APR 2 1914 



THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XXII. — The Rodadero (Cuzco, Peru),- — A Fault 

 Plane of Unusual Aspect • by Herbert E. Gregory.* 



Introduction. 



Overlooking the city of Cuzco, perched high on a jutting 

 eminence, stands the famous Inca fortress of Sacsahuaman. 

 The enormous size of the blocks used in its construction, the 

 incredible labor expended in quarrying and transporting, and 

 the skill exhibited in nicely fitting the stones into walls with- 

 out mortar, give this structure a unique place among the works 

 of prehistoric man. Seven hundred feet north of Sacsahuaman 

 and at a somewhat higher elevation is the equally famous 

 Rodadero, a grooved and polished rock mound in which have 

 been cut the " Seats of the Incas" (fig. 1). 



Topographically the Rodadero is one of four knobs or bosses of 

 intrusive igneous rock rimmed about with massive blue limestone 

 which forms the plateau overlooking Cuzco from the north. 

 Three of these intrusive masses are more or less cloaked with 

 vegetation and the products of weathering ; the Rodadero, how- 

 ever, is bare, singularly fresh and firm, and gives the impression 

 of a ledge of rock stripped and polished by the hand of man. 

 The insignificant influence of weathering revealed by the exposed 

 surface of the Rodadero is worthy of comment, in view of the 

 fact that at nearby localities disintegration of rock of identical 

 character has reached an advanced stage. Thus in the banks of 

 the Tullumayo the firmer portions of the basic igneous ledges 

 are separated into blocks with rounded edges, and other por- 

 tions consist of concentrically weathered bowlders embedded 

 in a disintegrated groundmass (fig. 2), and still other parts of 

 the original mass are represented by a yellow-white, clay-like 



* Geologist, Peruvian Expedition of 1912. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIT, No. 220.— April, 1914. 

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