Jg2 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th See. 



Marine 



The beds belonging to the Marine substage of the Clai- 

 borne, as seen on the Rio Grande, have a wide development, 

 the river flowing through them from the mouth of Espado 

 Creek to a point eight miles south of Laredo, a total dis- 

 tance of 50 miles, but this is at an angle to the dip of the beds. 

 They narrow rapidly to the south and where they cross the 

 Salado River west of Guerrero, their outcrop is not more 

 than five or six miles wide. East of Cerralvo, they occupy 

 the area between La Masa and the General Trevino Ranch. 



Like the Midway, the Marine beds are well developed in 

 the valley of the Salinas. Indeed, the valley of the San 

 Juan, of which the Salinas is a part, gives by far the most 

 complete and satisfactory section of the Atlantic coast type 

 of Eocene deposits which we have so far found in Mexico. 

 Two miles north of Herreras, there is a hill 170 feet in 

 height, known as Loma Larga, with an escarpment facing 

 west and south, the beds of which have a southeast dip. 

 The section is : 



Feet. 



Reynosa limestone 4 



White clay with small concretions. 6 



Yellow clay with interbedded micaceous sand- 

 stone 30 



Greenish sandy clay 10 



Blue shale and clay with septaria 120 



The fossils identified from this locality include Venericardia 

 planicosta V. potopacoensis^ and others. An exposure on the 

 railroad half a mile east of Herreras shows similar beds 

 with numerous fossils belonging to the Marine stage. This 

 section was repeated in an exposure on the river south of the 

 railroad. A mile east of Herreras, a long shallow cutting 

 on the railroad shows a section of grayish yellow clay and 

 thin calcareous sandstone. Here w-ere found quantities of 

 well preserved fossils, consisting principally of Venericardia 

 planicosta and V. potopacoensis. The position and dip of 

 these beds would place them at the base of, or below, the 



6 This form is well known in the Maryland Tertiaries but has not been found 

 previously in the Gulf Tertiaries. 



