ORIGIN OF HIGH PLAINS. 



133 



which in its turn has been decomposed by vegetation into nitrate of 

 soda. 2d. At a height of 7000 feet to 8000 feet in the desert of 

 Atacama, there are immense salt plains on a grand scale. 3d. At a 

 height of 13,000 feet, on the tableland, white crystalline salt is found 

 on the shores of swamps and lakes. This tableland is intersected by 

 ranges of hills which run north and south, and sometimes rise 2500 

 feet above the plateau. The intervening valleys are nearly level plains 

 often formed of gravel, resulting from the wearing down of the ridges. 

 Volcanic cones occasionally rise 6000 feet above the plain. The area 

 may be separated into west, central, and eastern parts, which are re- 

 spectively of Oolitic, Permian, and Silurian age. 



The Silurian rocks here form the higher chain of the Andes, rising 

 to 25,000 feet, and extending through the ranges which feed by the 

 melting of their snows tributaries of the Amazon and the La Plata. Far- 

 ther south in Chili the structure of the country is somewhat different. 

 In the north, mountains predominate, but in the south the mountains arc 



Fig. 44. Pass (Andes). 



subordinate to the plains. Mr. Darwin mentions that Santiago stands 

 on a plain 15 miles broad, and 1750 feet high, which has the undula- 

 tions of its surface parallel to the main valleys of the Andes, against 

 which chain it abruptly terminates. The surface of the plain is formed 

 of stratified pebbles, volcanic ashes, and clay. Southward it contracts 

 and expands successively three or four times, forming a series of basins 

 connected like a necklace. On the eastern side of the Andes, the 

 mountains are abrupt, and rise out of a slope like a talus which is 

 formed of rounded pebbles. This slope blends into a flat space 2700 

 feet above the sea, which is a few miles wide, and is bounded to the 

 east by an escarpment 80 feet high, running north and south, and 

 formed of rounded pebbles, obtained beyond all question by the sea 

 from the Andes, and rolled and rounded at their base, when the 

 higher tablelands had been so far raised from the sea as to connect 

 these mountains into a long and narrow island. As the South 

 American tablelands are followed northward into North America, their 



