2 MR. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [April 1914, 



the 'Montana,' or forested region of the Amazon basin, known 

 as the Bolivian Yungas. It is the partial result of two 

 years' geological exploration in Peru, undertaken on behalf 

 of Mr. W. E. Balston, F.Gr.S., who, being impressed during 

 his travels by the geological features of the country, was good 

 enough to send me out for the Oxford University Museum to 

 study them in greater detail. I was accompanied for fifteen 

 months by Mr. J. E. Thomas of Montana (U.S.A.), who, after 

 his valuable help in the field, I much regret, has been unable to 

 assist me further in the study and publication of our results. 



At Mr. Balston's suggestion, we commenced work in the south of 

 the district, following the route of the new Arica-La Paz railway, 

 which at the time of our visit was in course of construction. 

 Unfortunately, the line passes for the greater part of its length 

 over igneous rocks and barren sediments, furnishing little in the 

 way of palaeontological evidence, and thus compares somewhat 

 unfavourably with the more fossiliferous sections of our northern 

 traverses. This southern section, however, has a particular interest 

 on account of its historical associations, since it is practically 

 coincident with a section first described in 1842 by A. d'Orbigny, 

 who travelled to La Paz by the old Bolivian high road from Tacna. 

 The same route was followed later by P. J. A. Pissis in 1856 and 

 David Forbes in 18(37. These authors differed somewhat widely 

 in their interpretations of the section, and a summary is given in 

 Forbes's paper, which was published in the Journal of this Society. 

 Among more recent travellers who have contributed to our 

 knowledge of the district are Prof. Gr. Steinmann, with his 

 colleagues Dr. H. Hoek and Baron A. von Bistram, Prof. Hauthal. 

 Sir Martin Conway. Dr. J. W. Evans, and several others. 

 Local scientific men in Bolivia are now beginning to take an 

 interest in the geology of the country, and much useful work is 

 being done by Senores Posnansky, Sundt, and others under the able 

 leadership of Dr. Manuel V. Ballivian, Minister of Colonization and 

 Agriculture. Senor Escutti Orrego, an ardent Chilian geologist, 

 has also furnished us with a number of fossils from the Morro 

 de Arica, while another of his collections is preserved in the 

 British Museum (Natural History) at South Kensington. 



ii. greneeal outline of the physiography of the peiu vjan 

 and North Bolivian Andes. 



The Pacific border of this portion of the South American 

 continent falls naturally into three well-defined regions, differing 

 markedly in their physical and climatic conditions : — 

 (i) The rainless deserts of the coastal plains. 



(ii) The mountainous district, or ' Sierra,' comprising the ' Cordilleras ' 

 or chains of the Andes, with their intervening- high-level valle\ a 

 and plateaux, 

 (iii) The thickly-forested region, or ' Montana,' of the Amazon basin. 



(i) — The coastal deserts, as has been clearly shown by 

 Dr. Gr. I.Adams, in his 'Outline Review of Peruvian Greolosrv.' 



