xv The Continental Area 279 



structure and scenery from point to point. The highest peak 

 of the Andes is Aconcagua, 22,400 feet, in 33 S. ; but at 

 least thirteen other summits rise more than 19,000 feet above 

 the sea. Many of the passes, which mark the meeting of 

 the heads of transverse valleys of opposite slopes, are 

 elevated more than 14,000 feet, and the lowfest in a stretch 

 of 4000 miles is 11,400 feet above sea-level. Tertiary 

 sedimentary rocks form the slopes of the Andes, and are 

 overspread in many places by sheets of volcanic rock, while 

 the loftiest volcanic cones in the world shoot up in solitary 

 grandeur above the ridges. The Andes are young moun- 

 tains, geologically speaking, and are still growing. Every 

 little step of upheaval is accompanied by earthquakes ( 299), 

 which occur more frequently along the western margin of 

 South America than anywhere else. South of Aconcagua the 

 system consists of a single rugged ridge, which gradually 

 diminishes in height and in steepness toward the south, 

 where the sea has invaded its valleys forming the Chonos 

 Archipelago. From Aconcagua northward to the equator 

 the system forms two mountain ranges, one keeping close 

 by the Pacific coast, the other sweeping inland. Where 

 they diverge most widely the two mountain walls ericircle 

 a high plateau of internal drainage, which is as large as 

 Ireland, and its lowest part, 12,000 feet above the sea, is 

 occupied by the great Lake of Titicaca. Converging at 

 the northern extremity of the Titicaca Plateau the two 

 ranges wall in a longitudinal valley of great length, sloping 

 northward and traversed by rivers which escape by wild 

 gorges through the eastern ridge. From the equator north- 

 ward the ridges of the Andes diminish in height, unite in 

 the " Knot of Pasto," and then branch' into three spurs, 

 separated by the long valleys of the Magdalena and 

 Cauca sloping to the north. The eastern spur sweeps round 

 the north coast of South America, completing the framework 

 of the continent. Along its whole length the eastern ridge 

 of the Andes slopes down to the central low plain by a 

 succession of great terraces, and sends out many short 

 diverging mountain buttresses. Ores of silver, mercury, 

 and copper abound in these mountains, and coal-beds occur 



