E. Howe — Geology of the Isthmus of Panama. 223 



faulting and considerable folding have taken place so that the 

 relations of the various rocks are not perfectly clear. There 

 can be no question, however, that the breccias are a part of 

 the upper Culebra beds, for in several places a simple sedimen- 

 tary contact between the two kinds of rock was observed, and 

 it was even possible to collect hand specimens showing the 

 transition from fine shales to the breccia. The breccias are at 

 many places cemented by calcite of secondary origin. Speci- 

 mens collected at Gold Hill (Cerro Culebra) and Paraiso, 

 although considerably decomposed, may be recognized as made 

 up of fragments of pyroxene-andesite of different textures, 

 some of which have an abundant glassy groundmass. A boring 

 near Paraiso (Kilometer 58) showed similar rocks at several 

 horizons, while interstratih'ed with them were other beds of 

 sediments exactly like those in the lower or main part of the 

 Culebra section. It would appear, therefore, that toward the 

 close of Culebra sedimentation volcanic eruptions took place 

 and that the ejectamenta were laid down conformably on the 

 sediments ; a number of such eruptions evidently occurred 

 with intervals of quiescence, during which the deposition of the 

 sediments continued. The association of these breccias with 

 the Culebra sedimentary beds corresponds closely with that 

 of the JBohio breccias and conglomerates with the exception 

 that the Culebra breccias appear in the upper part of the sec- 

 tion while those at Bohio seem to be near the base. Unfortu- 

 nately the state of decomposition of the Bohio rocks prevents 

 satisfactory petrographical comparison between the two brec- 

 cias. In neither case does it seem possible to separate the 

 breccias from the sediments as distinct igneous formations. 



The relation of the Culebra sediments to the Obispo forma- 

 tion is more definitely shown than the Bohio rocks to the 

 Obispo on the Caribbean side of the isthmus. At the surface 

 the relations are not always clear, but it is possible by means of 

 borings made at frequent intervals to trace the Obispo breccias 

 from Bas Obispo south to a point «near Empire, where they 

 suddenly pitch in a southeast direction beneath the rapidly 

 thickening cover of the Culebra beds. There is a moderate 

 amount of local folding and faulting in the region and the 

 Culebra beds have a prevailing dip to the southeast, but their 

 inclination is much less than that of the southeasterly pitching 

 surface of the Obispo on which they rest. Near Corozal the 

 Obispo again appears at the surface, while between Corozal 

 and La Boca borings have shown that there are present, 

 beneath the alluvium, sandy shales precisely like many found 

 in the Culebra section. As mentioned later in discussing the 

 age of the Culebra beds, certain limestones that are thought 

 to belong to the upper part of the shale series rest on the 



