Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on May 31, 2016 THE QUARTERLY JOURNAL OP TttE GEOLOGICAL SOCIETY OF LONDON. VOL. LXX. GEOLOGICAL SECTIONS through the A~DES o/' PERU and BoLIvIx: I--From the COXST at ARICX in the NORT]t Of CHILE tO L2L PAZ ant'/ t)~e BOL1VIAN 'YUNGKS.' By JAMES ARCHIBALD DOUGLAS, M.A., B.Sc., F.G.S. (Read November 5th, 1913.) [PLxTES I-X.] CONTENTS. Page I. Introduction ......................................................... 1 II. General Outline of the Physiography of the Peruvian and North Bolivian Andes ............................................. 2 III. Main Topographical Features of the Country included in the First Section (Ariea to the Bolivian ' Yuugas ') ...... 3 IV. Geological Description of the Section from Arica to the Bolivian ' Yungas ' . ................................................ 6 (i) The Coastal Region and the Westmru Cordillera. (ii) The Altaplanicie, or High-Level Bolivian Plateau. (Hi) The Eastern Cordillera and the Amazon Slopes. V. General Summary and Conclusions .............................. 46 VI. Bibliography .............................................. : .......... 49 ]]. INTRODUCTION. THrS paper deals with the geological structure of a part of the South American Andes, as illustrated by a horizontal section drawn from the port of A rica in the extreme north of Chile (fomner!y Peruvian territory), across the mountain-ranges or ' Cordilleras,' to Q. J. G. S. No. 277. B Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on May 31, 2016 2 )IR. J. A. DOUGLAS ON GEOLOGICAL SECTIONS EApril 19 J4, the 'Montafia,' 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.G.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 detM1. I was accompanied for fifteen months by Mr. J. R. 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 palmontological evidence, and thus compares somewhat unfavourablv with the more fossiliferous sections of our northern traverses. ;_Phissouthern section, however, has a particular interest~ on account of its historical associations, since it is practically coincident with a section first described in 18r 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 1867. These authors differed somewhat widely in their interpretations of the section, and a smmnarv is given i~l Forbes's paper, which was published in the Journal ot~' this Society. Among more recent travellers who have contributed to our knowledge of the district are Prof. G. Steimnann, with his colleagues Dr. H. Hock and Baron A. yon Bistmm. 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 countr3", and much useful work is being done by Sefiores Posnansky, Sundt, and others under the able leadership of Dr. Manuel V. Ballivian, Minister of Coloniza~tion and Agriculture. Sefior Eseutti 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. GENEllAL OUTLINE O:~~ THE PII;YSIOGItAPHY OF TIlE PEUt'VLtX 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 valleys and plateaux. (iii) The thickly-forested region, or ' Montafia,' of the Amazon basin. (i)--The coastal deserts, as has been cleari~." shown by Dr. G. I. Adams, in his ' Outline Review of Peruvran Geology,' Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on May 31, 2016 "Vol. 7o.~ THROUGIt THE :kNDES OF PERU AND BOLIVIA.. 3 occupy three separate areas, between which the foothills are continued to the ocean border: namely, a northern dish'ict, including the plains of Trujillo and Yiura ; a south-central in the neighbourhood of Pisco ; and a southern, comprising the pampas of Mollendo and Tacna. (ii)--In Southern Peru and the district described in the present paper, the mountain-system of the Andes consists of two main chains--a western, the ' Cordillera Occidental,' which is the con- tinuation of the main Chillan range, and an eastern, the ' Cordillera Oriental.' Between these two lies the elevated plateau known as the 'Altaplanicie,' the site of Lake Titicaca. Northwards the chains unite in latitude 1i ~ S., forming the Vilcanota knot, to divide again into three main branches---recognized by Raimondi as a Western 'Cordillera,' following the general direction of ~he coast; a Central, separating the v.dleys of the Apurimac and Mantaro Rivers from that of the Urubamba or Vilcamayo ; and an Eastern separating the Urubamba from the Madre de Dios. In the neighbourhood of Cerro de Paseo the chains reunite to form a second mountain-kno% divkling northwards once more into three. Here the Western Cordiller~ at its southern end, where it diverges 9from the knot, splits into two minor parallel chains, the ' Cordillera Negra' and the ' CoMillera Blanca,' separated by the valley of Huaylas. This valley at first follows the strike of the chains to the north-west, but in latitude 9 ~ S. cuts the ' Cordillera Negra' almost at right angles, to enter the sea in the Bay of Santa north of Chimbote. The main Central Cordillera separates the valley of the Marafion from that of the Huallaga; while the Eastern separates the latter from the Ueayali. (ill)--The thiM region comprises the vast forested area of the Amazon basin, almost impenetrable save along the navigable rivers and the nan'ow trails leading to gold or rubber camps. The descent from the ' Sierra' is rapid, and the whole country soon becomes clothed in dense tropical vegetation. Rare open spaces, or pajonales, devoid of trees, enable one, however, to obtain occasional glimpses of the distant landscape. A great part of ~his country has been explored by Dr. J. W. Evans, who describes a nun{hcr of small parallel chains running in a north-westerly direction parallel to the main ' Cordillera.' The first of these, bounding the valley of the ]{iver Coroieo on the east, he has named the Cusali Range. III. MAI~~ TOPOGR:tPItlC-~-L FE_~TUI~ES OF THE COUNTRY II~'- CLUDED IN TIIE FIRST SECTION. (ARIcI TO THE BOLIVIAN 'Yv~-Gxs.') (See Map, P1. X, fig. 2.) Our first main traverse, the results of which are included in the following pages, was made through the extreme north of Chile, formerly Peruvian territory, and a part of Bolivia, following the present line of railwa.v as far as La Paz. whence it was continued ~2 Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on May 31, 2016 :MR. J. A. DOUGLAS Oh- GEOLOGICAL SECTIO~.NS [April i9~4, over the Eastern Cordillera into the district known as tile North Yungas. This railway commences at the port of Arica, which is situated in lat. 18 ~ 29 S. and long. 70 ~ 20' W., just at the point where the main trend of the Pacific coast-line of South America changes from a north-and-south direction to north-west by south-east. After following the shol~ for a few miles, it turns inland up the fertile valley of the Llutah River to Poconchile (kilometrc 37 ; altitude 1772 feet). From this point a steep winding ascent is made over desert foot- hills and the so-called 'Pampa Colomda' to Central (kilom. 70; alt. ~859 feet); it is continued eastwards with increasing grades and a 4O-kilometre stretch of rock-and-pinion line, over typical , bad lands' composed of numerous small and steep-sided dr3~ rocky valleys or qucbradas, until more or less level ground is reached al)ove Ituaylas (kih)m. 135 ; alt. 12,797 feet). We are here in the region of the Western Cordillera, a vast range of giant extinct volcanoes which rise abl~ptly from the high-level plateau. On the south are visible the mighty peaks of Sajalna (21,42~ feet) and the Payaehatas or Twins ; while close at hand, separated from us by the deep Jamiraya gorge, through which flow the head- watem of the Llutah River, stands the 'Nevada de Putre' or Taapaea (19,145 feet); see Pl. I. Following the line past the swamps of Aneachulpa and the salt lake Laguna Blanca to the Bolivian frontier, we pass in close proximity to the southern base of Mount Tacom (19,520 feet); while Mount Chupiquifia (18,855 feet) and other high peaks are seen succeeding one another in the north. After crossing the frontier (kilom. 205; alt. 13,321 feet), the railway makes a gradual descent to the valley of the Mauri River (kilom. 250; alt. 12,8~2 feet), which is followed for some 50 miles until it enters the Desaguadero at the village of Calacoto (kilom.