56 ME. J. A. DOL'ULAS OX GEOLOGICAL SECTIONS [vol. lxxvi, 



ever laid down over the coastal region in this area, and the deposits 

 of this age in the inter- Andean region may have been formed in a 

 bay or land-locked arm of the northern sea. Thus we may seek to 

 explain the gradual transition from the widespread fossiliferous 

 limestones of the north into the red sandstones and gypsiferous 

 marls of the south. The Cretaceous rocks shared in the folding 

 which took place during Tertiary times, and, as is also the case in 

 Bolivia, have been penetrated by later intrusions of diorite. The 

 fact that these marine deposits are now found at altitudes over 

 13,000 feet above sea-level gives some idea of the magnitude of 

 the Tertiary uplift. 



Volcanic activity continued with renewed vigour, and the giant 

 cones of the Western Cordillera were gradually built up to heights 

 even greater than they possess at the present day. Some of them, 

 although dormant or extinct, still retain, like El Misti, a well- 

 preserved crater ; others, like Chachani, have been dissected by 

 long-continued erosion, and retain little of their original form. 



With regard to the date and extent of recent uplift there is 

 conflicting evidence. It has already been argued that the presence 

 of an extinct mammalian fauna at 13,000 feet in the Desaguadero 

 district of Bolivia can only be explained by the assumption of a 

 considerable post-Pleistocene elevation, and it appears likely that 

 the gently-sloping rock-platform fringing the coast of Southern 

 Peru was formed at the same time. A still more recent elevation 

 is indicated by the raised beach at Mollendo ; but there appears to 

 be no conclusive proof that an} r extensive movement has occurred 

 during the period of human history. 



It thus becomes clear that the area now occupied by the folded 

 chains of the Andes has been subjected to repeated oscillations : 

 periods of uplift, marked by folding and erosion, having alternated 

 with periods of submergence, marked by the deposition of trans- 

 gressive formations. 



The general history of the Cordillera may be tabulated as 

 follows : — 



Deposition of Older Pakeozoic rocks, up to and including the Lower-Middle 



Devonian. 

 Upper Devonian uplift. 

 Permo- Carboniferous transgression. 

 Permo-Triassic uplift. 

 Jurassic transgression. 



Post-Jurassic uplift ; accompanied by batholitic invasion. 

 Cretaceous transgression. 

 Mid-Tertiary uplift (amounting to at least 14,000 feet), accompanied by 



a great outburst of volcanic activity. 

 Post- Pleistocene uplift (probably amounting to about 1500 feet). 



It may be as well to postpone any detailed discussion on the 

 nature of the folding, until the description of the structure of the 

 more northern districts of Peru has been completed. A brief 



