"22 ME. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [April 1914,, 



Patapatani arid Huaylas, where they form picturesque cliffs of a 

 beautiful red or crimson colour. 



Owing to the porous nature of these rocks, they have frequently 

 become mineralized by infiltrating solutions. In one case the 

 whole matrix of an agglomerate was found to be completely 

 impregnated - with, magnetite; more usually, the change has been 

 one of silicification, resulting in the conversion of the bulk of the 

 rock into chalcedony or opal. 



(A 19 ) Trachyte or trachy-andesite ; Mount Taapaca. 



Microscopic characters: Felspar in the form of large 

 tabular phenocrysts: some sanidine. but mostly an acid pi agio - 

 clase snowing Carlsbad and polysynthetic twinning, with common 

 zonary structure. Quartz is present sparingly in large pheno- 

 crysts. Hornblende is abundant in small idiomorphic crystals, 

 with strong dark-brown to yellow pleochroism. Abundant dark- 

 brown biotite. 



The accessory minerals include sphene, sparingly in character- 

 istic wedge-shaped crystals, with high birefringence; magnetite, 

 apatite, and augite (represented by a few small pale-green to 

 colourless crystals). 



The ground-mass is vesicular and glassy, with microlites of 

 felspar and hornblende, and grains of magnetite exhibiting 

 parallel arrangement along lines of flow. 



(y) The lavas of the third or andesitic group are character- 

 istic of the region between Titiri and the Bolivian frontier, where 

 they have been emitted from the volcanoes of Tacora and Chupi- 

 quina, and overlie the pale rhyolites. They are also met with as- 

 dykes cutting through the latter. One particularly good example 

 is to be seen on the banks of the small river near Titiri, where, 

 owing to the more rapid weathering of the porous rhyolites, the basic- 

 dyke stands out as a conspicuous wall showing well-developed 

 transverse prismatic jointing. 



These rocks, owing to their compact basaltic structure and their 

 dark coloration, present a very distinct appearance, and arc further 

 characterized microscopically by the presence of pyroxene as the 

 dominant ferromagnesian element. 



(A., 5 ) Pyroxene-andesite ; Mount Tacora. (PI. VII, fig. 4.) 



Macroscopic characters: A compact, black, basaltic-looking 

 rock, with minute laths of white felspar and occasional larger 

 phenocrysts of pyroxene. (Specific gravity =2'84j.) 



Microscopic characters: Plagioclase (acid labradorite) 

 occurs in tabular or lath-shaped, square-ended, idiomorphic pheno 

 crysts, showing Carlsbad and polysynthetic twinning, and inclusions 

 of the ground-mass arranged in zones. Little or no decomposition 

 is visible. 



Abundant pale-green to colourless augite occurs in small 

 idiomorphic crystals, frequently showing complicated twin inter- 

 growths. There is also a considerable amount of rhombic 



