Vol. 70.] THROUGH THE A^'DES OF PERU AND BOLIVIA. 21 



corrosion ; quartz also occurs as a secondary product, filling- vesicles 

 in the form of a mosaic. Sanidine subordinate to plagioclase, 

 which is mostly a 1 bite and oligoclase. 



Biotite is not very abundant, it is bent and broken, and there 

 is a little green hornblende posterior to the biotite. Apatite, 

 zircon, and magnetite occur sparingly. 



The ground-mass consists of a pale brownish glass crowded with 

 minute spherulites, which, in addition to showing black crosses 

 between crossed nicols, can also be recognized under a high power 

 in ordinary lio*ht. 



Abundant foreign xenoliths of a more basic type, composed 

 chiefly of plagioclase-crystals in a brownish vitreous or crypto - 

 crystalline ground-mass, without quartz, but with abundant 

 magnetite, occur throughout this rock. 



(A !3 ) Rhyolite; Patapatani, Llutah River. 



This rock underlies (A 4T ), and represents an older series. 



Macroscopic characters: An even-grained, compact, grey 

 rock, splitting easily in one direction along the lines of flow. 



The phenocrvsts are chiefly quartz and bronze-coloured mica 

 without much felspar, set in a vitreous base. Numerous inclusions 

 of clear black glass occur. 



Microscopic characters: Small phenocrvsts of quartz, 

 plagioclase (chiefly albite), and biotite ; the last-named is 

 much bent, and shows parallel arrangement along the lines of flow. 



Apatite occurs as inclusions in the biotite. Hornblende 

 occurs very sparingly ; it is greenish brown, and shows corrosion. 



Ground-mass of pale-brown isotropic glass, including corroded 

 patches of clear colourless glass with conspicuous flow-structure. 



(/3) The lavas included in the second or tr achy tic group 

 are typically absent along the line of railway, but attain their 

 maximum development in the region south of the Llutah Valley, 

 being especially characteristic of the district of Putre, where they 

 represent the last phase of activity of the volcanic cone of 

 Mount Taapaca. The sheets of lava which, with interbedded 

 tuffs, build up this huge cone have a very distinct facies, and 

 the typical rock referred to as ' Putre trachyte " is always easily 

 recognizable among the boulders carried down the Llutah River. 

 It is, however, never met with on the opposite or north side of the 

 valley, which therefore was probably in great part formed prior to 

 the eruption. Although quartz is occasionally present, it is never 

 so abundant as in the rhyolitic group, the most characteristic 

 feature of the rock being the numerous large, clear, porphyritic 

 crystals of felspar, which are set in a ground-mass of predominantly 

 grey colour. Microscopically, a distinguishing feature is the 

 presence of hornblende in addition to the biotite. 



Associated with these horn blende -trachytes or trachy- 

 andesites occur vast thicknesses of volcanic tuffs and agglo- 

 merates, which are well displayed in the river-section between 



