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



Chilarios the oyster beds are found with fragments of Ostrea 

 contracta. The development of the Fayette sands in the vi- 

 cinity of the Conchos River is quite extensive. They form 

 the western flank of the Sierra de Pomaranes and the con- 

 tinuation of these hills to the south and west of the river, 

 overlapping the lower beds of the Tertiary to a contact with 

 the Cretaceous (San Juan) limestones east of Burgos Peak. 

 It is only the erosion of the river which has brought to 

 light the few exposures of the lower beds of the Eocene, 

 which we have described as in its basin. The upper beds 

 of the Fayette which cross the river at and east of San 

 Diego and the San Pedro Ranch, are composed of massive 

 rough weathering sandstone with a few large oysters. These 

 extend up the river valley to within a mile of Alendez, where 

 they rest on the Yegua. They appear west of this in a hill 

 near Piedras. As nearly as we can determine, they form the 

 plateau north of the river. The same beds are seen forming 

 the plateau southward toward Burgos ; on a creek west of 

 Burgos, dipping west at high angle and overlain by Equus 

 beds; and are found again three miles east of Burgos. They 

 also form the body of Mt. Corcovada, where they show a 

 westerly dip. The road from Burgos to Cruillas has few 

 exposures and these only of yellow clays, but between 

 Cruillas and Choreras the gray sandstone of the Fayette oc- 

 curs in a hill, and it is found lying in low ridges a few' miles 

 southwest of Choreras. This is the last exposure we are able 

 to identify as Eocene in this area. 



To the west the Cretaceous limestones and shales come in 

 and continue down the Soto la Marina to Abasolo, while 

 from Choreras southward, the San Fernando practically 

 covers all the lower beds and rests directly upon the Cre- 

 taceous. 



Frio 



On the Rio Grande the Frio beds are found capping the 

 Fayette just south of Roma and disappearing under the Oak- 

 ville below Rio Grande City. They form the surface rocks, 

 where the Reynosa is absent, in the valley and to the north of 

 the San Juan River for 30 miles or more southwest of 

 Camargo. 



