Physical geography. By Mary Somerville ...

110 PH YSICAL GEOGRAPIHY. CIIAP. IX.
hollows, in which so many horses and other animals perish, that the
ground smells of musk, an odour peculiar to many South American
quadrupeds. From the flatness of the country too, the waters of
some affluents of the Orinoco are driven backwards by the floods of
that river, especially when aided by the wind, and form temporary
lakes. When the waters subside, these steppes, manured by the
sediment, are mantled with verdure, and produce ananas, with occasional groups of fan palm-trees, and mimosas skirt the rivers. When
the dry weather returns, the grass is burnt to powder; the air is
filled with dust raised by currents occasioned by difference of temperature, even where there is no wind. If by any accident a spark
of fire falls on the scorched plains, a conflagration spreads from river
to river, destroying every animal, and leaves the clayey soil sterile
for years, till vicissitudes of weather crumble the brick-like surface
into earth.
The Llanos lie between the equator and the Tropic of Cancer;
the mean annual temperature is about 84~ of Fahrenheit The heat
is most intense during the rainy season, when tremendous thunderstorms are of common occurrence.
GEOLOGY OF SOUTH AMERICA.
THE most remarkable circumstance in the geological features of
the South American continent is the vast development of volcanic
force, which is confined to the chain of the Andes, where it has
acquired a considerable breadth, as in the Peru-Bolivian portion, to
the part nearest the sea-coast. It would be wrong, however, to say
that there are no traces of modern volcanic action at a great distance
from the sea: it is one of those theories which recent discoveries
in both continents have proved the fallacy of. The volcanic vents
occur in the Andes in linear groups: the most southern of these is
that of Chile, extending from the latitude of Chiloe to that of Santiago, 42~ to 33~ S.: in this space exist five well-authenticated
1 Mr. Pentland found a very perfect volcanic crater, with well-marked
currents of lava issuing from it-a rare occurrence in the higher craters
of the Andes-not far from San Pedro de Cacha in the valley of the Yucay
(lat. 14~ 12', long. 71~ 15/ W., and at an elevation of 12,000 feet), near to
the ruins of the Temple of the Inga Viracocha, a monument and a locality
celebrated in Peruvian legend, the nearest point of the sea-coast being 175
miles distant. It is probable that some of the most celebrated mining districts of Alto Peru-Potosi, for instance, situated in a porphyry —have
been upheaved at a very recent period. Modern volcanic rocks are not
wanting in the valley of the Desaguadero; volcanic conglomerates exist in
the deep ravines round the city of La Paz, lat. 16~ 39'; and the mountain
of Litanias, which furnishes the building-stone for that Bolivian city (lat
16~ 42/, long. 68~ 19-/), is composed of a most perfect trachyte, and rises
to a height of 14,500 feet above, and at a distance of 160 miles from the
Piacific.