536 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



Oligocene — Continued. Feet thick. 



6012e-6. Dark-brown, carbonaceous shale with little lenses 

 of lignite. Thickness irregular but maximum 



about 4 



a. Lower part of formation, contains some sandy and 



pebbly layers 17 + 



Culebra formation — 



1. Dark friable carbonaceous shales and dark-gray granu- 

 lar tufaceous beds. A few thin lenses and beds that 

 contain much gravel. Fossiliferous. Upper part of 



Culebra formation. Total thickness about 130 



h. Loosely cemented gravel that seems to mark an un- 

 conformity between the Cucuracha formation on 

 top and the Culebra formation below it. Some 



limy sandstone lenses at base of gravel 10-25 



(Normal faults with a downthrow of 10 to 40 feet on 

 the north side.) 

 6012a~y. Few lenses of limy sandstone at base of gravel; Lepi- 



docyclina pana?nensis ? 3 



6013-?". 1 Friable shale and clay 35 



6012c-A. Lens of sandy limestone 5 



(Made special studies of and collections from the 

 uppermost of these limestones and the shales be- 

 tween them. Found Amphistegina and Lepidocy- 

 clina, many oysters, and some Turritella.) 



g. Friable shale and clay 16 



/. Limy sandstone 8 



e. Friable shale and clay 10 



d. Bed of light-colored, limy sandstone, similar to (6).. 5 



c. Bed of friable shale and clay 18 



b. Lower bed of limy sandstone and sandy limestone. 

 Light colored and fairly hard and well cemented. 



Fossiliferous 8 



a. Lower layer of friable shale and clay 15 



Section on west side of canal from Canal Commission station 1720, near Empire, to 1740, 



near Culebra. 



(Total thickness of section, about 530 feet.) 



Feet thick. 



8. Hard and finely cemented mass of dark andesitic tuff and brec- 

 cia. Locally contains basalt fragments up to several inches 

 in diameter, cemented into a mass. Rough bedding, with 

 dip at a considerable angle, but formation generally massive. 

 (Fault contact between (7) and (8) shows sheared zone several 

 inches wide. Downthrow apparently on south side.) 

 7. Light gray, buff after weathering, andesite breccia, a flow. 

 6. Light-colored, fairly fine-grained, limy tuff, somewhat clayey; 



distinctly bedded 20 



Oligocene (Culebra formation) : 



5. h. Soil and, locally, old waste dump material. About 15 



g. Bed of very loosely cemented gravel, somewhat weath- 

 ered because of proximity to surface. About 20 



1 About this horizon but on east side of cut. 



