rocks along the whole coast of Peru. 



Of earl}^ Mesozoic deposits in the South of Peru little appears 

 to be known, and I nowhere encountered beds of Triassic age. 

 Jurassic deposits, however, are of wide extent, and in the south of 

 the countr}^ "th^y form the foundation on which are situated the 

 giant volcanic cones of the Western Cordillera. 



Beds of Callovian and Oxfordian age are exposed along the coast 

 at Arica, Elsewhere the}^ are largely concealed by Tertiary and 

 recent volcanic material, and crop out only along the river- valleys. 

 In the latitude of Arequipa. beds of Inferior Oolite age are clearly 

 seen to have been folded and eroded before the deposition of the 

 overlying transgressive Cretaceous deposits. In Central Peru, 

 however, owing to the intensity of the subsequent Tertiary folding, 

 this post- Jurassic unlift is not so clearly displa3^ed. Fossiliferous 

 Liassic deposits are here of common occurrence, but no Middle or 

 Upper Jurassic rocks were encountered. 



The post-Jurassic uplift was accompanied by deep-seated in- 

 trusion, for numerous derived pebbles of plutonic origin were found 

 in the basement conglomerate of the Cretaceous sequence. During 

 Cretaceous times a wide marine transgression took place, and fossili- 

 ferous limestones chiefly of Albian and Cenomanian age are the 

 dominant feature of the Cordillera in Central Peru. 



When followed southwards these are seen to be replaced by red 

 gy jjsiferous sandstones and marls, ver}^ similar in appearance to our 

 British Triassic deposits, and obviously laid down under much the 

 same conditions, communication with the open sea having probably 

 been cut off: by an accumulation of volcanic material. Volcanic 

 activity had already broken out in Upper Jurassic times, as shown 

 by the interbedded pillow-lavas of the Morro de Arica. It was 

 renewed with increased vigour towards the close of the Cretaceous 

 Period, when vast accumulations of pyroclastic material (the so- 

 called ■ porphj^ritic conglomerate') were the chief product, and has 

 continued, more or less without inteiTuption, in the south of the 

 country down to the present day. Here recent volcanic cones, 

 many of them rising over 19,000 feet above sea -level, form the 

 do]mnant feature of the Western Cordillera, 



In Central Peru, although there is abundant proof of j)Ost- 

 Cretaceous volcanic activity, recent volcanoes are completely 

 w^anting. Evidence of deep-seated intrusion is, for this reason, 

 even better displayed here than in the south, where a thick mantle 

 of lava and tuff, in places as much as 12,000 feet thick, obscures 

 the underlying rocks. 



