GEO

 are undifferentiated in the southwestern part of the map area. These undifferentiated rocks, however, adjoin a strip along the canal where greater detail is shown than elsewhere on the map.
 CRETACEOUS(?) SYSTEM
 The oldest rocks in the Canal Zone and nearby, much older than any seen along the canal, are chiefly altered lavas and dioritic rocks. These extrusive and intrusive rocks are briefly described or mentioned under the heading "Igneous rocks.' Altered tuffs and other altered sedimentary rocks are associated with the lavas. None of these rocks, sedimentary or igneous, was studied during the field work and they were observed only casually. They crop out iu the eastern part of the Zone and extensively in Panami east of the Zone, making up the basement on which the Tertiary formations rest. This basement on the borders of Madden basin was designated the volcanic complex by Reeves and Ross (1930, p. 18).
 Everywhere the basement forms high, rugged forested uplands and mountains. The composition, structure, and age of these rocks are important aspects of Panamanian geology that remain to be studied. Contrary to the expectation of geologists not familiar with the tropics, the high-gradient streams in the rugged terrain characteristic of the basement offer a wealth of rock outcrops. Moreover, the streams are so numerous that a closely spaced network of outcrops is available.
 Strongly deformed altered tuffs were seen at a few localities. Three samples of different grain size, ranging from very fine-grained to very coarse and agglomeratic, were collected on the Transisthmian Highway 2 kilometers east-southeast of the bridge across Rio Gatfin. They were examined by W. S. Burbank, who found them to be moderately to strongly chloritized and carbonatized. The coarse-grained rock contains andesitic andlatiticfragmentsand some devitrified glass. The two samples of finer grain contain angular fragments of feldspar, pyroxene, iron oxides, and quartz. These rocks of finer grain are sheared and fractured, and cut by veinlets of calcite and a colorless mineral, probably a zeolite.
 Sedimentary and pyroclastic rocks of the basement complex at the Hyatt manganese prospects near Rio Boquer6n, the southernmost of which is shown at the north edge of the general geologic map (pl. 1), consist of siliceous limestone, quartzite, sheared agglomerate, and fine-grained tuff(?) altered to schist (Simons, in Roberts and Irving, 1957, p. 121, 124).
 Though the age of the basement rocks is unkownother than that they are older than the unconformably overlying middle and late Eocene deposits-they probably are Cretaceous, like widespread volcanic and


LOGY 13

 associated rocks throughout the Caribbean region (Woodring, 1954, p. 722-725). Late Cretaceous Foraminifera (Globotruncana and Gdmbelina) are reported to have been found in northwestern Panamd, near the Costa Rican border, in siliceous limestone that presumably represents the same major unit as the basement rocks of plate 1. The basement, however, may include rocks older and younger than Cretaceous. The altered tuffs sampled near Rio Gatdn are not the kinds of rocks that would be chosen as being likely to contain fossils. Yet thin sections of the three types sampled show indeterminable Foraminifera and Radiolaria. If further work on the basement rocks is undertaken, it doubtless is only a matter of time until identifiable fossils are found. The siliceous limestones are particularly promising for microscopic fossils.
 EOCENE SERIES
 GATUNCILLO FORMATION
 Formations of Paleocene and early Eocene age are unknown in the central Panama area. They may, however, be represented in the basement complex or by overlapped deposits in the structurally deeper parts of the area. The oldest known Tertiary formation is the Gatuncillo formation, which lies directly on the Cretaceous(?) basement.
 The Gatuncillo formation was named by Thompson (1944, p. 12-13) as the Gatuncillo shale. The type region is in the valley of Rio Gatuncillo on the east limb of the Quebrancha syneline. The name Tranquilla shale has priority, but that name was defined inadequately, principally on the basis of foraminiferal samples from a locality in Madden basin later flooded by Madden Dam (Coryell and Embich, 1937, p. 289; for location of Tranquilla see Reeves and Ross, 1930, pl. 5).
 The Gatuncillo crops out in the eastern part of the Canal Zone and east of the Zone. It forms rolling lowlands, which stand in contrast to the rugged uplands characteristic of the basement complex. The thickness of the Gatuncillo is estimated to range from 150 to 800 meters. The formation unconformably overlies the Cretaceous(?) basement. In the type region and in other areas wherever the succession is complete, the Gatuncillo is conformably overlain by the Bohio formation. In Madden basin, however, the Bohio is overlapped by the Caimito formation. In the Pacific coastal area the Gatuncillo does not appear between the basement and the Bohio formation, being overlapped by the Bohio. Collections of larger Foraminifera sent to T. W. Vaughan many years ago by A. A. Olsson and R. A. Terry indicate that Eocene deposits reappear farther east in the Pacific coastal area in the valley of Rio Bayano, 45 kilometers east-