of the Cuzco Valley, Peru. 93 



chipped, broken, and pitted, and fragments may be pried from 

 columns and lintels with a pen knife. Disfiguration from 

 weathering is however partly compensated by increased 

 attractiveness of color. 



Ichchu-Orcco. 



The igneous mass of Ichchu-Orcco is topographically a 

 cuesta; its inner face, fronting Cerro Pachatucsa, is a cliff 

 340 feet high, and its top slopes southward at the rate of about 

 200 feet to the mile. Neither craters nor lava streams are 

 found, the surface of the rock is worn into knolls and ridges 

 thinly coated with erratics. The absence of features associated 

 with volcanic outflows is not, however, indicative of intrusive 

 origin or of great age, for the entire mass has been glaciated, 

 the evidences of ice work being particularly clear along the 

 west side of the cuesta. The geologic relations of Ichchu-Orcco 

 were not determined. Its history is probably similar to that 

 of Huaccoto, a local volcanic outflow of late Tertiary times. 



The igneous material composing Ichchu-Orcco includes two 

 varieties. One is a dense, glistening black rock devoid of 

 cleavage or flow structure, slightly porphyritic with biotite 

 phenocrysts, and very resistant to weathering. In the cliff it 

 presents a rude columnar structure and breaks out into blocks 

 2 to 4 feet in diameter, controlled by a system of nearly hori- 

 zontal joints. The other variety is a gray, highly micaceous 

 rock found at and near the surface. This type generally con- 

 tains minute open cavities even in fresh specimens, and the 

 more weathered portions are commonly amygdaloidal. Bowl- 

 ders found in the stream bed below the igneous cliffs and pre- 

 sumably from this mass, although possibly from the ash of 

 Pachatucsa, exhibit the scoriaceous texture characteristic of 

 surface flows. 



Microscopically the two types of rocks are identical in com- 

 position, except for difference in weathering and in amount 

 of glass present. The component crystals are plagioclase, 

 biotite, pyroxene, apatite, and iron ore set in a groundmass of 

 minute crystals and glass. The plagioclase occurs as laths, 

 square prisms, and six-sided crystal sections; its composition 

 is intermediate between andesine and labradorite, with about 

 equal quantities of albite and anorthite molecules as deter- 

 mined by the Michel-Levy method. Brown biotite is present 

 as shreds, crystals, and crossed twins. Pyroxene of two varie- 

 ties is found — extremely scarce monoclinic diopside of pale- 



