26 MR. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [vol. lxxvi, 



growth, subsidiary plagioclase, a little biotite mostly altered to 

 green chlorite and epidote, some magnetite and apatite, and a 

 small amount of tourmaline with characteristic blue and brown 

 pleochroism. 



The last-named mineral, however, is never abundant in this area, 

 in which respect the rocks of the complex differ in a marked degree 

 from those of the plutonic batholite of the Arica-La Paz section, 

 where tourmaline-pegmatites are of common occurrence ; moreover,, 

 it is found here in the form of small isolated crystals, and not in 

 the coarse radial aggregates so typical of that district. 



The several stages in the general history of the complex may be 

 summarized as follows : — 



(1) The intrusion, probably into Mesozoic strata, of the Quishuarani 



granodiorite, comparable with that of the Cachendo foot-hills. 



(2) Its invasion by a residual acid magma. 



(3) Its subjection to powerful earth-stresses, which produced intense 



dynamic metamorphism, with the local development of gneissic struc- 

 ture, and culminated in : - 



(4) Fracture, with the intrusion of the augite-diorite of Huaico. 



(5) Formation of numerous quartz-epidote veins. 



(6) Intrusion of the Tiabaya granite (adamellite) comparable with that of 



Huagri, producing contact-metamorphism of the diorite. 



(7) Final intrusion of minor acid veins containing tourmaline. 



(2) The Geological Structure of the Country between 

 Arequipa and Puno. 



The Uchumayo Valley, as has already been mentioned, forms a 

 natural line of division between the Cerros de la Caldera and the 

 lofty peaks of the Western Cordillera. On its south-west side 

 lies the plutonic complex just described, while on the north-east is 

 found a thick accumulation of bedded volcanic tuffs, ejected from 

 Mount Chachani and El Misti. 



Prior to the cutting of the valley, these tuffs must have extended 

 far towards the coast, for their scattered remains are frequently 

 visible lying as a denuded mantle on the older rocks. Isolated 

 blocks of lava are even met with as far as Vitor ; but, as a rule, 

 the eruptions seem to have been chiefly of an explosive nature,, 

 resulting in the formation of well-bedded compact tuffs, pure white 

 or of a pink colour, containing abundant fragments of pumice. 

 These tuffs are extensively quarried in the neighbourhood of 

 Arequipa for building-purposes. 



True lavas are more or less confined to the vicinity of the moun- 

 tains. They have already been fully described by Dr. F. H. Hatch, 

 and it is therefore hardly necessary to make more than a passing 

 reference to them here. That author has shown that most of the 

 lavas of the Arequipa district belong to the andesite group, and he 

 has further subdivided them, according to their dominant ferro- 

 magnesian mineral, into hornblende-andesites, augite-andesites, 

 hornblende-augite-andesites, and andesites with hypersthene in 

 addition to the former minerals. 



