[ 871 ] 

 XCV. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 472.] 



April 20th, 1921.— Mr. E. D. Oldham; F.E.S., President, 

 in the Chair. 

 HPHE following communications wore read : — 



1. ' Geological Sections through the Andes of Peru and Bolivia : 

 III — Prom Callao to the Eiver Perene.' By James Archibald 

 Douglas, M.A., B.Sc, F.G-.S. 



This paper is the third of a series by the writer dealing with the 

 geological structure of the Andes of Peru. 



The ancient rocks of the coastal Cordillera of the south are no 

 longer met with in Central Peru, and the zone of Mesozoic rocks 

 extends to the Pacific coast, which is here formed of shallow- water 

 deposits of Lower Cretaceous age. 



The granodioritic batholite which forms the core of the Andes 

 is encountered in the neighbourhood of Lima, and again almost at 

 the summit of the range. 



The western flanks of the Cordillera are characterized by a 

 great development of Cretaceous porphyritic agglomerate ; while 

 the normal calcareous facies is the dominant feature of the high- 

 level regions. 



The intensity of the Tertiary folding has obscured the effects of 

 the post-Jurassic uplift, previously shown to occur in the south, 

 and it is only on palseontological evidence that a break in the 

 sequence of Mesozoic deposits can be determined. 



Intrusions of andesite and dacite in the form of volcanic necks 

 are of common occurrence ; but there is no trace of recent volcanic 

 activity in this area. 



The fossiliferous Devonian and Permo-Carboniferous deposits of 

 the Titicaca district are not continued into the region here described. 

 The rocks of Palaeozoic aspect which form the eastern flanks of the 

 Cordillera are, for the greater part, unf ossilif erous, and have largely 

 been converted into phyllites and mica-schists, penetrated by 

 granite, which has also shared in the metamorphism. On the Eio 

 Perene another and much bigger mass of red granite is met with. 

 This is essentially a rock of ' alkaline ' character, as distinct from 

 the ' calcic ' granodiorites of the Cordillera. It is suggested that 

 its origin, like that of a similar rock in the coastal Cordillera of 

 the south, dates from a very early period, antecedent to the uplift 

 of the mountain -ranges. 



2. 'The Valentian Series.' By Prof. Owen Thomas Jones, 

 M.A., D.Sc, F.G.S. 



Owing to the great variation in the lithological and fauna 1 

 characters of the Valentian rocks of different districts, their 

 classification and correlation has always presented difficulty. ^ ith 

 the hope of clearing up some of the ambiguities in the classifi- 

 cations in current use, the history of the nomenclature from the 



