part 1] THROUGH THE ANDES OF PERU AND BOLIVIA. 11 



(A 99) Near its junction with the gneiss the rock becomes converted into 

 a fine-grained, confused, fibrous aggregate of hornblende, biotite, and 

 chlorite, with abundant grains of magnetite and a little felspar. 



Numerous small light-coloured areas of circular outline are also visible 

 in a hand-specimen. These, when viewed microscopically, are found to 

 be composed of a patchwork of clear tremolite-crystals with an occasional 

 large grain of magnetite. Their outline is sharply defined, and they 

 appear to be xenoliths that have undergone metamorphism. 



(A 97) Dolerite. Mollendo (kil. 2). (PI. II, fig. 1.) 



Microscopic characters :- -This rock, which occurs in the form of dark, 

 compact, fine-grained dykes penetrating the gneiss, is composed of small 

 porphyritic crystals of plagioclase and augite embedded in a micro- 

 crystalline ground-mass. 



The plagioclase-phenocrysts are idiomorphic to the augite : they have 

 an elongate lath-shaped habit, and are of a peculiar brownish hue which 

 is due to ' schillerization," for under a high magnification they are seen to 

 be crowded with ultramicroscopic dust-like inclusions. They exhibit 

 Carlsbad twinning in conjunction with that on the albite and pericline 

 laws, and are comparatively fresh, although after their separation they 

 have undergone a certain amount of corrosion by the residual magma, 

 • with the production of a clear selvage of irregular outline and free 

 from schiller inclusions. Extinction-angles indicate a range from oligo- 

 clase to andesine. 



The augite is a colourless or very pale-green variety, showing numerous 

 interpenetrating twin-growths. It tends to wrap round the felspar- 

 crystals in a subophitic manner, but never completely encloses them. In 

 many instances it has been entirely replaced by scaly aggregates of 

 serpentinous and chloritic material, though some unaltered phenocrysts 

 still remain to indicate the original nature of these peculiar pseudomorphs. 



The ground-mass approaches in structure the ' intersertal ' of Kosen- 

 busch, consisting of a network of small felspar-laths (chiefly oligoclase) 

 enclosing granules of augite and magnetite, and a small amount of 

 residual glass. 



The foot-hills bounding the coastal region between Tambo and 

 Cachendo are composed mainly of holocrystalline pmtonic rocks of 

 granitic texture, which are well exposed in the numerous cuttings 

 on the railway. These rocks are usually pink or grey, and consist 

 essentially of quartz, felspar, hornblende, and subsidiary biotite. 

 They have the composition of typical granodiorites ; plagioclase 

 is, as a rule, in considerable excess of orthoclase, the percentage 

 of ferromagnesian minerals is not high, and quartz is frequently 

 present in some abundance. 



A parallel arrangement of the hornblende-crystals is noticeable 

 in some localities, which suggests slight movement during consoli- 

 dation. An incipient development of gneissic structure due to 

 earth-stresses, however, must also be' regarded as a possible 

 explanation, for similar rocks, probably of the same age, were 

 discovered farther inland, showing every stage of dynamic 

 metamorphism from a coarse-grained plutonic rock to a finely- 

 banded gneiss. 



In places, overlying beds of white and reddish quartzite are ob- 

 served, the age of which, in the complete absence of paheontological 

 evidence, cannot at present be determined. On comparison with 

 similar rocks from the Arica section, however, it is logical to suppose 



