552 The Andes and the Amazons. 



400; while the Huallaga has probably a swifter current 

 than any of the southern affluents. 



. The basin of the Great River is principally inclosed by 

 the sedimentary slopes of the Andes and the metamorphic 

 regions of the Casiquiare and Central Brazil. 



As the rise of the Andes was the creation of the Ama- 

 zons, the study of the Mountain should precede that of the 

 Ri^er. Indeed, the structure of the basin can not be un- 

 derstood without a knowledge of the "rim." The geology 

 of the Andes is not sufficiently advanced to warrant a clas- 

 sification of the ranges with respect to their periods of ele- 

 vation. Yet it is very probable that the coast cordillera 

 was the first to emerge, and very certain that the eastern 

 did not reach its present elevation until after the Creta- 

 ceous age. The characteristic rocks of the maritime range 

 are trachytes and porphyries ; of the oriental, sandstones 

 and slates. 



The profile of the Andes of Northern Pei-n, on the fol- 

 lowing page, gives the relative heights of the ranges and 

 the main formations. I found no fossils in the Pacasmaj^o 

 beach ; but at Payta, farther north, there are many, among 

 them Turritella Patagonica, Sow. (which Darwin found 

 also on the coasts of Patagonia and Chile), and Pecten 

 viodisonus, Say, and Crej>idula fornicata, Say, identical 

 with Miocene species on the east coast. The beach was 

 therefore raised in late or post Tertiary times ; and there 

 is evidence that a subsidence lias taken place since the 

 Conquest,* for an Incarial road, with side-walls intended 



* I have elsevvheve called attention to the singular fact that every succes- 

 sive measurement of the Andes gives a reduced elevation, tempting one to 

 believe that either the chain is sinking or the atmospheric pressure increas- 

 ing. Thus, Humboldt (1803) made the altitude of Quito 9570 feet; the 

 writer (1867), 9520; Reiss and Stiibel (1870), 9350. Pichincha, according 

 to Humboldt, is 15,922 ; according to the writer, 15,827 ; according to Reiss 

 and Stiibel, 15,704. In 1827, Pentland very carefully estimated the altitude 

 of Lake Titicaca at 12,795 feet, and Friesach, in 1858, determined it to be 



