100 The Cuzco V'alley, Peru. 



As viewed in the ledge the rock from Rumiccolca is of two 

 types, which, however, grade into each other — (1) a massive 

 rock of light-gray tone, microcrystalline except for the presence 

 of flakes of biotite which dot the surface; (2) a banded rock 

 with alternate belts of white, light gray or pink, and dark gray 

 or black. The bands occur as individuals or in series and vary 

 in width from threads to ribbons 2 to 3 inches wide. Both 

 types of the rock are in places amygdaloidal, and in the banded 

 variety lithophysse are common. These evidences of former 

 fluidal condition are supplemented by the parallel arrangement 

 of part of the biotite. Fragments of sandstone are incorporated 

 within the lava. 



Microscopic examination shows the rock to consist of plagio- 

 clase, pyroxene, hornblende, biotite, and iron ore set in a dense 

 glassy groundmass. Transparent tridymite traversed by 

 numerous cracks fills the larger cavities. The laths of plagio- 

 clase twinned according to the Carlsbad law were determined 

 as bytownite. Hypersthene is the variety of pyroxene present ; 

 it occurs as distinct pale-green pleochroic prisms and also as 

 irregular masses included within the hornblende. All the 

 hornblende present is the brown variety common in basalts and 

 is much corroded. Biotite of characteristic aspect is a promi- 

 nent constituent. The texture is hyalopilitic to nuoidal and the 

 rock may be classed with the hypersthene basalts. 33 



Tinta. 



The volcano of Tinta, 75 kilometers southeast of the Rumic- 

 colca, is a conspicuous landmark on the Juliaca-Cuzco Railroad. 

 The topographic forms of the cone and the sprawling outflows 

 of lava are but slightly modified by erosion. Weathering of 

 the black scoriaceous lava is insignificant, and the cavities 

 within the rock are almost free of secondary filling. 



The lava of the Tinta volcano is a pyroxene andesite. The 

 constituent minerals are oligoclase, augite, hypersthene, biotite, 

 apatite, and iron ore embedded in a dark microcrystalline and 

 glassy matrix. Both fluoidal and amygdaloidal textures are 

 present. A detailed description of the Tinta lava is given by 

 Duenas. 34 



33 Under the name "labradorita a biotita" Duenas (op. cit., p. 192) 

 describes a rock "que afloran-en la seccion comprendida entre Urcos y 

 Sierra Bella" which may have been collected from the Rumiccolca quarry. 



34 Op. cit., pp. 190-191. 



Yale University, 



New Haven, Conn., October, 1915. 



