Vol. 70.] THROUGH THE ANDES OF PERU AND BOLIVIA. 23 



pyroxene, displaying lower interference-colours than the augite 

 and straight extinction. The cross-sections are ahnost square, and 

 the prism-faces are hardly developed. 



Magnetite is abundant, especially as minute granules in the 

 ground-mass. One or two rounded xenocrysts of hornblende 

 occur, with strong corrosion-borders. 



The ground-mass exhibits hyalopilitic structure, consisting of 

 brown isotropic glass crowded with parallel microlites of felspar 

 and granules of magnetite. 



(iij The Altaplanicie or High-Level Bolivian Plateau. 



Between the Western and Eastern Cordilleras lies the high-level 

 Bolivian plateau known as the Altaplanicie. Though broken up 

 by numerous minor ranges, the general surface of the country has 

 a mean elevation of about 13,000 feet. The drainage of nearly 

 the whole of this vast area is effected by the Desaguadero and 

 its tributaries. This river flows southwards from Lake Titicaca 

 into Lake Poopoo, from which there is no visible outlet ; its fall, 

 moreover, is so slight, that in times of great rainfall the surrounding 

 country becomes flooded for hundreds of square miles, and it is 

 difficult to say in which direction the water is flowing. It may 

 be spoken of, therefore, as constituting an internal system of 

 drainage, distinct from the rivers of the Pacific and Atlantic 

 slopes. The extreme eastern limit of the Altaplanicie, however, 

 is drained by the river of La Paz, which cuts through the Eastern 

 Cordillera to join the Atlantic system. 



Though largely concealed by wide tracts of alluvium, the base- 

 ment of the eastern part of the Altaplanicie is seen to be formed 

 of strongly-folded Palaeozoic sediments, which are overlain on 

 the west by transgressive Cretaceous deposits ; these, in turn, are 

 covered by extensive sheets of volcanic material, which are probably 

 of post-Miocene age. To the last-mentioned deposits I shall refer 

 as the ' Mauri Volcanic Series,' from their extreme development in 

 the western part of the area, which is drained by the Mauri River. 



0) The Mauri Volcanic Series. (PL IV, fig. 2.) 



Having passed through the Western Cordillera and crossed the 

 Bolivian frontier, one notes that the whole surface of the country 

 is still continuously covered by extensive deposits of volcanic 

 origin ; and, where these are cut by river-erosion, the valleys are 

 flanked by monotonous flat-topped hills, which impart a very 

 peculiar and characteristic aspect to the landscape. Though 

 apparently a plane surface, in many cases it presents examples of 

 honeycomb weathering on an immense scale ; and, crossing the hill- 

 tops, one may wander for miles through a perfect labyrinth of 

 natural rooms and corridors, which have an extraordinarily artificial 

 appearance. This type of country continues down the Cano and 

 Mauri Rivers for a distance of some 60 miles. Although these rocks 



