GOLD AND SILVER DEPOSITS — LINDGREN. 157 



near the Colombian boundary has been worked from the seventeenth 

 to the twentieth century and the deposit is contained in Tertiary 

 andesite. 1 



THE SOUTH AMERICAN CORDILLEKA. 



General features. — From Cape Horn to Colombia the South Ameri- 

 can Cordillera or Andes forms a continuous chain closely following 

 the coast. Its width ranges from 100 miles near Magellan Strait to 

 500 miles in the latitude of Bolivia. North of Bolivia it again con- 

 tracts to a width of about 300 miles. It is thus, considering its 

 length, a narrow mountain chain, but nevertheless generally made up 

 of three longitudinal units. In the north they are known as the 

 eastern, central, and western cordillera or by other local names. In 

 Peru they are spoken of as the Coast, Sierra, and Montana regions, 

 the last being the eastern slope of the Andes. In the south there are 

 locally four subdivisions — the coast range, the western and the east- 

 ern cordillera, and the pre-Cordilleras or front ranges. Between the 

 eastern and western range lies, in Bolivia, the high plateau or "Alti- 

 planicie." In places, as in northern Chile and Bolivia, the western 

 range itself partakes of the character of a plateau. 2 



Two ranges stand out by reason of great altitudes, both being rich 

 in mineral deposits. One is the Sierra Blanca of northern Peru, in 

 the western cordillera; the other is the Cordillera Real of eastern 

 Bolivia which includes the high summits of Sorata and Illimani. 



III. GEOLOGY OF SOUTH AMERICA. 

 INTRODUCTION. 



It will be admitted that it is no easy task to condense in a few 

 pages what is known of the geology of a continent ; and for the im- 

 perfections and omissions in this account I must therefore ask the 

 indulgence of the reader. 



Broadly speaking, the most prominent formations of the Andes are 

 the Cretaceous sediments, which extend almost without interruption 

 from northern Colombia to Tierra del Fuego. Of scarcely less im- 

 portance though smaller in area, are the Tertiary and Recent lava 

 flows and the intrusive masses of early Tertiary age. No great in- 

 trusions of Cretaceous age appear to exist in South America, al- 

 though the volcanic activity in the Jurassic and Cretaceous was in- 

 tense and yielded heavy masses of lava flows intercalated in these 

 formations. 



1 Malcolm Maclaren (Gold, London. 1908). 



2 Isaiah Bowman : Physiography of the Central Andes (American Journal of Science, 

 4th ser., vol. xxviii, pp. 197 and 373 (1899)). 



