[ 223 ] 

 XXIV. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from vol. xxvi. p. 548.] 



November 5th, 1913. — Dr. Aubrey Strahan, F.R.S., President ; 

 in the Chair. 



r piIE following communication was read : — 



' Geological Sections through the Andes of Peru and Bolivia/ 

 By James Archibald Douglas, M.A., B.Sc, F.G.S. 



This paper deals with the geological structure of the South 

 American Andes, as illustrated by a horizontal section drawn from 

 the port of Arica in the extreme north of Chile (formerly Peruvian 

 territory) across the mountain-ranges or 'Cordilleras' to the forested 

 region of the Amazon slopes, in the district known as the Bolivian 

 'Yungas,' following the route of the new Arica-La Paz railway, 

 which was under course of construction at the time of the author's 

 visit. 



It is the partial result of two years' geological exploration in 

 Peru, undertaken on behalf of Mr.*W. e! Balston, F.Gr.S., for the 

 Oxford University Museum. After a description of the general 

 physiography of the Peruvian Andes, the topographical features of 

 the country traversed by the railway are discussed in some detail. 



Its geological structure is then described under three headings : — 



(1) The Mesozoic sediments of the coastal region with their contempo- 



raneous igneous rocks, the intruded core of granodiorite, and the 

 overlying recent volcanic rocks of the Western Cordillera. 



(2) The volcanic rocks of the Mauri River, the Mesozoic and Palaeozoic 



sediments of the ' Altaplanicie ' and the Titicaca district ; the line of 

 dioritic intrusions, and the Pleistocene gravels of the Desaguadero 

 River. 



(3) The Palaeozoic rocks and granitic core of the Eastern Cordillera and 



the Amazon slopes. 



(1) The Mesozoic stratified rocks are well exposed in the 'Morro 

 de Arica,' where fossils occur which indicate an Upper Jurassic 

 (Callovian) age. They are interbedded with thick sheets of basic 

 enstatite-andesite, showing well marked ' pillow '-structure ; this 

 rock is remarkably fresh, and free from albitization. 



Similar stratified rocks are traced up the Llutah and Palca river- 

 valleys. In the former they are penetrated by a thick intrusion of 

 quartz-hypersthene-norite, which, it is suggested, is the plutonic 

 equivalent of the pillow-lava of the coast. 



The erosion of the river- valleys that has brought to light the 

 Jurassic sediments has also laid bare the underlying plutonic mass 

 of granodiorite, which may be regarded as the deep-seated core 

 of the Western ' Cordillera.' This plutonic mass appears to have 

 been intruded in the form of a batholite in post- Cretaceous times. 



The Western Cordillera is essentially a volcanic range, formed 

 of numerous more or less isolated, snow-capped, dormant and 

 extinct volcanoes, attaining heights of 19,000 to 20.000 feet. The 



