part 1] THROUGH THE ANDES OF PERU AND BOLIVIA. 9 



The junction of diorite and gneiss is well seen in one of the rail- 

 way cuttings at kilometre 2, where the former rock along the 

 chilled margin assumes the aspect of a fine-grained amphibolite, 

 while the gneiss has undergone little change. A similar occurrence 

 is to be noted at kil. 13, where the latter rock is again pene- 

 trated b} r a second and smaller mass of diorite with associated 

 basic d} r kes. 



The acid magma has consolidated as a coarse-grained pink granite 

 (A 91), containing clear smoke-coloured quartz, pink felspar, and 

 a small percentage of biotite and chlorite. 



The rock is much fractured and weathered. Its junction with 

 the gneiss is plainly seen at Mejia, the latter rock being penetrated 

 in every direction by numerous veins of pegmatite. The presence 

 on the Pacific coast of this typically ' alkaline ' type of rock, con- 

 taining, as it does, abundant microcline and few ferromagnesian 

 minerals, in a district essentially characterized by rocks of a ' calcic ' 

 facies, is of considerable interest. It would appear to support the 

 theory of a remote, possibly Archaean, age for the gneiss and 

 granite, by the suggestion that their formation preceded the 

 development of the forces which gave rise to the uplift of the 

 Andes, with the accompanying intrusion of piutonic rocks of 

 ' Pacific ' type. 



That a generalization of distribution of so-called Atlantic and 

 Pacific provinces cannot be applied to the older rocks, has already 

 been proved by Dr. Gr. T. Prior l in the case of the Great Rift 

 Valley of British East Africa, where a younger foyaitic-theralitic 

 series overlies older crystalline rocks of Pacific kindred. In the 

 present instance the reverse is the case, rocks of an alkaline facies 

 being succeeded by a younger calcic group. 



Petrographical Description of the Rocks of the 

 Coastal Cordillera. 



(A 71) Gneiss. Mollendo (kil. 2). 



Microscopic characters : — This rock consists essentially of a granular 

 mosaic of clear quartz, felspar, and certain coloured minerals, showing- 

 marked parallel structure in their arrangement ; the effects of dynamic 

 metamorphism have further residted in the crushing or bending of many 

 of the constituents. 



Quartz is abundant, and exhibits pronounced strain-shadows. 



The felspars are slightly turbid owing to alteration, which has resulted 

 in a confused mass of sericite-flakes ; they comprise microcline, ortho- 

 clase, and plagioclase (near andesine), the last-named frequently showing 

 bending- and fracture of the twin-lamellas. The microcline structure, 

 although possibly in some cases original, often appears to be a pheno- 

 menon due to strain-pressure acting on the orthoclase. 



The micas are not abundant ; muscovite and biotite are present in 

 about equal proportions, the latter showing alteration into chlorite and 

 epidote. 



A colourless or faintly pink garnet is abundant ; the crystals are 



1 ' Contributions to the Petrology of British East Africa ' Min. Mag. vol. xiii 

 (1901 1903) pp. 228 63. 



