Vol. V] DUMBLE— TERTIARY DEPOSITS IN NORTHEASTERN MEXICO ]^g3 



beds exposed in the Loma Larga. East of this cutting we 

 found gray sandstones and shales and brown, ferruginous 

 sandstones which extend east within a mile of La Laja. 

 Among the fossils found in them were Ostrea alahamiensis, 0. 

 selldformis, Venericardia planicosta, Pyriila, sp. (?), Cas- 

 sidaria, sp. (?) and many smaller forms. Continuing south 

 of the Pesqueria, there is found three miles southeast of 

 Comitas a succession of low ridges caused by the upturned 

 edges of sandstones of this age with an abundant fauna, 

 which, however, is very firmly embedded in the calcareous 

 sands. On the San Juan River west of China and between 

 Barranca and San Juan, the Marine beds of clays and sands 

 with Ostrea alahamiensis are found. The road from San 

 Juan to Vaqueria shows many exposures of these beds. 

 A hill half a mile west of La Ciga gives, at its base, a good 

 section of the clays and sands with Venericardia potopacoen- 

 sis and other forms. At Jaboncillos Ranch, 20 miles south of 

 San Juan, a calcareous band was found, composed largely of 

 Venericardia potopacoensis and this stratum was followed 

 for 10 miles in a southeasterly direction toward the Conchos. 

 It dips N. E, 10°. While no fossils were found which posi- 

 tively identified them as Marine, it is probable that the yellow 

 clays and sands along the river east of Vaqueria belong to 

 this substage. 



Yegna 



The beds belonging to the Yegua substage form the surface 

 rocks on the Rio Grande from eight miles south of Laredo 

 nearly to the mouth of the Salado River. Their eastern 

 border in this area is found west of Guerrero on the Salado 

 and extending southward, crossing the road from Mier to 

 Monterey, a short distance east of the Borregas Ranch ; the 

 Brownsville-Monterey railroad, 6 miles east of La Laja; the 

 San Juan River just west of China, and the Conchos west of 

 Mendez. Its fossils are principally oysters and are usually 

 poorly preserved. They are probably Ostrea alahamiensis. 

 At the foot of a hill a short distance east of Borregas Ranch, 

 there is an exposure of beds of purple sandstones and clays. 

 one of which is composed almost entirely of the cannon-ball 

 concretions of the Yegua. From these beds we collected the 



