90 H. E. Gregory — .-1 Geologic Eeconnaissancc 



prisms; only in the freshest rock are its original characters 

 preserved; in many specimens it is represented by chlorite, 

 rarely by uralitic hornblende. Hornblende with green color, 

 characteristic cleavage, and crystal outline was observed in only 

 one section. Apatite is sparingly distributed as small prisms 

 and as inclusions in titanite. Black, opaque ilmenite with 

 rhombohedral twinning is displayed as small laths and squares. 

 Titanite, which appears to be both primary and secondary in 

 origin, occurs in irregularly shaped fragments with sharp angles 

 and with characteristic optical properties. Of the secondary 

 minerals chlorite developed after augite and probably also 

 representing hornblende and biotite attains typical develop- 

 ment. Colorless to dirty-brown leucoxene is found about the 

 borders and along cleavage planes of the ilmenite crystals. The 

 common calcite and rare zoisite and antigorite present their 

 usual features. Epidote is extremely abundant as grains 

 scattered through the rock, as isolated clusters, and as a coat- 

 ing for cleavage planes ; one thin section examined contains 

 practically nothing but epidote. The rock is noncrystalline 

 with granitic to porphyritic texture and may be classed as an 

 augite diorite porphyry. 



Two inconspicuous knobs about 2 miles north of El Rodadero 

 are composed of diorite porphyry in less-altered condition than 

 similar rock at Sacsahuaman. The texture is coarse, and partly 

 decomposed phenocrysts of augite assume larger sizes than the 

 minerals of the groundmass. Under the microscope oligoclase 

 feldspar with inclusions" of apatite is seen to be the most 

 abundant mineral ; orthoclase is rare. Hornblende is well- 

 defined six-sided crystals of dark-yellow tone and shreds of 

 pleochroic biotite are present; the augite is much decomposed 

 and relatively rare. Scapolite, iron ore, muscovite developed 

 after orthoclase, and chlorite occur. 



Detached outcrops in the bed of the Huatanay west of 

 Sacsahuaman were not studied, but they appear superficially 

 to resemble closely in composition the rock of El Rodadero. 



Stone from the Sacsahuaman mass taken from quarries near 

 the city was highly prized by builders of the Inca dynasty. 

 The best-preserved walls of ancient Cuzco, in which the stones 

 are fitted and polished with incredible skill, are formed of this 

 material (fig. 41). 



