6 STUDIES IN GEOLOGY, No. i 



GEOLOGY 

 EARLIER DESCRIPTIONS 



Certain outstanding features of the geology of Corocoro 

 have been recognized by all who have been there. There 

 are two series of rocks having a strike west of north dipping 

 away from a fault that runs parallel to the valley of the 

 Corocoro River north of the city and lies on the west side 

 of the valley. Both series of beds are prevailingly red in 

 color and are made up of a thick sequence of shales, sand- 

 stones, and conglomerates. The series of westerly-dipping 

 beds contains more sandstones and conglomerates than the 

 series of easterly-dipping beds. Numerous beds of both 

 series are impregnated with copper ores. 



The copper-bearing beds of the westerly-dipping series 

 are called "vetas" and those of the easterly-dipping beds 

 "ramos," and as a matter of convenience, these names have 

 also become attached to the rocks themselves. The term 

 "veta" is Spanish for vein and "ramo" the Spanish for 

 branch, terms that have neither genetic nor descriptive sig- 

 nificance in this application. They have been so generally 

 adopted, however, that it seems best to retain them as the 

 names of the respective groups of ore bodies ; and the series 

 of rocks in which each occurs will, therefore, be called the 

 Vetas series and the Ramos series, respectively. Since the 

 sandstone beds are thicker and more prominent in the 

 former series than in the latter, the vetas are said to average 

 thicker than the ramos, a difference implied in the name. 



Forbes (p. 42) describes the section of the Vetas forma- 

 tion from the Pontezuelo River eastward to the fault as 

 follows : 



"Starting from the westward, over a series of fine-grained red 

 sandstones, we come upon some coarser and more gritty strata in 

 which are embedded several seams containing copper . . . . ; 

 pebbly conglomerates are then passed over, some of which are also 

 impregnated with copper ; and we then arrive at the Veta de Buen 

 Pastor, a fine-grained sandstone, impregnated not only with copper, 

 but also with native silver. ..... The succeeding strata 



