382 I. Bowman — Physiography of the Central Andes. 



and is flanked by metamorphosed sedimentary rocks — schists, 

 slates, quartzites, and the like — of Paleozoic age (Cambrian to 

 Lower Devonian), which dip away from the central axis.* 

 The whole series forms an involved anticlinal with a core of 

 granitic rock in the center. It is this granitic core which by 

 virtue of its superior hardness gives rise to the celebrated line 

 of lofty peaks running from Ancohuma to Illimani. Fifty 

 miles northeast of the main axis thus outlined are the Cusali 

 mountains (fig. 15), whose trend parallels that of the Cordillera 

 Real. They are composed of slates, sandstones and limestones 

 of Paleozoic age. For nearly another fifty miles are similar 

 parallel ranges of sandstone which terminate at the Bali-Susi 

 range, the last of the Andes mountains toward the northeast.f 

 Beyond is the great basin of the Amazon, which at even this 

 far inland point is but seven hundred feet above the level 

 of the sea. The Mesozoic sediments which form these out- 

 lying ranges are either unaltered or but slightly altered ; and 

 neither in structure nor physical condition do they constitute 

 an integral portion of the Cordillera Real. 



Southward across the La Paz valley the axis of the Cor- 

 dillera Peal is continued in the Nevadas de Araca, Nevadas 

 de Quimsa Cruz, and Nevadas de Vera Cruz (sometimes 

 erroneously called the Santa Vela Cruz). These form a line 

 of heights as definite in trend and structure as the Cordillera 

 Peal and continue the definite features of the latter southward 

 for nearly fifty miles. They are described by Steinmann and 

 Hoek as the direct extension of Illimani, not only as to geologic 

 but also as to geographic character.^ No volcanic material 

 is found in either of the mountain groups indicated. The 

 lofty snow-covered peaks are not a line of extinct volcanoes, 

 but a central core of highly resistant rock whose superior 

 hardness and greater initial elevation have preserved it from 

 the ultimate effects of the great denudation elsewhere recog- 

 nized. 



The baseleveled surface developed about the western border 

 of the Cordillera Peal can be clearly identified from El Cum- 

 bre, north of La Paz. Looking south, southeast and southwest 

 from this position (about 13,000 ft. elevation), one sees in the 

 foreground the long slopes of the ampitheatral valley head 

 (in which lies the city of La Paz) descending to the valley 

 iloor. In the middle distance, fig. 16, is the upper edge of 

 the valley, descending to the right as the piedmont alluvial 



*Sievers, W., Slid- und Mittel-Amerika, pp. 381, 382, 1903. 



f Expedition to Caupolican Bolivia, 1901-1902, by J. W. Evans (Geog. 

 Jour., vol. xxii, p. 631, 1903). 



\ Erlauterung zur Routenkarte der Expedition Steinmann, Hoek, v. Bis- 

 tran, in den Anden von Bolivien, 1903-4 (Petr. Geogr. Mitteilnngen, Heft 

 1, p. 16, 1906). 



