526 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



The localities for the fossils are indicated as the station numbers 

 in the United States National Museum record book for Cenozoic 

 invertebrate fossils. In order to obviate confusion, as the Canal 

 Commission also had station numbers, the United States National 

 Museum numbers are printed in heavy-faced types in the descrip- 

 tions of the exposures, while the Canal Commission numbers are 

 printed in italics. 



SEDIMENTARY FORMATIONS. 



Eocene (?). 



BAS OBISPO FORMATION. 



The Bas Obispo formation is the oldest Isthmian formation, so far 

 as is known, and although referred doubtfully to the Eocene it may 

 be of pre-Tertiary age. It was formed of rock fragments and ash 

 blown from old volcanic vents. The debris settled over the sur- 

 rounding region and was subsequently cemented into fairly hard 

 rock by the slow-acting piocesses of rock cementation. Locally it 

 shows some rough bedding and some rounded water-worn fragments. 

 In composition it belongs to the andesitic tuff group of rocks, 

 although locally it might be classed as andesitic breccia. It outcrops 

 extensively at Bas Obispo and near old Panama, and small outcrops 

 rise above the alluvium near Miraflores and Diablo Ridge. 



LAS CASCADAS AGGLOMERATE. 



The Las Cascadas agglomerate also had its origin in fragmental 

 rock material blown from volcanic vents and later washed down into 

 different beds and masses. It rests unconf ormably on the Bas Obispo 

 formation, is much less consolidated and cemented than the latter, and 

 is of much more recent origin. It is a greenish to gray, basic agglom- 

 erate, which contains large and small subangular fragments in a 

 fine-grained groundmass of volcanic clay and tuff. The whole is 

 arranged in massive to roughly bedded deposits, often unconformable 

 with each other. Interbedded with these deposits are andesitic 

 flow-breccias, some fine-grained grayish and some coarse-grained 

 dark andesitic flows, and a few easily crumbled lava-mud flows which 

 show columnar jointing where exposed in the canal. The whole is cut 

 by large and small basalt dikes. The formation outcrops extensively 

 along the canal between Empire and Las Cascadas. 



Oligocene. 



bohio conglomerate. 



The Bohio conglomerate consists of beds containing water-worn 

 cobblestones and pebbles. These beds are separated from each other 

 by layers of sandstone and clay rock. The lower part of the forma- 

 tion contains more cobbles and pebbles than the upper part and seems 



