part 1] THROUGH THE ANDES OF PERU AND BOLIVIA 7 



Its sides are steep, often precipitous, and the river descends in a 

 series of falls and boiling rapids to join the Inambari a few miles 

 below the rubber camp of Chaquirnayo. 



Numerous small tributary streams enter the main river in the 

 form of hanging valleys, at the mouths of which are fine examples 

 of ' cones de dejection.' These are remarkably well developed, and 

 control the mighty rush of water in the most striking manner, 

 forcing it repeatedly frorn one side of the valley to the other. 



The trail enters the dense tropical vegetation of the ' Montana ' 

 or forest-region at Ollachea (alt. 8860 feet), and, after several 

 times crossing the river by frail swinging bridges, reaches its 

 termination on the Inambari River at Puerto Seddon (alt. 1575 

 feet). 



III. Geological Description op the Section prom 

 Mollendo to the Inambari River. (See PL VI.) 



For the purposes of geological description, it has been found 

 convenient to divide this section into three parts : — 



(1) Mollendo to Arequipa : comprising- the ancient metamorphic and in- 



trusive rocks of the coastal Cordillera, the plutonic complex of the 

 Cerros de la Caldera, and the fragmentary remains of a Mesozoic 

 cover. 



(2) Arequipa to Puno : comprising- the volcanic rocks ejected from the 



giant cones which fringe the Western Cordillera ; the Mesozoic and 

 Paheozoic sediments of the inter- Andean region corresponding- to the 

 Altaplanicie of Bolivia ; the dioritic intrusions of the Maravillas dis- 

 trict ; the lava-flows of post-Cretaceous age in the neighbourhood of 

 Puno, and the alluvial flats of Lake Titicaca. 



(3) Tirapata to the Inambari River : comprising- the Palasozoic sediments 



and plutonic rocks of the Eastern Cordillera and the Amazon slopes. 



(1) The Geological Structure of the Country between 

 Mollendo and Arequipa. 



In a former paper dealing with the geology of a section from 

 Arica to the Bolivian Yungas, I described the coast of Northern 

 Chile as being composed of stratified Mesozoic deposits associated 

 with igneous rocks of contemporaneous origin ; this series, with its 

 intruded core of granodiorite, was shown to form the basement on 

 which are situated the volcanic cones of the Western Cordillera. 



If the coast be followed southwards, however, this Mesozoic zone 

 is soon found to be cut off from the ocean by an intervening zone 

 of ancient crystalline rocks, comprising chiefly granites, gneisses, 

 and mica-schists. Rocks of this nature are well exposed at Mejil- 

 lones, Coquimbo, Valparaiso, and other intermediate ports ; they 

 constitute the coastal Cordillera of Chile. 



North of Arica, the Peruvian coast-line bends sharply towards 

 the north-west. This change of direction, though accompanied by 

 a corresponding deflection in the strike, is sufficiently great once 

 more to bring to light rocks that may be regarded as forming the 

 continuation of the coastal Cordillera of the south. Such rocks 



