128 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



the San Juan. From Mendez the shales were traced west- 

 ward around the south end of El Abra Hills to Micos and 

 San Dieguito, where they hold the same relation to the Tam- 

 asopa limestone that they do west of Cruz. They lie against 

 the upturned edges of the limestone and extend to consider- 

 able heights above the valley. 



The section along the railroad between Micos and Las Pal- 

 mas is typical, showing the Tamasopa, San Juan, and Papa- 

 gallos in their usual relations but disturbed and faulted, and 

 a kilometre west of Las Palmas the Papagallos shales come 

 in sight resting against the massive Tamasopa limestone with 

 its rudistes fossils. 



There can, therefore, be little doubt that the beds be- 

 tween the scarp of Tamasopa limestone at Micos and the 

 El Abra Hills are the direct continuation of the San Juan 

 and Papagallos of the north. East of El Abra Hills, how- 

 ever, later beds may also be present. 



Bose says of this locality: 



"On leaving San Mateo the road turns again to the east 

 to descend to the large plateau of Valles. This plateau, cov- 

 ered by small hills, represents a broken up scale of Neo-Cre- 

 taceous shales Above Valles the structure be- 

 comes very simple. The Neo-Cretaceous beds are slightly in- 

 clined toward the east and between Valles and El Abra the 

 shales rest almost horizontally upon the Rudistes limestone." 



From the San Juan Mountains in Coahuila to the railroad 

 line at Valles is nearly 400 miles, and throughout this entire 

 distance, along the face of the Sierra Madres the San Juan 

 and Papagallos formations preserve their lithological charac- 

 teristics and their general relations to the Tamasopa lime- 

 stone. Numerous exposures in the valley between the Sierra 

 Madres and the Tamaulipas range show materials apparently 

 identical with the Papagallos, and both San Juan and Papa- 

 gallos (and probably Tamasopa) occur east of the valley in 

 the San Carlos Mountains. Wells drilled at Ebano, Topila, 

 and Panuco also prove that the same relations continue along 

 the floor of the valley in that vicinity, as platy limestones en- 

 tirely similar to the San Juan are found overlying the Tam- 

 asopa and underlying the blue shale. 



