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



These rocks are best studied along the railway, where the}' are 

 well displayed between Poconchile and the ' Pampa Colorada,' and 

 again from Central to Puquios. (PL IV, fig. 1.) 



The following description of a few typical examples will serve 

 to illustrate their general facies : — 



(A e ) Rhyolite; quarries near Molino. 



Macroscopic characters: This rock consists essentially of 

 phenocrysts of quartz, glass}' clear felspar, and black hexagonal 

 plates of mica set in a fine-grained pink matrix. 



Microscopic characters : Idiomorphic phenocrysts of 

 quartz, sanidine, and acid plagioclase with poly synthetic 

 twinning and occasional zonaiy banding. Dark-brown biotite is 

 fairly abundant, much broken and bent, and exhibits resorption - 

 borders. Apatite occurs sparingly, in small needles. 



The ground-mass is cryptocrystalline to vitreous, with evident 

 flow-structure. Abundant spherulitic or axiolitic growths of 

 delicate fibres occur lining irregular cavities, the centres of which 

 seem often to be filled by a fine quartz-mosaic or possibly tridymite. . 



(A 7 ) Rhyolite; cuttings above Poconchile. 



Macroscopic characters: Similar to (A 6 ), but of a greyish 

 colour and with copper-coloured mica. 



Microscopic characters: Similar to (A 6 ) ; ground-mass 

 cryptocrystalline, with obscure grains of quartz and felspar 

 bereft of definite outlines. No flow- structure visible. 



(A 46 ) Rhyolite ; Kilometre 110. 



Macroscopic characters: Phenocrysts of quartz, glass}" 

 clear felspar, and black mica, in a purplish-red vitreous matrix. 



Microscopic characters: Idiomorphic phenocrysts of 

 quartz, sanidine, and acid plagioclase. Biotite not very 

 abundant, deep reddish-brown in colour, with corrosion -borders. 

 Magnetite occurs sparingly. 



The ground-mass shows marked flow- structure, and consists of 

 brown isotropic glass alternating with cryptocrystalline bands. 

 Microspherulitic structure is occasionally developed. 



South of the railway, thousands of feet of lava and tuff, 

 presenting a wonderful variety of colours, are again visible in the 

 Jamiraya gorge, lying horizontally upon the steeply-dipping 

 Jurassic shales and the granodiorite core. 



In many respects, these rocks are essentially similar to those 

 described above, but from their position are obviously of somewhat 

 earlier date. Hornblende is occasionally met with, although biotite 

 is still the dominant ferromagnesian mineral, and there is an 

 abundance of quartz. 



(A 47 ) Rhyolite; below Pa tapatani, Llutah River. 



Macroscopic characters: This rock is almost identical 

 in appearance with (A 1C ), but has a greyish colour. 



Microscopic characters: Large phenocrysts of quartz 

 occur, frequently with crystal outline, but more usually rounded by 



