GEOLOGY AND PALEONTOLOGY OF CANAL ZONE


may be suitable for cement. According to an oral communication from Thompson, the widespread limestones of the Gatuncillo formation are too high in magnesia.
 Rock for construction.-MacDonald (1915, p. 35-38) adequately covered construction material used in the building of the canal. The great quarry in dacite on the west face of Ancon Hill, the quarry in basalt at Sosa Hill adjoining the Balboa docks, and the quarry in basalt on the west side of the Panama Railroad 3 kilometers northwest of Monte Lirio, are reminders of the construction period.
 Since MacDonald wrote his account, a quarry has been opened in hornblende andesite on the Chiva Chiva Road 6% kilometers northeast of Pedro Miguel.
 OIL POSSIBILITIES
 Though the central Panami. area of Tertiary marine sedimentary formations is small and the total thickness of marine strata is moderate, the oil possibilities deserve consideration, especially since the discovery in 1956 of oil in the Costa Rican part of the Bocas del Toro area. The southeastern end of the Costa Rican part of the Bocas del Toro area is shown in figure 1. At the time of writing (Sept., 1956) the discovery well, Union Oil Co. No. 2 Cocoles, located 1% kilometers north of the international boundary, had just been completed and stratigraphic data had not been released.
 In many respects the stratigraphic succession in the southeastern part of the Bocas del Toro area is similar to that in the central Panamd area, which includes the Canal Zone. On the islands of Bocas del Toro Archipelago a thin section of carbonate and other rocks of Pliocene age crops out. They are underlain in the archipelago and on the mainland by Miocene strata, designated by Olsson the Gatun stage or formation (Olsson, 1922, p. 10-16). The upper part of Olsson's Gatun consists of carbonate and fine-grained, calcareous detrital rocks of late Miocene age, correlated with the upper part of the Gatun formation in the western area of the present report. The lower part of Olsson's Gatun corresponds to the late middle Miocene middle part of the Gatun in the Canal Zone and also to the late middle Miocene upper part in the eastern area, although the depth-facies in the Bocas del Toro basin is shallower than that of the upper part in the Canal Zone. The equivalent of the early middle Miocene lower part of the Gatun in the Canal Zone is missing at the outcrop in the Bocas del Toro area or is partly represented by nonmarine conglomerate, rocks of finer


grain containing land plants, and lignite. Olsson's Gatun rests unconformably on the Uscari formation (or better Uscari shale), which consists almost entirely of moderately deep-water fine-grained rocks (Olsson, 1922, p. 10). Light oil issues from fractures in strongly deformed strata of the Uscari in the type region along Uscari Creek. The Uscari is of late Oligocene and early, Miocene age and corresponds in age to the Caimito formation of Madden basin. The oldest outcropping Tertiary strata in the southeastern part of the Bocas del Toro area are limestones, probably of both Oligocene' and Eocene age. The presence of subsurface marine strata of Eocene age is a reasonable expectation. There are two important differences between the two areas. No oil seeps have been found in the central Panama area and nothing in the Oligocene and lower Miocene outcrop section closely resembles the almost uniformly fine-grained rocks of the Uscari shale.
 Three districts in the part of the central PanamiA area covered by plate 1 are promising for testing oil possibilities: the Gatun Lake district, Madden basin, and the Pacific coastal district east of Panama City.
 Gatun Lake district.-Estimates of outcrop and probable maximum subsurface thickness in the Gatun Lake district are as follows.

Estimated outcrop and probable maximum subsurface thickness of
 sedimentary rock formations in Gatun Lake district


 Formation
Chagres sandstone Deposits of late Miocene age Gatun formation Deposits of early Miocene age Caimito formation Bohio formation--- - -- - Marine member of Bohio(?) formation
Gatuncillo formation

 Total


 subsurface outcrop thickness thickness in
 in meters meters
 250 250 Overlapped 100 250 500 Overlapped 300 300 400 300 300

 100 250 25+ 300

 1, 225+ 2, 400


 The outcropping formations in the Gatun Lake district are marine, with the exception of the Bohio formation, which is nonmarine throughout most of the area. On Barro Colorado Island, however, the upper part of the Bohio includes thin marine tongues of somewhat calcareous, medium-grained subgraywacke, suggesting that the nonmarine coarse-grained rocks are replaced seaward by marine rocks of finer grain.
 The upper part of the Gatun formation consists of more or less calcareous, sandy and silty rocks containing


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