e deposits have been found either
in the eastern Andes or in the plains of Bolivia, but freshwater beds
of Tertiary and later date occupy a wide area. The recent deposits,
which cover so large a part of the depression between the Eastern and
the Western Cordillera, appear to be partly of torrential origin, like
the talus-fans at the foot of mountain ranges in other dry regions;
but Lakes Titicaca and Pampa Aullaguas (Poopo) were undoubtedly at one
time rather more extensive than they are to-day. The volcanoes of
Bolivia lie almost entirely in the Western Cordillera--the great
summits of the eastern range, such as Illimani and Sorata, being
formed of Palaeozoic rocks with granitic and other intrusions. The
gold, silver and tin of Bolivia occur chiefly in the Palaeozoic rocks
of the eastern ranges. The copper belongs mostly to the red sandstone
series.

_Climate._--Bolivia lies wholly within the torrid zone, and variations
in temperature are therefore due to elevation, mountain barriers and
prevailing winds. The country possesses every gradation of temperature,
from that of the tropical lowlands to the Arctic cold of the snow-capped
peaks directly above. This vertical arrangement of climatic zones is
modified to some extent (less than in Argentina) by varying rainfall
conditions, which are governed by the high mountain ranges crossing one
corner of the republic, and also by the prevailing winds. The trade
winds give to S. Bolivia a wet and dry season similar to that of N.
Argentina. Farther north, and east of the Cordillera Oriental, rains
fall throughout the year, though the summer months (November-March) are
usually described as the rainy season. On the west side of the
Cordillera, which extracts the moisture from the prevailing easterly
winds, the elevated plateaus have a limited rainfall in the north, which
diminishes toward the south until the surface becomes absolutely barren.
Brief and furious rain-storms sometimes sweep the northern plateau, but
these are not frequent and occur during a short season only. Electrical
wind storms are frequent in these high altitudes.

Bolivia has a wide range of temperature between places of the same
latitude. The natives designate the Bolivian climatic zones as
_yungas, valle_ or _medio yungas, cabezera de valle, puna_ and _puna
brava_. The _yungas_ comprises all the lowlands and the mountain
valleys up to an elevation of 5000 ft. The temperatur