part 3] TiEROUGii the a:ndes op pehu and boliyia. 2o7 



It is thus seen to form an almost continuous line for more than 

 750 miles, and throughout the whole of this extent the rocks of 

 which it is composed were found to possess a uniform petrological 

 character, consisting for the most part of coarse-grained acid 

 granodiorites, frequently associated with more basic diorites, and in 

 places showing a transition to the adamellite-monzonite group. 



Kocks of alkaline facies are totally lacking, and thus a marked 

 distinction can be drawn between these calcic rocks of the central 

 core and, on the one hand, the true granites of the coastal 

 Cordillera, on the other, the granites and syenites of the eastern 

 slopes. This distinction, moreover, is not only one of petrological 

 character, but also probably one of geological age. 



In a former paper evidence was put forward to show that the 

 batholithic intrusion took place, in part at least, in pre- Cretaceous 

 times ; but in the district here described it is clearly of post- 

 Cretaceous date, and doubtless accompanied the great Tertiary 

 folding of the Andes. 



The relationship between the sedimentary and the pkitonic rocks 

 is well displayed in the Cerro San Cristobal, a hill close to the 

 northern outskirts of Lima. The gi'anodiorite is here seen to cut 

 .across the bedding-planes of steeply-dipping Cretaceous strata, 

 which furnish abundant evidence of having been metamorphosed 

 along the margin of contact. From this point it is more or less 

 continuously exposed along the valley of the Himac nearly as far 

 as Matucana, and, although not met with again along our actual 

 line of section, it crops out once more near the summit of the range 

 in the Morococha Valley. 



Throughout this lateral extent it does not consist of one type 

 of rock alone, but on the contrary suggests two distinct phases of 

 instrusion,^ the first represented by a diorite, the second by a more 

 acid granodiorite. The former is chiefly developed in the district 

 between Santa Clara and Nana, and again west of Chosica. 



In the granodiorite, moreover, occur numerous basic inclusions 

 which appear to be xenoliths derived from the earlier diorite. 

 Further, both types of rocks are penetrated by a later series of 

 minor acid intrusions. 



In studying the sequence of events indicated by these rocks, it 

 is natural to make comparison with the evidence of successive 

 periods of intrusion afforded by the corresponding plutonic complex 

 of the Cerros de la Caldera, in the Arequipa district farther south. 

 The diorite was there shown to be the second of three phases of 

 intrusion, and it appears probable that this and the subsequent 

 third phase are the equivalent of the two phases represented here. 

 In each case they are succeeded by a phase of minor intrusions. 



It may be recalled that it was to rocks of the first phase, 

 unrepresented in the district now described, that I sought to trace 

 the origin of the pebbles of granodiorite found in the basement 



^ Two successive phases of intritsion, of similar character to the above, 

 were also observed in the plutonic rocks of the Trujillo district farther north. 



v'2 



