STKATIGRAPHY * 731 



.there is likewise a marked unconformity between the Totora series, of 

 which the Espejos formation is a part, and the overlying Bermejo series. 



L08 MONO® SHALE 



The oldest formation exposed within the Front Eanges sonth of the 

 twentieth parallel is a succession of shales and intercalated sandstones, 

 to which the name Los Monos shale is applied because of the excellent 

 exposures in the Quebrada de Los Monos in the Sierra de Aguarague 5 

 miles southwest of Villamontes. Other outcrops of this formation have 

 been observed in the gorge through which Eio Vitiacua traverses the 

 northern part of the Sierra de Aguarague near Machareti, and at the 

 point where Eio Parapiti emerges from its canyon through the Cuestas 

 de Pipi, a few miles west of the town of Pipi. It is also probable that 

 the lowest beds in the Sierra de Mandiyuti, at the locality shown in 

 figure 12, where Eio Cuebo crosses the Mandiyuti fault, and the lower 

 strata exposed in the escarpment at the east side of the Cuestas de Oquita 

 near Eio Parapiti should be referred to this formation, although these 

 beds are much more sandy than the strata at the type locality. 



The Los Monos shale is blue gray to black, thin-bedded and fissile, 

 containing an abundance of tiny flakes of muscovite mica and apparently 

 rather rich in carbonaceous matter. Interbedded with the shale there 

 are occasional layers of micaceous and argillaceous sandstone ; toward the 

 top of the formation, these intercalations become more frequent and 

 thicker; the shales also are more sanely and exhibit a dark brown color, 

 while the interbedded sandstones are much coarser and even conglomer- 

 atic. 



The base of the Los Monos shale has nowhere been observed and its 

 thickness can not be stated. At the type locality the exposed beds aggre- 

 gate a thickness of 1,000 to 1,200 feet. 



In the Vitiacua gorge near Machareti about 800 feet of the Los Monos 

 shale are exposed in the canyon walls immediately downstream from 

 El Chorro. At this point the Vitiacua-Machareti trail regains the 

 canyon floor, following its detour past the narrows. Here the Los Monos 

 shales are more arenaceous and are interbedded with thin sandstones, 

 but the general dark color, carbonaceous content, and abundance of mica 

 make quite sure the correlation with- the formation at its type locality. 

 No fossils were found in the Los Monos shale during the progress of 

 the reconnaissance studies on which this report is based. Dr. Bonarelli, 

 however, describes 7 a trilobite, Dalmanites (?) yentlandi, and a pele- 



7 G. Bonarelli, 1921, p. 54. 



