30 Bulletin 30 



shales Lignite. 2^ in. 



of Yellow sandy clay shales. 7 



Guayubin. Lignite. 3^ in. 



Nivaje Blue shales. Depth 



shale. unknown. 



Comparisons of Gabb' s Conclusions with Ours 



The Gravels. — Gabb recognizes but one set of gravels which 

 forms the top of his Sabaneta section and is correlated with the 

 Mao gravels. 



Our section shows that in addition to these gravels there are 

 older gravels intermingled with the blue clays. In short there 

 are superficial, probably Quaternary, gravels and Tertiary gravels. 

 The surface gravels are described in the Sketch of our Expedition 

 (pages 3-4, Bulletin 29) as reddish and extending all the way from 

 Las Matas to Sabaneta, and they form the gravelly knolls on top 

 of the savannahs around Sabaneta. These gravels also occur in 

 the river terraces. The Tertiary gravels noted in our Sabaneta 

 section were also found in the Cana section Zones H and I and 

 lie near the base of the formations studied by us. 



The Yellow Shales of Guayubin. — The differentiation into 

 yellow shales is not a good one because the blue . shales weather 

 yellow and their surface exposures are always yellow. To Gabb 

 all yellow shales are Guayubin shales but quite probably the 

 Guayubin shales are simply weathered equivalents of the blue 

 shale. However as Guayubin was rebel we could not study the 

 formations there satisfactorily. 



As noted (Bulletin 29, page 7) in the region traversed by 

 our party the so-called blue and yellow shales were really hard 

 clays not consolidated nor laminated. Hence we have through- 

 out this memoir called them clays. 



The Blue Shales. — These are also called the blue shales of 

 Santiago and the Nivaje shale. The latter name was given be- 

 cause of their typical development on the Nivaje arroyo which 

 enters the Rio Yaqui near Santiago. See Sketch Map facing page 

 10, Bulletin 29. Most unfortunately, the Revolution prevented 



