18 
ORIGIN OF THE ANDES 
developed and it extends from here southward in an unbroken 
mass from the knot of Vilcanota to Cape Horn. Lying east of the 
Western Range, and extending from Vilcanota almost to latitude 
22° south, is the Titicaca-Poopo basin, or the altaplanicie of Bol¬ 
ivia. This is bounded on the east by the Cordillera Oriental of 
southern Peru (Los Andes of Paz Soldan), and these continue 
southward, as the Cordillera Real of Bolivia, fanning out south 
of Tres Cruces to form the broken country constituting the De¬ 
partments of Cochabamba and western Chuquisaca, and extending 
eastward as far as Santa Cruz de la Sierra, nearly 500 miles from 
the Pacific coast. To the east they gradually decline and disappear 
beneath the Amazon and Paraguay plains. To the south they 
decline and virtually disappear in southern Bolivia. Peru has 
a low Coast Range in the northern and southern parts of the 
state largely made up of granitic rocks (diorite and granodiorite). 
In southern Peru and Bolivia the Eastern Andes consist largely 
of Siluric and Devonic shales. They contain some marine Carbonic 
beds and shallow-water Late Cretacic deposits as well as Pliocene 
continental deposits, together with Pliocene granitic batholiths and 
porphyritic rocks. In their eastern border region they contain 
some Ordovicic formations, and toward Santa Cruz a northward 
continuation of the Permian tillites of Argentina. The rocks 
of the Altaplanicie are Devonic, Carbonic, Cretacic, and Pliocene 
to Recent in age. The Western Range, so far as known, consists 
entirely of Mesozoic sediments and very late Tertiary volcanics, 
and they retain this character southward to the Antarctic, but 
with some continental Rhaetic beds toward the south. 
Chile has one Andean system (Los Andes), a direct continua¬ 
tion of the Maritime, or Cordillera Occidental of Peru and Bolivia. 
It is a single, broad, montane zone, the watershed of which to the 
southward has been shifted eastward by glacial action, as noted 
independently by P. Quensel ^ and B. Willis with the conse¬ 
quence that several rivers pass westward in canons amid the high 
peaks along the crest, and flow into the Pacific. From about 
latitude 30'’ to the Tropic of Capricorn Los Andes are said to 
show a zone of faulting separating them from the graben lying 
3 On the Influence of the Ice Age on the Continental Watershed of Patagonia: 
Bull. Geol. Inst. Upsala, Vol. IX, 1910. 
4 Physiography of the Cordillera de los Andes between latitudes 39“ and 44" 
South: Compte rendu, XII Cong Geol. Int. pp. 733-7S6, 1914. 
