Reports & Proceedings — Geological Society of London. 573 



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 

 Hiver. (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 I'emarkably 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 moi-e or less isolated, snow-capped, dormant, and extinct 

 volcanoes, attaining heights of 19,000 to 20,000 feet. The enormous 

 amount of volcanic material emitted from these cones has almost 

 completely concealed the underlying rocks. 



The lavas can be resolved into three main groups, characterized by 

 their dominant ferromagnesian mineral, succeeding one another in 

 age according to a law of increasing basicity: («) acid rhyolites 

 and tuffs with biotite ; {h) trachytes and trachy-andesites with 

 hornblende, typically developed in the district of Mount Taapaca; 

 {c) andesites and basalts with pyroxenes, forming the cones of Mounts 

 Tacora and Chupiquiiia. 



(2) The western part of the high-level Bolivian plateau, or 

 'Altaplanicie', is almost entirely covered by vast horizontal sheets 

 of volcanic ash, tujff, and pumiceous lava, described as the Mauri 

 Yolcanic Series. These rocks have often the appearance of 'trass', 

 and it is suggested that they have been formed in large part as 

 subaqueous deposits. The occurrence in an interbedded layer of 

 gravel of a fragment of a jaw of ^ JVesodon', almost identical in 

 appearance with specimens from the Miocene beds of Santa Cruz, 

 affords the only clue for an estimation of their age. They are overlain 

 on the east by gravel deposits of the Desaguadero Hiver, the 

 highest terrace of which was found to contain remains of Mastodon, 

 Megatherium, Scelidothernwi, and other Pleistocene vertebrates. 



Prom beneath these superficial deposits crops out a series of un- 

 fossiliferous red and chocolate- coloured sandstones and conglomerates. 

 After comparison with other districts on the north, these are divided 

 into two groups — a younger gypsiferous sandstone and marl series 

 of Cretaceous age, broken through by a line of dioritic intrusions and 

 resting with pseudoconformity on an older Permo-Carboniferous 

 group. The latter ends abruptly along a fault-line against vertical 



