Hershey.] 



Istlimits of Panama. 



243 



they may have been forced up into mountain masses, had been 

 reduced by erosion to a comparatively low land much of which was 

 a slightly rolling plain — a Cretaceous (?) peneplain — and upon this 

 comparatively plain surface after submergence, the Santiago sand- 

 stone and shale were deposited and the breccia-conglomerate 

 formed by combined wave and current action. 



There are, perhaps, nowhere on the American Continent two for- 

 mations, so widely separated, so nearly resembling each other as the 

 Santiago series on the Isthmus of Panama and the Chico series in 

 the western part of Shasta County, California. They are identical 

 in composition, texture, lithification, color and relations to older 

 and newer strata. It seems hardly possible that all the conditions 

 of their deposition and subsequent history could have been so 

 nearly alike unless they date from about the same period. We will, 

 therefore, provisionally correlate these formations and assume that 

 the deposition of the Santiago formation closed the Cretaceous 

 period. 



The Tertiary Basal Conglomerate. — On the Aguadulce-Santiago 

 plain, apparently occupying broad shallow depressions in the 

 Santiago formation, there are large patches usually elongated from 

 northwest to southeast and having widths of one to five miles, of a 

 series of newer and softer formations. Southwest from Santiago 

 the basal formation is a conglomerate of dark dull red and brown 

 color, often well lithified and always distinctly a rock as distin- 

 guished from mere gravel. In places it contains a few imperfect 

 fossils. It is exposed to considerable thickness and may be 50 to 

 100 feet, but evidently varies much from place to place. It always 

 dips decidedly but rarely very steeply toward the center of struc- 

 tural depressions. Eastward from Santiago it is reduced to 10 to 

 25 feet of a rather coarse, moderately well indurated, heavily- 

 bedded sandstone of a dark purplish tint. 



The Tertiary Red Shale. — Resting conformably upon the con- 

 glomerate is a thin formation (10 to 50 feet, averaging about 25 

 feet) of soft shale and clay, bright red and dark reddish brown in 

 color, without fossils, and generally laminated but readily weather- 

 ing into structureless clay. It is persistent throughout the plain 

 wherever its horizon is exposed. 



