72 



II. E. Gregory — A Geologic Reconnaissance 



The Yucay formation consists of limestone of two types — 

 (1) a black to gray massive variety, firm and compact, break- 

 ing* with conchoidal fracture; (2) a drab-gray, thin-bedded, 

 slightly arenaceous rock. The massive variety constitutes the 

 bulk of the formation; the laminated type occurs in masses 

 of small dimensions. Both types of rock develop light-gray 

 tones on weathering and both are sparingly fossiliferous. 

 The strata are traversed by joints and much broken by normal 

 and thrust faults of a few inches to a few feet displacement; 



Fig. 36. 



Fig. 36. Typical exposure of Yucay limestone, Kkallachaca Pampa. 



slickensided surfaces are common. An exposed ledge reveals 

 brecciation carried to an extreme; even a hand specimen may 

 consist of 10 to 15 angular fragments firmly re-cemented. 

 The entire Yucay formation may be described as consisting 

 of finely brecciated limestone broken into joint and fault 

 blocks which in turn form knobs and ridges tilted at various 

 angles and in nearly every direction. The rock outcrops are 

 not unlike the broken surface of a lava flow (fig. 36). 



Under such conditions measurements of strikes and dips 

 have only local value and the determination of the attitude 

 of the formation as a whole was found impossible in the time 

 at my disposal. Moreover the intricate faulting of the sand- 

 stone beds inclosing the limestone has obscured the relations 

 between the Yucay formation and the Huayllabamba forma- 



