16 
ORIGIN OF THE ANDES 
years ago I pointed out the surprising similarity in the geologic 
history of this whole region from the Isthmus of Panama to 
Graham Eand.^ 
So far as I have observed the Andes do not exhibit the Alpine 
type of structure, with recumbent folds and notable overthrusts, 
and this is in accordance with the observations of other students. 
I did not see overthrusting except at one locality near Potosi, 
Bolivia, and the movements of elevation and depression appear 
to have been mainly vertical, rather than the result of tangential 
forces. 
In a general way the Andean region can be divided into an 
Eastern Range and a Western Range, with a series of high valleys, 
or plateaus, lying between, this arrangement attaining its most 
typical development in southern Peru and in Bolivia. 
In Colombia the Andean system consists of three well-defined 
ranges, separated by valleys deep enough to be in the Tropical 
faunal and floral zone. These are the Eastern, Central, and 
Western Andes, separated by the Magdalena and Cauca valleys 
respectively. The Magdalena valley is nowhere less than 30 miles 
wide and suggests a garden. The Cauca valley is 20 to 30 miles 
wide, and the Central and Western Andes which bound it are di¬ 
rectly continued southward to form the Ecuador segment of the 
system. The Eastern Andes of Colombia appear to represent 
an additional unit trending more to the East of North and con¬ 
stitute the Cordillera de Bogota, which, in latitude 6° 30' North 
and longitude 73° West, forms a knot from which a northern 
branch dies out west of Lake Maracaibo, and an easternly branch, 
the Cordillera de Merida, which forms the watershed of Venezu¬ 
ela and which probably connects with the Antillean arc. Its rocks 
appear to be metamorphic, Cretacic and Tertiary sediments, and 
Tertiary intrusives. In Colombia the Western Andes do not 
reach the snow-line, but have no passes below 4900 feet. The 
Central Andes average about 4000 feet higher, have several snow- 
clad peaks, and no passes below 10,000 feet. The Eastern Andes 
have several snow-clad peaks. The Central and Western Andes 
so far as known consist of ancient igneous and metamorphic 
rocks and Mesozoic and Tertiary sediments. 
The Baudo Mountains, or Coast Range of Colombia, forming 
the divide between the Pacific and the valley of the Rio Atrato 
2 Bull. Geol. Soc. America, Vol XXIX, pp. 637-648, 1918. 
