part 8] CIEOLOUTCAL SECTIONS TTEEOUGMI THE ANDES. 249 



zones were there recognized, Avhich, for the purposes of comparison, 

 may be briefly restated as comj^rising 



(1) A coastal desert fringing the Pacific Ocean, and bomided on the west by 



fragmentary remains of an ancient coastal Cordillera formed of 

 gneisses and plutonic rocks, probably of Archaean age. 



(2) A giant range of recent volcanic cones, capi^ing 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 occupied by Lake Titicaca Avith the longitudinal 

 valleys of the Desaguadero and the Vilcamayo Rivers ; a region 

 formed chiefly of the later Palaaozoic and transgressive Mesozoic 

 rocks. 



(4) An Eastern Cordillera (the Cordillera Real of Bolivia), formed of granite 



and early Palaeozoic rocks, and bounded on the east by a sharp 

 descent to the forested region of the Montana. 



From the description given in the following pages, it will be seen 

 that these salient featm-es which find expression in the south no 

 longer prevail in Central Peru. The mountainous region here 

 extends almost to the shores of the Pacifie, to the exclusion of the 

 coastal desert. 



The zone of recent volcanoes is entirely wanting, and Mesozoic 

 sediments with their intrusive batholithic core here play the chief 

 part in determining the configuration of the Cordillera, outcrops of 

 plutonic rock being met with in some cases almost at the summit 

 of the range. 



The inter-Andean region is represented in the latitude of Lima 

 by the narrow longitudinal valle}'' of the Mantaro, here, not far 

 from its source, forming but an insignificant 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 Cordillera. Farther 

 north, the longitudinal depressions of the Maraiion 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 Kaimondi as a 

 ' mountain -knot.' 



It is im^^ortant to lay stress upon this fact, for Eduard Suess ^ 

 in his summary of the structure of the Andes refers to the range 

 east of Lima as ' the first loiiy chain of the Andes ' — a statement 

 that suggests the presence of other chains parallel to it on the east, 

 which in reality do not exist. 



II. GrEOLOGTCAL DeSCEIPTIOX OF THE SECTION FEOM 



Callao to the EiyEE, Pere]S"E. 



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 ' Engl, transl. vol. iv (1909) pt. 5, ch. xiii, p. 468. 



