E. Howe — Geology of the Isthmus of Panama. 237 



the process of faulting, the sediments, previously deformed by 

 the intrusions of basalt, were still further twisted and now dip 

 at angles of sixty degrees or more to the northeast into Gold 

 Hill. ' 



Between Culebra and the Pacific the structure is simple. 

 The Culebra beds, frequently intruded by broad dikes of basalt, 

 are gradually carried by their prevailing southeast dip beneath 

 the level of the Bio Grande valley not far from Pedro ILiguel. 

 Prom Miraflores to the outskirts of the city of Panama the acid 

 tuffs have been locally folded, but in the neighborhood of 

 Ancon Hill and under the town their structure, as already 

 mentioned, appears to be due to initial dips of the beds depos- 

 ited on the flanks of the old Ancon volcano. That is, in 

 tracing the beds from north of Ancon around the hill to a point 

 on the shore close to Sosa Hill the strike is found to swing 

 through an angle of nearly one hundred and eighty degrees ; 

 the dips range from five to fifteen degrees, the steeper inclina- 

 tion being found near the base of Ancon Hill. 



Beviewing these facts, the structure of the isthmus appears 

 to be characterized by a dominant arch or broad anticline with 

 its axial trend between east-west and northeast-southwest, the 

 crest of the arch being in the south-central part of the isthmus 

 near Bas Obispo. The northern limb of the anticline is of 

 moderate inclination, while the southern limb, near the crest of 

 the arch at least, is steeper. The limestones and calcareous 

 sandstones of the upper Chagres are nearly at the crest of the 

 fold and their prevailing southwest to west dip suggests that 

 the anticline pitches to the southwest. TVhether this is a broad 

 structure or comparatively local is not known, no evidence 

 having been found in the country southwest of the Canal Zone. 

 It is probable that the westward pitch is more than a mere 

 local cross fold and due to the same uplift that formed the San 

 Bias Pange to the east. Unfortunately there is little or no 

 trustworthy information as to the geology of these mountains 

 other than that they are believed to have a core of granular 

 rock- intruded in late Tertiary time. It must be borne in 

 mind, however, that there is evidence of a decided orogenic 

 movement in this region in late Eocene time that caused the 

 unconformity at the base of the Pena Blanca Orhitoides beds ; 

 it is not impossible that the supposed southwest pitch of the 

 anticline may be due to the earlier deformation. 

 Washington, D. C. 



*Hill, op. cit., pp. 211-213. 



