6* ME. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [April 1914,. 



through it the Arica-La Paz Railway ; wherefore Mr. H. Schu- 

 macher, of Tacna. the locating engineer of the line, cut a narrow 

 trail, with native labour, from one end to the other, and ac- 

 complished its survey for the first time. The danger and cost of 

 construction, how T ever, were found to be too great, so the scheme 

 was abandoned ; and at the time of our visit landslides had almost 

 obliterated the old trail, which in most places was nothing more 

 than a narrow ledge cut in the vertical face of rock. 



It is now T , therefore, impossible to go more than a short distance 

 up or down stream from the two spots to which a descent can be 

 made. One cannot but regret the loss to science of so magnificent 

 a natural section ; for, the line to the north being now completed, 

 no one is ever likely again to spend time or money in constructing 

 another trail in so inaccessible a region. 



After visiting the Jamiraya gorge, we crossed from Pocon chile 

 to Tacna, and proceeded inland up the valley of Palca — for the 

 purpose of comparing our results with the section described by 

 A. d'Orbigny, Pissis. and Forbes. 



We may turn now to the eastern part of the main section. 

 In the following year (1911) we journeyed from Puno round the 

 western shores of Lake Titicaca to the Copacabana Peninsula and 

 the Island of the Sun ; and thence crossing the Straits of Tiquina, 

 we proceeded southwards to La Paz. 



Finally, on two separate occasions we explored the country south 

 of Calacoto along the Desaguadero Biver, in the neighbourhood of 

 Callapa and Ulloma. 



IV. Geological Description of the Section eeom Aeica to 

 the Bolivian •' Ytjk'gas.' (See section, PI. X, tig. 1.) 



A most complete and detailed description of this district was 

 given by David Forbes, in a paper published in 1861 on the Geology 

 of Bolivia & Southern Peru 1 ; and, as much of his work is in 

 close agreement with my own observations, I shall here confine 

 myself (in order to avoid useless repetition) mainly to those points 

 concerning which new results have been obtained, or where the 

 interpretations of that author appear to be open to criticism. 



In dealing with a section of such magnitude, I have deemed it 

 advisable to describe the formations in the order in which they are 

 met with in traversing the country, but not in the actual order 

 of superposition, since the main structural features seem to be 

 of wider interest than the mere stratigraphical sequence, and 

 my ultimate object is a comparison of a number of sections 

 through the Peruvian Andes. As each formation is met with, 

 however, those fossils that are of interest will be discussed in the 

 palaeontological notes. 



The petrographical descriptions of the microscopic rock-sections 

 must only be regarded as preliminary observations ; for, in the 



' See Bibliography, § VI, p. 49. 



