38 FISHES OF WESTERN SOUTH AMERICA 
Coming northward through Chile we encounter a single range of mountains 
most of the way, and elevations increasing from 3000 feet in the South to passes 
of 17000-18000 feet in middle Chile, and Aconcagua at 23000 feet. In the latitude 
of northern Chile and southern Bolivia a new range begins to rise out of the Argen- 
tine plain and run in a parallel line. The Chilean range becomes the Western 
Cordillera, and the new one the Eastern. In southern Bolivia, the valley between 
these cordilleras is raised to more than 12000 feet by an immense volcanic area 
which blocks the exit, and holds back the waters of the Titicaca-Poop6é basin from 
creating an exit southward. The western range on the Bolivian-Chilean border 
is largely obliterated by volcanic formations, while the eastern rises higher and 
higher to a crest of 22000 feet north of La Paz, and the great Sierra Nevada over- 
looking Lake Titicaca. North of that lake the two cordilleras draw nearer together 
until the Nudo de Vilcanota is reached, tying them together by transverse ridges 
running East and West, elevation above 14000 feet. 
Departing northward from this high intercordilleran bridge, the ranges swing 
more northwestward, continuing at great elevations, and separated by great gorges. 
From here we may recognize three ranges. The westernmost, the Occidental, is 
separated from the Central by the Rio Apurimac, and the Eastern from the Central 
Cordilleras by the Vileanota and the Urubamba rivers. Unlike the southern inter- 
cordilleran basin, these of the middle section of Peru have made or retained their 
water-gaps to the outside, draining to the Amazon. The three cordilleras approach 
one another again and form a third transverse bridge, the Nudo de Pasco, draining 
north and south. Northward of Cerro de Pasco the three ranges are separated 
as before, by the rivers Maranon and Huallaga, not to reunite within our territory, 
but to be bridged across further northward, at the Nudo de Ampato of Ecuador. 
At about the eastern extremity of the Nudo de Pasco a chain of mountains 
strikes eastward toward the Amazon valley, contrary to the usual trend of the 
ranges elsewhere. This is the Cadena de los Cerros de la Sal. 
(4) GroGRAPHIC REGIONS OF WeEsT-CENTRAL SOUTH AMERICA 
With the physical background described above, the terrain is seen to be of 
a maximum relief, with its axis transverse to the course of prevailing winds, and, 
although wholly within the Torrid Zone, should have given rise to distinct bio- 
geographical provinces. The accompanying diagram (Table I) is an attempt to 
analyze the geological, or rather, physiographical basis for such provinces. — First 
of all we must recognize the north-south lines of demarcation mentioned in all 
the books, the Western Cordillera and the Oriental, and the three resulting general 
regions, the Coast, the Sierra, and the Montana trending northwest-southeastward. 
In the fifteen hundred miles here considered, extending nearly from the Equator 
to the Tropic of Capricorn, latitude becomes a factor, and at the same time two 
great physical barriers are set up in an east-west direction, the Nudo de Pasco 
and the Nudo de Vilcanota mentioned above, the former above 14000 feet, the 
latter nearly as much. 
The coastal provinces are not sharply divided, yet are sufficiently unlike that 
