224 Geological Society. 



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 : — 



(i) Acid rhyolites and tuffs with biotite. 



(ii) Trachytes and trachy-andesites with hornblende, typically developed 

 in the district of Mount Taapaca, 

 . (hi) 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, tuff, and pumiceous lava, described as the Mauri 

 Volcanic 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 ' Nesodon," almost identical in 

 ap>pearance 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- depo sits of the Desaguadero 

 River, the highest terrace of which was found to contain remains of 

 Mastodon, Megatherium, Scelidotherium, and other Pleistocene 

 vertebrates. 



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

 fossiliferous red and chocolate-coloured sandstones and conglome- 

 rates. 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 pseudoconforinity on an older Permo- 

 Carboniferous group. The latter ends abruptly along a fault-line 

 against vertical shales and quartzites, containing a few characteristic 

 Lower Devonian fossils. These Devonian beds, though much con- 

 cealed by alluvial plains, form the basement of the eastern part of 

 the ' Altaplanicie.' 



The Carboniferous formation is nowhere exposed along the line 

 of section ; but an account is given of its development in the region 

 of Lake Titicaca, where the limestones contain an Upper Carbo- 

 niferous or Permo- Carboniferous marine fauna. 



A short discussion is entered into on the theory of the recent 

 elevation of the Andes and the origin of Lake Titicaca. 



(3) The Eastern Cordillera rises to heights of over 22,000 feet, 

 being composed chiefly of steeply- dipping Devonian slates and 

 quartzites, though many of the unfossiliferous black slates and 

 greywackes of the eastern slopes most probably may be referred to 

 the Silurian, or even to an older formation. Outcrops of granite are 

 of rare occurrence along the line of section, although there is reason 

 to. suppose that this rock forms the core of most of the high peaks. 

 As marine Lower Carboniferous and LTpper Devonian rocks are 

 absent from this district, it is suggested that the granitic core was 

 intruded during a period of land-elevation at this time. 



