03 II. E. Gregory — A Geofogic Reconnaissance 



developed and amygdaloidal cavities filled with calcite and 

 barite are seen; in other places the rock has a dense ground- 

 mass in which phenocrysts of biotite, commonly in well-formed 

 crystals, are prominent. Parts of the flows are massive and 

 microcrystalline. Inclusions of qnartzite and of slate and 

 segregations of biotite and of magnetite were noted. 



The Huaccoto lava may be classed as hypersthene andesite; 

 its constituent minerals, as revealed by the microscope, are 

 plagioclase, biotite, pyroxene, iron ore, and apatite. The 

 plagioclase, which ranges from andesine to labradorite, occurs 

 as laths and as square prisms with both albite and Carlsbad 

 twinning. The biotite occurs in relatively large brown shreds 

 with characteristic pleochroic colors. The pyroxene crystals, 

 which belong to the orthorhombic variety hypersthene, are 

 displayed as minute prisms ranging in pleochroic colors from 

 greenish yellow to reddish yellow. Iron ore in tiny grains is 

 scattered throughout the section, and prisms of apatite were 

 noted as inclusions in the feldspar. The texture of the rock 

 is hyalopilitic ; crystals of feldspar, hypersthene, and biotite 

 are embedded in a groundmass of cryptocrystalline fragments 

 and glass. 31 



The rock from Huaccoto is well adapted for construction. 

 Flow structure aided by jointing favors separation into slabs 

 suitable for paving, curb stones, and door facings ; softness 

 and uniformity of texture of selected blocks render it suitable 

 for carved stone work for exterior and interior decoration, and 

 the red color of weathered portions is attractive. These 

 qualities have made the Huaccoto volcano the seat of a quarry- 

 ing industry for hundreds of years. Rudely paved roads con- 

 nect the numerous quarries, and the ruins of workmen's huts 

 are surrounded by piles of chips and partly finished blocks. 

 During the Spanish building epoch in particular the Huaccoto 

 lava was extensively used. The fagades of the churches at San 

 Geronimo and San Sebastian and of the Jesuit monastery now 

 used by the University of Cuzco were probably built of stone 

 from this locality. Even at the present time a few small 

 quarries are in operation, the stone being carried by llama, 

 burro, or cargador down the difficult trail to villages 3,000 

 feet below. As compared to the diorite extensively used by 

 the Incas, the basalt from Huaccoto weathers rapidly in the 

 Cuzco climate and where found in ancient buildings it is 



31 The Huaccoto lava lias been previously described by Dueuas (op. cit., 

 pp. 192-193). 



