98 



H. E. Gregory— A Geologic Reconnaissance 



specimen taken from a dike at the south end of the Sierra- 

 Bella bridge the plagioclase was found to he andesine near 

 oligoclase, with the formula Ab ;i An x . Considered as a whole 

 the igneous rocks at Sierra-Bella may he classed as andesites. 



Rumiccolca. 



The lava field at Rumiccolca possesses features of unusual 

 interest. It is the largest exposure of igneous rock within the 



Fig. 43. 





Fig. 43. Inca wall of hyper sthene andesite, Rumiccolca quarry. Hiram 

 Bingham, photo. 



Cuzco region ; its presence is responsible for the position of 

 the lower Huatanay River; stone from Rumiccolca was exten- 

 sively used by Inca and Spanish builders and the present 

 quarries 32 are the principal source of rock used in modern 

 construction (fig. 43). Lava flows from at least three centers 

 within this area retain their original forms. Their surfaces 

 are marked by mounds and hollows, and small flows extend 

 like fingers from the sides of the larger masses. Caves and 

 steam cavities are present, and in certain places broken blocks 

 of scoria from the original ropy surface may be seen. Along 



32 For a picturesque description of the ancient quarries at Eumiccolca 

 see Squier, Peru, pp. 418- 419. 



