710 ABSTRACTS OF PAPERS 



conglomerate. It is dark gray in color, except locally, where many brown 

 specks indicating fragments of organic material are shown. (6) Mostly fine, 

 soft sandstone for about 100 feet thick, containing a few fossile. (c) Light- 

 colored, indurated clay beds. The formation is very extensive and constitutes 

 the foundation on which the Gatun locks are built. The upper part of the 

 formation weathers into red clay.' and. except where this is <:-ut through by 

 streams, it covers the solid rock to a depth of 20 to 25 feet. Large collections 

 of fossils have been made from these beds, and from a study of them it is 

 known that the formation was deposited in Oligocene time. The light-colored, 

 indurated clay beds forming the upper part of the Gatun series seem to corre- 

 spond to the Panama formation, while the Caimito formation is probably the 

 equivalent of the middle and the lower Gatun beds. 



Number 10. Caribbean Limestone 



The Caribbean limestone (Gabb's antillite) is a sandy fragmental limestone, 

 locally a coquina or shell marl. It fringes the Caribbean coast, forming low 

 bluffs on many of the headlands. Near the river mouths and the lower ends 

 of valleys it is generally absent. It outcrops at Toro Point, west of Gatun 

 Dam, at the mouth of the Chagres River, and is the rock from which Fort San 

 Lorenzo was built. From its fossils it is provisionally referred to the Pliocene. 

 In Costa Rica this limestone fringes the Caribbean shore in many places, and 

 inland from it are argillite beds of the same age. 



Number 11. Pleistocene Formations 



These consists of (o) bench gravels up to 100 feet above present river levels ; 

 (b) Swamp formations filling old channels to depths of 375 feet below present 

 sealevels; (c) river gravels 10 feet above present flood-plain levels; old sea 

 beaches 6 to 10 feet above present beach levels; (d) shoals, beaches, and 

 present river alluvium. 



The old channels mentioned under (&) were cut out by the rivers when the 

 land stood at least 400 feet higher than it now is. It then slowly sank to 

 about 8 feet below its present level. Its last motion was upward 8 feet, as 

 shown by the geologically very recently raised beaches west of Limon Bay. 

 and at many other points westward into Costa Rica. 



Igxeous Rocks 



The groups of igneous rocks are: (1) Quartz diorites and granitic rocks, 

 such as that of Cocovi Island and the granitic float locally found in the Chagres 

 River wash, indicating granite outcrops somewhere within the river valley: 

 (2) andesitic rocks, as at Point Farfan, opposite Balboa, locally in Culebra 

 Cut. and at other places within the Chagres River basin, as evidenced by the 

 andesite pebbles in the wash of that river: (3) rhyolitic rocks, as at Ancon 

 Hill, the rock used in the concrete of the Pacific locks: (4) basalts, as at 

 1'araiso. the top part of Gold Hill, near Rio Grande, and locally as dikes in 

 Culebra Cut. as well as at many other places within the Canal Zone; (5) mud 

 lava hows and breccias, as shown l.x-ally between Empire and Las Cascades: 

 meta-sediments and breccias, some of which may have been pushed up 

 cold, as crown masses on top of basalt intrusions. Geneticallv these latter 





