Societies and Academies

Abstract

LONDON. Geological Society, February 4. Mr. G. W. Lamplugh, president, in the chair.—J. A. Douglas: Geological sections through the Andes of Peru and Bolivia: ii., from the Port of Mollendo to the Inambari River. The paper gives a description of a geological section across the Andes of southern Peru, from the port of Mollendo to the Inambari River, a tributary of the Madie 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 srneiss 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 strongly 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 discovered in the district here described.