54 



MB. J. A. DOUGLAS ON GEOLOGICAL SECTIONS [vol. lxXVl, 



modified by assimilation of silica during its passage through the 

 Devonian quartzites. 



The Devonian series, which crops out in the neighbourhood of 

 Coniri in the south, and is continued beneath the Eastern Altaplanicie 

 of the Viacha district to form the flanks of the Eastern Cordillera, 

 is again exposed here between Maravillas and Las Huertas. The 

 beds are highly f ossiferous, the fauna (of Middle Devonian age) 

 being equivalent to that of the Gonularia Shales of Steinmann, 

 which have been correlated Avith the Lower Hamilton Shales of 

 North America. 



In the district west of Puno the remnants of a once-extensive 

 sheet of basaltic lava, which appears to have been extruded in the 

 form of a fissure-eruption, are found capping a series of buttes in 

 the neighbourhood of Vilque. 



The former extension of Lake Titicaca to the north is further 

 shown by the wide alluvial tracts of the Juliaca area. 



The Pernio- Carboniferous fauna of Bolivia has not been 

 discovered in the district now described; but rocks of similar litho- 

 logical character, and probably of the same age, are met with on 

 the shores of the lake near Puno. and again, east of Tirapata, 

 where they overlie Devonian beds. 



Fossiliferous beds of Lower Carboniferous age, representing the 

 highest part of the Avonian sequence, occur near Macusani, where 

 they lie unconformably on a much-faulted and folded series of 

 older Palasozoic rocks. 



The granitic core of the Bolivian Cordillera does not appear to 

 extend into Peru, or at least is unrepresented along the line of 

 section ; an interesting series of alkaline igneous rocks, however, 

 many of them nepheline-bearing, is met with in the Carabaya 

 district near Ollachea. These, locally, show signs of intense 

 dynamic crushing, and are associated with andalusite-mica-schists, 

 which have the appearance of being highly-altered Palaeozoic 

 sediments. 



The Palaeozoic shales of the Caupolican district in Bolivia, de- 

 scribed by Dr. J. W. Evans, are again met with in the Inambari 

 district of Peru, and in the valley of the Chaquimayo Eiver yield 

 well-preserved graptolites of Llanvirn age. 



An account having been given of the characters and distribution 

 of the rocks which build up the Cordilleras of Southern Peru, it 

 remains to apply the evidence thus accumulated towards elucidating 

 the history of the folded chains. 



Ancient igneous rocks, probably of Archaean age, fringe the coast 

 at Mollendo ; but, just as is the case in Bolivia, Palaeozoic sediments 

 older than the Devonian are confined to the eastern flanks of the 

 Andes drained by the head-waters of the Amazon. After having 

 been penetrated by plutonic intrusions, these rocks underwent a 

 period of orogenic movement, and were subjected to intense dynamic 

 metamorphism. 



It is suggested that this folding took place prior to Carboniferous 



