of the Cuzco Valley, Peru. 89 



of the cliff within the city; along Rio Huatanay (Zappij its 

 full height is in view and the precipitous ravine of Tullumayn 

 trenches the rock slope from top to bottom. The rock compos- 

 ing the igneous mass is best exposed for study at the top of 

 the cliff, where four knobs or bosses of igneous rock, including 

 El Eodadero, rise above the general surface. All parts of the 

 igneous mass are essentially alike in composition, but the rock 

 presents very different aspects as the result of weathering and 

 the development of secondary minerals. At El Rodadero it 

 appears fresh and firm and the ledge retains the slickensides of 

 an ancient fault plane ; in the banks of the Tullumayo groups 

 of concentrically weathered bowlders embedded in a disinte- 

 grated matrix represent the rock ; in the banks of the Huatanay 

 complete decomposition has taken place and only a yellow-white 

 paste containing weathered crystals of feldspar and augite 

 remains. Large masses have been converted into epidote. 

 Hand specimens of the less-altered rock are dark gray in color, 

 with a greenish tinge when viewed in certain lights ; weathered 

 fragments are light green or even yellowish and marked with 

 bright-green spots. On the whole the rock is massive, dense, 

 fine grained, and of granitic texture ; but even in the freshest 

 hand specimens dark-colored crystals of augite and minute 

 clusters of epidote may be distinguished, thus producing a 

 porphyritic appearance. In certain localities aggregates of 

 yellow-green epidote varying in size from bodies as large as 

 a pea to clusters 8 or 10 inches in diameter are irregularly 

 distributed through the mass and are particularly prominent 

 along joint planes. At a few points the rock contains inclu- 

 sions 1 to 6 inches in diameter of white and pink marble and 

 of red, brown, gray, and pink quartzite. 



When examined under the microscope the rock from Sac- 

 sahuaman and El Rodadero is found to consist of plagioclase, 

 orthoclase, augite, titanite, apatite, ilmenite, leucoxene, epidote, 

 calcite, chlorite, zoisite, and antigorite. The plagioclase occurs 

 in broad tabular forms of two generations and exhibits albite 

 twinning. Though kaolinized and corroded, probably by 

 hydrothermal action, the plagioclase is subject to analysis by 

 optical methods and found to range in six different sections 

 from albite to andesine with an average composition of about 

 Ab 2 An-^ — that is, to be slightly more basic than oligoclase. 

 The orthoclase is small in amount, has irregular outlines, and 

 in most sections is replaced by secondary minerals. The augite 

 is green to colorless and occurs in tabular forms and stout 



