708 ABSTRACTS OF PAPERS 



of igneous origin. Of the former the oldest two formations contain no fossils, 

 so their age is unknown. The lower part of the Bohio conglomerate is also 

 of unknown age. The other formations range from Lower Oligocene to late 

 Pleistocene. The Miocene is, so far as known, not represented in the rock 

 succession. Correlation across the Isthmus is a difficult matter because of 

 sparsity of outcrops ; hence further work may slightly modify the conclusions 

 reached herein. 



Number 1. Bas Obispo Formation 



The Bas Obispo formation, the oldest Isthmian formation so far known, and 

 probably of pre-Tertiary age, is a volcanic breccia with some local meta-con- 

 glomerate of andesitic composition. It outcrops extensively at Bas Obispo and 

 near Old Panama, and small outcrops rise above the alluvium near Miraflores 

 and Diablo Ridge. 



Number 2. Las Cascades Agglomerate 



The Las Cascades agglomerate overlies the basal breccia and outcrops ex- 

 tensively along the canal between Empire and Las Cascades. It consists of 

 greenish to gray basic agglomerate, which contains large and small subangular 

 fragments in a fine-grained ground-mass of volcanic clay and tuff. The whole 

 is arranged in massive to roughly bedded unconformable deposits, interbedded 

 with which 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. 



Number 3. Bohio Conglomerate 



The Bohio conglomerate consists of two members. The lowermost of these 

 is, so far, of indeterminable age, but seems to be considerably younger than 

 the Las Cascades agglomerate. It consists of gravel, cobbles, and boulders up 

 to several feet in diameter, fairly well bedded, but locally massive. Inter- 

 bedded with the coarser material are sandstone and argillite layers. The 

 uppermost member is of the same general composition, but contains some beds 

 of dark gray foraminiferal marls probably of early Oligocene age. The cobbles 

 and boulders in both formations are cherts, andesites, and diorites in composi- 

 tion, and were derived from the older intrusive masses of igneous rock, now 

 found at intervals along the central part of the Isthmian region. The forma- 

 tion is probably nearly 1,000 feet thick, and it outcrops extensively in the 

 vicinity of Bohio and between Gamboa bridge and Caimito Junction. 



Number 4. The Culebra Formatiox 



The Culebra formation is the oldest series that has, so far, yielded many 

 determinable fossils. Doctors W. H. Dall and T. W. Vaughan regard its fauna 

 ns possibly marine Eocene, but very likely Lower Oligocene. When they have 

 carefully studied the collections, the question of age will be cleared up. The 

 formation consists of an upper and a lower member. The lower member (a) 

 consists of dark, well laminated beds of soft shales, marls, and carbonaceous 

 clays, with some pebbly, sandy, and tufaceous layers, and a few thin beds of 

 lignitic shale. It outcrops in Culebra Cut, near Culebra, and near Pedro 

 Miguel. The upper member (&) consists of beds and lenses of sandy lime- 



