Vol. V] DUMBLE— TERTIARY DEPOSITS IN NORTHEASTERN MEXICO \yg 



course to its crossing of the railroad and Pesqueria River, 

 a short distance west of Herreras, and then southeastward 

 again to near Vaqueria on the Conchos, which is near its 

 southernmost exposure. 



Along the Rio Grande the Midway with its fossiliferous 

 beds is fairly persistent for some distance and forms the base 

 of the hills to a point a mile or more south of the mouth 

 of Penitas Creek. But it is not often the surface rock, as it is 

 usually covered by the Wilcox or Carrizo. 



The Wilcox appears only in limited areas, having prob- 

 ably been subjected to erosion before the deposition of the 

 Carrizo and while it occasionally appears on this river be- 

 tween the Midway and Carrizo, there are many exposures in 

 which it is lacking and the Carrizo rests directly upon the 

 Midway. This is well shown in the Cerrita Prieta and the 

 hills to the south. 



South of the river the brown, fossiliferous sandstone of the 

 Midway is well exposed for several miles and it is seen 

 again at a crossing of Amole Creek nine miles southwest of 

 Perros Bravos. 



On Amole Creek the Wilcox appears below the Carrizo in 

 places, while at others the Carrizo Sands rest directly upon 

 the Midway or even on the Escondido beds. 



West of Amole Creek the Midway sandstone is underlain 

 by shaly sandstones with small oysters, and these are under- 

 lain by the Escondido beds with Ostrea cortex. Along the 

 western side of the Ceja del Pescado, a low range of hills, 

 lying south of Azulejo, there appears a series of shales and 

 sandstones weathering to a dark brown, clayey soil carry- 

 ing the distinctive boulders of the Midway, and broken frag- 

 ments of Ostrea cortex as found at base of the Midway else- 

 where. This range of hills stretches southward for some 

 miles. 



The Carrizo, as has been stated, has a very wide exten- 

 sion in the northern part of the area and forms the top and 

 eastern slope of the various small groups and ranges of hills 

 north of the Arroyo Agua Verde and of the longer range 

 known as the Ceja del Macha and Ceja Madre, which extends 

 from Azulejo nearly to the Salado River. 



South of the Salado, the conditions seem to be different 



December 31, 1915. 



