826 Geological Society. 



February 4th.— Mr. G-. W. Lamplngh, F.R.S., President, 

 in the Chair 



The following communication was read: — 



• Geological Sections through the Andes of Peru and Bolivia : 

 II — From the Port of Mollendo to the Inambari River.' By 

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



The paper gives a description of a geological section across the 

 Andes of Southern Peru, from the port of Mollendo to the Inam- 

 bari River, a tributary of the Madre de Dios. 



The deflection of the Pacific coast-line of South America north 

 of Arica towards the north-west brings to light a zone of ancient 

 granite and gneiss comparable with the rocks of the coastal 

 Cordillera of Chile. These rocks are shown to be of ' alkaline ' 

 type, and are contrasted with the ' calcic ' granodiorites forming 

 the batholitic core of the Western Cordillera. It is suggested that 

 their formation preceded the uplift of the folded chains. 



The Jurassic zone of Northern Chile has been almost entirely 

 stripped from the underlying plutonic core, but its continuation 

 has been proved at more than one locality, and in the inter- Andean 

 region stronglv-folded fossiliferous beds of Bajocian age are found 

 beneath an unconformable Cretaceous series. The batholitic core 

 is shown to comprise at least three distinct phases of plutonic 

 intrusion, represented by granodiorites, diorites, and adamellites. 



The volcanic cones of the Western Cordillera have given rise to 

 an extensive series of lavas and tuffs comparable with the Mauri- 

 River Series of Bolivia. 



Cretaceous limestones here take the place of the red gypsiferous 

 sandstones farther south, and are transgressive on to Devonian 

 rocks. The latter contain abundant fossils of Lower Hamilton 

 age- 



The post- Cretaceous line of dioritic intrusion, formerly described 

 as running through Coro Coro and Comanche, once more appears 

 on the line of section. 



The Permo-Carboniferous fauna of Bolivia has not been dis- 

 covered in the district here described, but beds of similar lithological 

 character are found overlying fossiliferous limestones assigned to 

 the highest part of the Avonian sequence. The eastern flanks of 

 the Cordillera are composed of a great thickness of barren shales, 

 slates, phyllites, and mica-schists, the only fossils discovered being 

 graptolites of Llanvirn age. 



This area is further characterized by a well-marked ' alkaline.' 

 province of igneous rocks, comprising elDeolite-syenite-porphyry 

 and rocks closely related to laurvildte, ditroite, and durbachite. A 

 comparison is made with a section drawn through Northern Chile 

 and Bolivia (from Arica to the Bolivian Yungas), and an attempt 

 is made to reconstruct the history of the Cordillera. 



