E. Howe — Geology of the Isthmus of Panama. 227 



Marls of Pena Blanca. 



[Lower Oligocene.) 



Less than a quarter of a mile west of the railway station 

 and quarries at Bohio at the base of a low hill are outcrops of 

 a hard, light yellowish marl. A quarter of a mile still farther 

 west the same rock has been exposed in excavations for a 

 diversion channel made by the French. The rock as a whole 

 may be described as a thick-bedded marl, buff to cream-colored, 

 and containing foraminifera in greater or less abundance, the 

 most typical being Orbitoides fortisi. Sparsely disseminated 

 through the rock are minute specks of a dark silicate and frag- 

 ments of feldspar. So far as is known, the only occurrences of 

 this rock are at the localities mentioned between Bohio and 

 Pena Blanca. 



JSTo where in the vicinity of Bohio was I able to discover the 

 relation of the Pena Blanca rock to the Bohio conglomerate or 

 breccia, nor is any direct evidence found in the borings, but 

 from the general field relations it would seem that the marls 

 are younger than the breccias, since less than half a mile east 

 of the marls borings have indicated the presence of the brec- 

 cias and conglomerates many feet below the observed elevation 

 of the marls, while as shown at the lock site exposures, the 

 Bohio rocks dip about fifteen degrees in the the direction of the 

 marls. Unless a fault of considerable magnitude occurs 

 between the two localities, of which there is no evidence, the 

 marls cannot be older than the Bohio beds, nor is there any 

 indication that they were contemporaneous deposits, borings 

 between Pena Blanca and Yamos Yamos having passed 

 through rocks of the Yamos Yamos facies of the Bohio alone. 

 Hill states* that he found the foraminiferal marls in uncom- 

 f ormable contact with the conglomerates at a locality known as 

 Pena Negra, one mile below Bohio. I was unable to identify 

 this point, but Hill's observation is in entire accord with my 

 opinion that the foraminiferal beds of Pena Blanca rest uncom- 

 formably on the rocks of the Bohio formation. 



Hill regarded the Pena Blanca beds as older than the Yamos 

 Yamos, basing his opinion on the northwest dip of the Yamos 

 Yamos beds and the position of the foraminiferal marls south- 

 east of them. I have shown, however, that in all probability 

 the Yamos Yamos beds and the Bohio conglomerates belong 

 to the same formation, and the discordance between the Pena 

 Blanca and Yamos Yamos beds is to be considered as another 

 manifestation of the same unconformity observed by Hill at 

 Pena Negra. This view agrees with Dr. Dall's opinion that the 

 Pena Blanca marls, on account of the presence of the charac- 

 *Op. cit., pp. 178-179. 



