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



and the deposits may be reached from the Pacific side by 

 ascending the Chorrera River. The locality is indefinite, but 

 cannot be far from the head waters of the Trinidad, where 

 similar lignites have been reported. 



Structure. 



The broader features of the geological structure of the 

 isthmus are simple. In the Canal Zone the sedimentary rocks 

 of the Atlantic side dip toward the coast at moderate angles 

 while on the southern side of the isthmus the inclination of 

 the Culebra beds is in the direction of the Pacific. It has not 

 been possible to discover any marked characteristic structure 

 in the Obispo breccias ; in the central region the few contem- 

 poraneous flows appear to be nearly horizontal, while no evidence 

 of stratification can be made out in the occurrences near 

 Panama and Las Sabanas. 



When examined in greater detail it is found that the incli- 

 nation of the older sediments on the Atlantic side of the 

 isthmus is greater than that of the Monkey Hill beds. The 

 strikes also are different ; that of the Bohio formation, best 

 shown at Bohio and Yamos Yamos, being about northeast- 

 southwest, while the Monkey Hill beds at Gatun strike more 

 nearly east and west. This agrees with the evidence of uncon- 

 formity between the Bohio formation and the Orbitoides beds 

 of Pena Blanca. No indication of marked faulting has been 

 discovered in the region north of Empire, although local frac- 

 turing and minor dislocation have occurred at places where 

 the folding appears to have been pronounced as at Yamos 

 Yamos, according to Hill, and to a minor degree at Gatun. 



The acid tuffs at San Pablo are inclined to the northwest at 

 angles of from five to ten degrees ; with the exception of 

 isolated patches of Bohio conglomerate near Matachin these 

 are the last beds encountered, following the canal southward, 

 in which any structure can be made out until the Culebra beds 

 are reached near Empire. 



South of Empire greater structural complexit} 7 exists. The 

 Culebra beds are locally folded, but have a general south- 

 easterly dip of from ten to fifteen degrees. At several points 

 the Culebra cut is crossed by small faults, the downthrows 

 of from ten to fifty feet being to the southeast. At the deepest 

 part of the cut, where the canal passes between Gold Hill and 

 Contractors Hill, the Culebra shales with the breccias in the 

 upper part of the section have been intruded by basalt and 

 since intrusion have suffered faulting. The mass of Gold Hill 

 has dropped as a block or wedge between the beds on either 

 side, being bounded on the north and south by faults ; during 



