88 II. E. Gregory — A Geologic Reconnaissance 



assistance lias been given by James S. Stewart, graduate student 

 in the geological department of Yale University, working under 

 the supervision of Professor L. V. Pirsson. 



Ttica-Ttica Pass. 



Overlooking the ancient highway leading through the pass 

 between Cuzco and Anta is an elongated knob of igneous rock, 

 part of the encircling wall of the Cuzco Basin. The ledge is 

 largely concealed by a superficial cover of weathered material, 

 4 to 10 feet deep, in which are included large bowlders and 

 fragments of blue limestone so thoroughly recemented in places 

 as to present the appearance of a stratum of conglomerate. 

 The igneous rock composing the mass is greenish gray in tone ; 

 its texture is microcrystalline except for the presence of min- 

 erals of secondary origin, which are abundant, especially in 

 weathered specimens. Epidote occurs in irregular lumps as 

 much as a quarter of an inch in diameter; and the joints in 

 the rock are lined with films of epidote and with sheets, specks, 

 and stellate masses of glistening black hematite, which give to 

 the rock a unique appearance. 



When viewed in thin section the rock from Ttica-Ttica Pass 

 is seen to consist of orthoclase, plagioclase, apatite, titanite, 

 and iron ore associated with minerals of secondary origin — 

 epidote, muscovite, chlorite, calcite, and hematite. The plagio- 

 clase, which greatly exceeds the orthoclase in amount, is in 

 large part albite with less abundant oligoclase ; it occurs as 

 small tabular phenocrysts and as irregular prisms in the ground- 

 mass. The crystals of apatite are unusually large for rocks 

 of this class. The few grains of titanite and the very abun- 

 dant chlorite present no uncommon features. As the rock 

 consists essentially of alkali feldspars it may be termed a 

 syenite. . 



A quarry on the south slope of the igneous knob has furnished 

 stone for constructing the aqueduct which supplies water to 

 Cuzco and appears well adapted for this purpose. 



SacsaJmaman and El Rodadero. 



The famous Inca fortress of Sacsahuaman, built of enormous 

 limestone blocks, overlooks the city of Cuzco from a height of 

 600 feet above the Plaza. The base on which it rests is part 

 of a cliff of igneous rock wdiich for a distance of nearly a mile 

 borders the city on the north. The pavements of steeply 

 inclined streets and ancient terraced gardens conceal the face 



