11. Geol, Oolcai, Sectmns /Hrottyh /He Axl)Gs

2~G MR. ft. A. DOU(ILAS O.N* GEOI, OGICAL SECTIOn's [voI. lxxvii, 11. GEOl,OOlCAI, SECTmNs /hrottyh /he Axl)gs (f PEI~U a~td BoLIvlx: III--~','o,n the PORT of C~I.LAO 10 the lllVER PERE:,-S.. By Jasrr.s ARCnlm~LD DOVt;I.AS, M.A., B.Sc.. F.G.S. (Read April 20th. 1921.) :PLATES XV-XX.] CONTENTS. Page I, Introduction .........................................................246 IL Geological Description of the Section from Callao to the River Perene ...................................................... 249 III. General Summary and Conclusions ........................... 276 IV. Biblio~phy ...................................................... 281 I. INTRODUCTION. THIs paper deals with the geological structure of tile Andes of Central Peru, as represented by a horizontal section drawn from the island of San Lorenzo off the Pacific coast at Callao to the River Perene, a tril)utar,v of the Apurimac, one of tile headwaters of the Ucavali. It is a third contribution t() a series of papers written as (he result of a two years' expedition to South America (1910-12) undm~aken on behalf of tile late Mr. W. E. Balston, F.G.S. The district traversed is one of the best-known and most populous in Peru, for through it passes the northern Transandine route from Lima, the capital of the countlT, to the port of Iquitos on the Amazon, whence steamers sail direct to Europe. The first half of the section follows the line of the central railway of Peru. This is the well-known Oroya line, famous not only as the highest milwsy in the world, but also as a wonderful feat of engineering, rising as it does to an altitude of over 15,000 feet in a course of 100 miles. The line ascends the steep- sided valley of the Rimae by means of a series of sinuous curves and zigzag 'switchbacks,' cut for the greater part in the face of the solid rock. After passing through no fewer than 65 tunnels it reaches the summit of the watershed between the Atlantic and the Pacific river-systems at Ticlio, 15,665 feet above sea-level, and thence descending to 12,000 feet terminates at the iunction of Oroya. From Ticlio a branch-line leads to the important mining centre of Morococha, situated in a valley remarkable for the 1)erfection of its glacial topo~phv. At Orova it makes connexion with lines from the celebrated eopper-mines'of Cerro de Pasco and from the a~ricultural districts of Jauja and Htmncavo farther south. In addition, it furnishes an outlet for the coffee ~'owing region of the interior. Leaving the railway at 0roya, the section is continued eastwards part 3] Tm~O~GH ThE ~t~DES OF PERU .~.XD BOLI~IA. 247 along the route to Chanehonlavo.. After the longitudinal valley of the Mantaro has been crossed, a second and subsidiary watershed is met with at 1t,000 feet, from which a steep descent is made down the Tarma valley. About 25 miles beyond the village of that name, at the Talnl~o of Huaeapistana, are encountered the first signs of luxuriant vegetation, denoting the western limit of the Montafia district. The scenery at this point is very similar to that of a typical valley in the Swiss Alps; but with a further descent the temperature rapidly increases, and the tropical cha- racter of the flora becomes pronounced. At San tlamon the Tarma River is joined by the Tulomayo to form the Chanchomayo, and this in turn unites with the Pauear- tambo to form the River Perene, one of the headwaters of the Amazonian s~'stem. This point, which forms the eastern limit of the section, although distant lnore than 2000 miles from the Atlantic coast, is only 1145 feet above sea-level. The earliest-recorded observations on the geology of the Lima district were those made by Charles Darwin during the voyage of the ' Beagle' in lS35, which dealt chiefly with the evidence as to recent uplift afforded by the kitchen-middens of the island of San Lorenzo. Little attention, however, seems to have been paid to the structure of the mountainous region of the mainland until the beginning of the present century, when the first definite account of its geological featm'es, illustrated by a section across the Cordillera, was published by Prof. Gustav Steinmann as the result of a journey made in 1904 from Lima to the Chanehomayo River. 1 Of late years our knowledge of the district has been further enriched by the writings of Prof. Carlos I. Lisson, of Lima, who has made a detailed study of the country in the ilmnediate neighbourhood of the capital. In view of the fact that my route across this part of the Andes was identical with that taken by Steinmann, although continued some miles farther towards the east, anything more than a sum- marized account of my observations does not appear to be necessary, except in so far as they afford additional evidence for the facts already recorded. In many eases, however, the views of that author are not in accordance with mine, and I have therefore been led to give an independent interpretation of the section, in order o illustrate the points upon which we appear to differ. It is only fair to add that more than onee I have been able to strengthen or confirm my views on obscure facts by reference at a later date to his paper mentioned above. A preliminary studv of the physical features of this part of the country shows that they differ markedly in eharaeter frmia those prevailing in the South of Peru and the North of Chile. as deseribed by me in former papers of this series. Four principal topographic See Bibliography, p. 282, w IV, No. 46. % ~ -, ,0 "fI' e ,~f oo . "6 P pJ ~4 ~ pj el ~N a,-.1 6:G ~'~,!:' , ~~:i!, part 3~ GEOLOGICALSECTIO.NS TIIROIgGtI THE ANDES. 249 zones were there recognized, which, for the purposes of comparison, may be briefly restated as comprising (1) A coastal desert fringing the Pacific Ocean, aM bounded on the west by fragmentary remMns of an ancient coastul Cordillera formed of gneisses and plutonic rocks, probably of Archsean age. (2) A giant range of recent volcanic cones, capping the Mesozoic rocks of the Western Cordillera, and forming its outstanding topographic feature. (3) An inter-Andean high-level region, comprising the Altaplanicie of Bolivia and the area occapied by Lake Titicaca with the lon~tudinal valleys of the Desaguadero and the Vileamayo Rivers; a region formed chiefly of the later Palaeozoic and transgressive Mesozoic rocks. (4) An Eastern Cordillera (the Cordillera Real of Bolivia), formed of granite and early PMmozoic rocks, and bounded on the east by a sharp descent to the forested region of the Montafia. From the description given in the following pages, it will be seen that these salient features which find expression in the south no longer prevail in Central Peru. The mountainous region here extends ahnost to the shores of the Pacific, to the exclusion of the coastal desert. The zone of recent volcanoes is entirely wanting, and Mesozoic sediments with their intrusive batholithie core here play the chief part in determining the configuration of the Cordillera, outcrops of plutonic rock being met with in some eases ahnost at the summit of the lange. The inter-Andean region is represented in the latitude of Lima by the narrow longitudinal valley of the Mantaro, here, not far fl"om its source, forming but an {nsignificant feature of the topo- graphy; while the low divide which bounds it on the east can in no way be regarded as constituting an eastern Cordille~,n. Farther north, the longitudinal depressions of the Marafion and Huallaga rivers once more tend to divide the range into a series of parallel chains; but along the line of section here described they unite to form a single Cordillera, aptly described by Raimondi as a ' mountain-knot.' It is important to lay stress upon tlns fact, for Eduard Suess 1 in his summary of the structure of the Andes refers to the range east of Lima as ' the first lofty chain of the Andes'--a statement that suggests the presence of other chains paralM to it on the east, which in reality do not exist. II. GEOLOGICAL DESCRIP'rIOX OF THE SECTION FI~OM CALLAO TO THE ~RIVER PERENE. Before commencing the description of the geological structure of the country traversed by the present section, I think it necessary to refer once more to the crystalline gneisses and associated igneous rocks, which indicate the existence of an ancient coastal Cordillera 1 , The Face of the Earth ' Eng]. transl, vol. iv (1909) pt. 5, ch. xiii, p. 468. 2,~0 MR. J. A. DOUtILAS ON ~IEOI,OI;ICAi, SECTIONs ~-vol. lxxvii, in the South ~,f Peru, represented at the present day bv isolated fragments bounding the shores of the Pacific. l iocks ,4 this nature are said t,v Pr~,f. Steinmann to extend as far m,rth as Pisco ; but, as the result of ol,servations made during a visit to that locality, I am led to question the accuntey of such a statement. In the peninsula of Paraeas. a few miles south of the port, the n,cks m~ the coast were found to consist of coal-bearing" quartzites of Pala.o- zoic age, .viehlin'~.- numerous, plant-remains. These beds are over- lain unconformal,lv, by marine Tertiary deposits, pierced by. later igneous intrusions; bt[t i could lind no trace of the existence of ancient igneous rocks or crystalline gneisses such as occur farther south.

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