part 1] GEOLOGICAL SECTIONS THROUGH THE ANDES. 3 



result of work undertaken on the Balston Expedition to Peru. It 

 deals with a second and parallel section drawn through the south of 

 Peru from the port of Mollendo to the River Inamhari, a tributary of 

 the Madre de Dios, one of the head- waters of the Amazon. That 

 part of the country illustrated in the western half of the section, 

 which includes the important town of Arequipa and the port of 

 Puno on Lake Titicaca, is accessible by the Southern Railway of 

 Peru, of which we were enabled to make full use during the course 

 of our work from Arequipa to the coast, through the kindness of 

 Mr. McCulloch, manager for the Peruvian Corporation. I have 

 also to thank Mr. F. A. Cony, chief engineer of the line, for the 

 -original from which the sketch-map (fig. 1) was taken. 



For the continuation of the section beyond Puno through the 

 Eastern Cordillera, it Avas found necessary to proceed northwards as 

 far as Tirapata, a station on the Puno-Cuzco railway, whence access 

 could be gained, by the trail of the Inamhari Rubber Company, to 

 the forested region of the Montana. 



The railway between Puno and Tirapata, however, runs more or 

 less parallel to the strike of the chain ; and, although the eastern 

 part of the section from Tirapata to the Inambari River has a more 

 northerly trend, and is described under a separate heading, it may 

 be regarded as being practically continuous with that part which 

 lies between Mollendo and Puno. 



The journey northwards was done by road, in order that we 

 might examine the intervening country. 



After completion of the Arica-La Paz section we travelled 

 northwards from Tacna to Arequipa, a distance of about 150 miles, 

 ■over the coastal deserts here known as the Pampa de Clemesi and 

 the Pampa de Islay, hoping thereby to obtain an occasional glimpse 

 of the underlying rocks which might prove of assistance to us in 

 .an attempt to correlate the two sections. With the same object 

 in view, we made a further journey southwards from Puno round 

 the western shores of Lake Titicaca to the peninsula of Copacabana 

 .and the Island of the Sun. 



Although the district described in the following pages is one of 

 the best-known areas in Peru, little attention seems to have been 

 paid to its geological features, unless account is taken of the 

 explorations made by those interested solely in its mineral wealth. 



In 1886, Dr. F. H. Hatch gave a petrographical description of 

 the volcanic rocks of the Western Cordillera in the neighbourhood 

 • of Arequipa; but it is not until comparatively recent years that our 

 knowledge of the district has been increased by the publications of 

 the following authors: — Dr. George I. Adams (1908), Mr. V. F. 

 Marsters (1909), Prof. R. Hauthal (1911), and Prof. Isaiah Row- 

 man (1909), all of whom have confined themselves mainly to an 

 account of the coastal regions. 



I must repeat my thanks to Mr. W. E. Balston for enabling me 

 to undertake the work of investigating the country, and to 

 .Mr. J. R. Thomas for his assistance and companionship in the field. 



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