of the Cuzco Valley, Peru. 95 



clase, the chief mineral, associated with biotite and iron ore- 

 Quartz, calcite, and chlorite occur as secondary minerals. 

 Examination of the least-altered crystals proved the plagioclase 

 to be andesite — in one of the thin sections andesine and 

 labradorite. In one of the specimens studied the iron ore is 

 seen to have resulted from resorption of hornblende in a rock 

 whose original components were probably hornblende pheno- 

 crysts set in a plagioclase groundmass. Igneous fragments 

 embedded in the volcanic tuff and breccia have essentially the 

 same composition as the dikes, and the igneous portion of the 

 whole complex may be assigned therefore to the andesite family. 

 The sedimentary rocks associated with the igneous masses at 

 Atasccasa are further described on p. 59. 



Chchiraura. 



Along the east bank of Rio Chchiraura, below the village 

 from which the stream receives its name, igneous rock may 

 be seen penetrating the Huayllabamba sandstones. In the 

 lower portion of the valley the rock is dense and massive; 

 farther up it becomes a tuff. The rock, where fresh, is black 

 to purple in tone, with a dense groundmass in which feldspars 

 are distributed as phenocrysts. Weathered surfaces bring out 

 the porphyritic texture, showing white laths and dark cubes 

 embedded in a gray groundmass. 



As revealed by the microscope the dense variety consists of 

 plagioclase feldspar, iron ore, apatite, chlorite, and calcite. 

 The plagioclase, which occurs as phenocrysts and also as 

 minute laths, was determined as labradorite. Apatite in tiny 

 prisms is included in the feldspar. The chlorite and perhaps 

 also the iron ore have probably been derived from a ferro- 

 magnesian glassy matrix. The texture is hyalopilitic, but the 

 glass of the groundmass has been greatly modified. The 

 original rock was doubtless a basic andesite ; it is so completely 

 altered that closer classification is impossible. Sections pre- 

 pared from rock near the upper surface of the Chchiraura 

 mass consist essentially of lapilli and ash derived from andesite. 

 The feldspars are albite and oligoclase with lesser amounts 

 of orthoclase and andesine. These crystals of feldspar rest 

 in a groundmass of minute feldspar crystals and abundant 

 grains of iron ore. The iron is secondary in origin, being 

 probably derived from a ferromagnesian glass in the matrix. 

 Calcite, muscovite, chlorite, and a little quartz complete the 



