278 ME. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [vol. Ixxvii, 



and has been in places converted into a banded gneiss. At San 

 Eauion a further great mass of plutonic rock is met with, which 

 was proved to extend for some distance beyond the eastern limit of 

 the section. This is essentially a rock of 'Atlantic' facies, as 

 distinct from the ' Pacific ' types which alone are displayed in the 

 Cordillera. It is suggested that the origin of this Perene granite, 

 like that of a similar rock occurring in the coastal C'ordillera of 

 the south, dates from a very earl}' period antecedent to the uplift 

 of the Andes. 



In conclusion, a brief synopsis may be given of the general 

 distribution of the rocks which build up the Cordilleras in this 

 part of South America, as shown in this and two other parallel 

 sections previously described hj me. The earliest rocks are met 

 with in the fragments of a coastal Cordillera which fringe the 

 Pacific coast in Southern Peru, between MoUendo and Pisco. 

 These consist of ancient crystalline gneisses and associated plutonic 

 rocks. The latter are of alkaline facies, and are quite distinct 

 from the calcic types which everyAvhere characterize the develop- 

 ment of igneous activity in the main Cordilleras. They thus 

 belong to what maybe regarded as a definite extra -Andean igneous 

 province, dating possibly from Archaean times. 



If we now make a complete traverse of the Cordilleras, we again 

 meet Avith rocks of similar character cropping out along the foot 

 of the eastern slopes, in the forested region drained by the head- 

 waters of the Amazon. Thus, for example, in the Perene district 

 of Central Peru, Ave find an ancient granite closely resembling that 

 which occurs on the coast at Mollendo ; while farther south in the 

 Inambari region, elseolite-syenites are the dominant type. The 

 latter are probably of early Palaeozoic age. 



The later transgressive deposits, Avhich form the bulk of the 

 folded chains, are thought to haA'e been laid doAvn in a geosyncline 

 bounded by two resistant horsts, composed largely of these ancient 

 plutonic rocks, between AA'hich they have been compressed and 

 eleA'ated to the position that thej' occupy at the present day. This 

 compression, hoAvever, has been of intermittent character, periods 

 of uplift, characterized by folding and erosion, haA'ing alternated 

 AA'ith periods of submergence, characterized by rencAA^ed deposition. 

 Moreover, the compression has not everywhere been of the same 

 magnitude. Where it has been greatest, as shown b}' a diminution 

 in the breadth of the Cordillera, there, as might be expected, the 

 folding has reached the greatest degree of complexity. Where it 

 has been least, there gentle anticlines and synclines are the rule. 

 Again, it has probably not been uniform in point of time through- 

 out the length of the Cordillei-a. 



This is suggested by the lithological change exhibited by certain 

 formations when they are traced from south to north. Thus, 

 the typical shalloAv-Avater type of Cretaceous deposits in Southern 

 Peru, characterized by the predominance of red sandstones and 

 mai-ls with abundant seams of gypsum and rock-salt, is replaced 



