44 STUDIES IN GEOLOGY, No. i 



most part filled with threads of native copper and there is not 

 the slightest doubt but that the remains were found in place 

 in the Ramos beds, although Forbes, with his preconceived 

 notion that these beds were of Permian age, is obliged to 

 predicate that the Macrauchenia fell into a deep fissure which 

 subsequently became thoroughly consolidated (p. 47.) 



The genus Macrauchenia was formerly thought to repre- 

 sent an ancestral Auchenia type, but later studies, particularly 

 of the rich Tertiary faunas of Argentina have demonstrated 

 that it represents the last stage of evolution of the wholly 

 extinct and exclusively South American mammalian order 

 Litopterna. This genus, which is confined to the Pliocene 

 and Pleistocene, spread in late Tertiary times from the 

 Argentine plains over much of the Andean region. Closely 

 related species are recorded from Tarija in southern Bolivia 

 and from UUoma on the Desaguadero River, 35 km. south 

 of Corocoro, the latter occurrence in beds that are probably 

 the same age as the Desaguadero series at Corocoro. The 

 Tarija beds are considered by Scott as Upper Pliocene in 

 age and correlated with the similar beds in the Argentine 

 province of Catamarca which are involved in the Andean 

 movements. 



The paleontologic evidence of the age of the Ramos series, 

 thus furnishes the strongest sort of evidence that these beds 

 are of late Pliocene or Pleistocene age. 



STRATIGRAPHIC RELATIONS OF VETAS AND RAMOS 



The Ramos formation has generally been considered 

 younger than the Vetas, But this interpretation has been 

 either a mere guess or based on an erroneous conception of 

 the geology of the district, and hence has not been supported 

 by facts. 



The meager paleontologic evidence indicates that the 

 Vetas are Pliocene and the Ramos either Pliocene or Pleisto- 

 cene. The former determination rests on floral evidence, 

 the latter on a vertebrate form, hence no sharp comparison 

 can be made as to relative age. That the Ramos are younger 



