Vol. 70.] THROUGH THE ANDES OF PERU AND BOLIVIA. 15 



highly-acid pegmatite-veins, consisting essentially of quartz, ortho- 

 clase, and tourmaline, with a little topaz. 



The basic type appears to be more completely developed towards 

 the west of the main core, and ascending the river- valley becomes 

 progressively more acid. 



The following description represents the general facies of the 

 rock between Tinares and Cata : — 



(A 9 ) Macroscopic characters : A noncrystalline even- 

 grained rock of granitic texture and greyish colour, with rather 

 a high percentage of ferromagnesian minerals. 



Pale-green to white plagioclase, pink orthoclase, the former 

 predominating ; dark-green hornblende and black biotite. 

 Quartz not very obvious or abundant, but never completely absent, 

 as Forbes erroneously stated. (Specific gravity = 2 - 73.) 



Microscopic characters : Holocrystalline, hypidiomorphic 

 structure. 



Quartz not very abundant, allotriomorphic. 



Orthoclase turbid, and plagioclase fairly clear in ordinary 

 light ; the latter in excess over the former, and showing fine 

 polysynthetie twinning, chiefly albite to oligoclase, with slight 

 incipient alteration into sericite. 



Biotite abundant, pleochroism deep brown to pale yellow; 

 .slight decomposition to chlorite and epidote. 



Hornblende, pale green to colourless. Apatite occurs 

 -sparingly as inclusions in the biotite. Epidote after biotite, with 

 •canary-yellow pleochroism. Magnetite is fairly abundant. 



(A n ) Palmani, Llutah River. 



A rock similar to that just described, but of a more acid nature. 



The felspars are present in about equal proportions, and the 

 orthoclase, which occasionally tends to be porphyritic, gives a 

 decided pink colour to the hand-specimen. Quartz is more 

 abundant, but never very obvious. A slightly lower percentage 

 of biotite and hornblende is found. (Specific gravity = 2 - 68.) 



Microscopic characters : Quartz fairly plentiful, often 

 forming pegmatitic intergrowths with the felspar. 



Orthoclase equal to, or in excess of, the plagioclase. and 

 commonly intergrown as pert hit e. The alteration of the fel- 

 spars, which usually commences in the interior of the crystals, 

 results in the formation of flakes of sericite. 



Biotite. hornblende, epidote, and magnetite as in A n . 



Some inclusions of zircon, with pleochroic halos, occur in the 

 biotite. 



(A 4t ) Tourmaline-pegmatite; Cata, Llutah River. 



This vein consists essentially of clear quartz, pink orthoclase, 

 and tourmaline in individual crystals or radiating aggregates. 



Microscopic characters : Quartz clear, fluid inclusions 

 abundant with mobile bubbles, pegmatitic intergrowth witli 



