532 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



of the formation weathers into red clay, and it covers the solid rock 

 to a depth of 10 to 25 feet, except where it is cut through by streams. 



PANAMA FORMATION. 



The Panama formation is a light-colored well-bedded tuff some- 

 what acid in composition. Locally it contains some argillaceous 

 beds. It outcrops extensively from Mirafiores to Panama and in a 

 few other places. The formation is at least 400 feet thick and over- 

 lies the Caimito formation. It seems to be relatively porous, fairly 

 well bedded, somewhat jointed, and of a crumbly or friable nature. 



Pliocene, 

 toro limestone. 



The Toro 1 limestone is sandy and fragmental, and locally is a 

 coquina or shell marl. Its type locality is at Toro Point, but similar 

 appearing limestone fringes the Caribbean coast, except where 

 large valleys have caused it to be eroded or covered with alluvium. 

 In places it forms low bluffs or headlands, expecially at Toro Point, 

 west of the Gatun Dam, and at the mouth of Chagres River. It is 

 the rock from which Fort San Lorenzo was built. Rock from this 

 formation was used as a hearting for the Toro Point Breakwater. 



CHAGRES SANDSTONE.* 



The name Chagres sandstone is proposed for a massive, coarse- 

 grained, rather soft sandstone, that locally shows stratification and 

 considerable cross-bedding. Some of the beds contain a few poorly 

 preserved bivalves and dark to brownish fragments of organic matter 

 are not uncommon. This appears to be a land or beach deposit, 

 and is probably as much as a thousand feet or even more thick. 

 It is tilted oceahward at angles ranging from 5 to 20 degrees and lies 

 on top of the Toro limestone on which it may or may not be con- 

 formable. It forms the hills overlooking the coast between Toro 

 Point and the mouth of Chagres River. 



Pleistocene. 



The Pleistocene formations consist of (a) swamp deposits, black 

 soil, and silt, filling old channels to depths of 325 feet below the 

 present sea level; (b) river gravels up to 10 feet above the present 

 normal river levels; (c) old sea beaches 6 to 10 feet above the present 

 beach level; and (d) bars, beaches, and the present river alluvium. 



DESCRIPTION OF LOCAL SECTIONS ACROSS THE ISTHMUS OF PANAMA. 



The black-faced numbers in the following descriptions are the station numbers in 

 the United States National Museum register of Cenozoic invertebrate collections 



i Called Caribbean limestone by the author in reports of Isthmian Canal Commission for 1912 and 1913. 

 See p. 63 and p. 570 of report for 1913. 

 2 The name and description of this formation were added to the proof of this article. 



