n side of this depression there are red sandstones with
coal-seams, possibly Tertiary; the floor and the eastern side consist
chiefly of ancient crystalline and schistose rocks. The Central
Cordillera is the direct continuation of the Eastern Cordillera of
Ecuador, and is formed chiefly of gneiss and other crystalline rocks,
but sedimentary deposits of Cretaceous age also occur. Finally the
Eastern branch, known as the Cordillera of Bogota, is composed almost
entirely of Cretaceous beds thrown into a series of regular
anticlinals and synclinals similar to those of the Jura Mountains. The
older rocks occasionally appear in the centre of the anticlinals. In
all these branches of the Andes the folds run approximately in the
direction of the chains, but the Sierra de Santa Marta appears to
belong to a totally distinct system of folding, the direction of the
folds being from west to east, bending gradually towards the
south-east. Although volcanoes are by no means absent, they are much
less important than in Ecuador, and their products take a far smaller
share in the formation of the Andes. In Ecuador the depression between
the Eastern and Western Cordilleras is almost entirely filled with
modern lavas and agglomerates; in Colombia the corresponding Cauca
depression is almost free from such deposits. In the Central
Cordillera volcanoes extend to about 5 deg. N.; in the Western Cordillera
they barely enter within the limits of Colombia; in the Cordillera of
Bogota they are entirely absent.[1]

_Climate._--Were it not for the high altitudes of western Colombia,
high temperatures would prevail over the whole country, except where
modified by the north-east trade winds and the cold ocean current
which sweeps up the western coast. The elevated plateaus and summits
of the Andes are responsible, however, for many important and profound
modifications in climate, not only in respect to the lower
temperatures of the higher elevations, but also in respect to the
higher temperatures of the sheltered lowland valleys and the varying
climatic conditions of the neighbouring plains. The republic lies
almost wholly within the north torrid zone, a comparatively small part
of the forested Amazonian plain extending beyond the equator into the
south torrid zone. The great Andean barrier which crosses the republic
from the south to north acts as a condenser to the prevailing east