GEOLOGY AND PALEONTOLOGY OF CANAL ZONE


fossils. The volcanic rocks are intruded by dioritic rocks. This basal complex is probably of Cretaceous age, but so far all that is known about its age, in the region covered by plave 1, is that it is older than middle Eocene. It is much older than MacDoaald's (1913, pl. 4) igneous complex-a map term lie used for volcanic and intrusive rocks of Oligocene and early Miocene age.
 The middle and upper Eocene Gatuncillo formation rests with marked unconformity on the basement rocks. It has been identified east of the Canal Zone and in the eastern part of the Zone. Lithologically it is more uniform than the Oligocene and lower Miocene formations.
 Formations of known or inferred Oligocene age are of wide extent and very heterogeneous. They are marine and nonmarine, volcanic and nonvolcanic, at least nonvolcanic aside from tuffaceous debris. They represent all of Oligocene time and for the most part are conformable to each other and to the Gatuncillo formation. In the Pacific coastal area, however, the Bohio formation overlaps the Gatuncillo formation and in the northern part of Madden basin the Caimito formation overlaps the Bohio. Except in the Gaillard Cut area, the Oligocene formations are marine or partly marine: the Bohio and Caimito formations. Though the Bohio formation (the older of the two) appears to be for the most part nonmarine, marine strata are found in it at different horizons. Isolated outcrops in the Gatun Lake area, inferred to represent a tongue of marine strata in the lower part of the Bohio formation, contain mollusks considered of eadly Oligocene age and larger Foraminifera assigned to the late Eocene. Smaller Foraminifera of early Oligocene age are found in the basal part of the Bohio in the Quebrancha syncline. In both the Gatun Lake and Pacific coastal areas thin marine deposits in the upper part of the Bohio contain late Oligocene fossils. The Caimito formation overlies the Bohio and is for the most part marine. Late Oligocene fossils, particularly larger Foraminifera, are widespread in the Caimito. In Madden basin, however, the lower part of the formation is late Oligocene and the upper part is early Miocene.
 In the Gaillard Cut area the place of the Bobio and Caimito formations apparently is taken by the wholly nonmarine and volcanic Bas Obispo formation and Las Cascadas agglomerate. They are considered Oligocene(?) on the basis of their inferred relations to the Bohio and Caimito formations. The Bas Obispo formation-the older of the two-seems to grade northwestward into the Bohio formation. The Las Cascadas agglomerate is thought to be the equivalent of the lower part, or perhaps all, of the Caimito formation of the Gatun Lake area.


 The upper part of the Caimito formation of Madden basin, including the formally named Chilibrillo limestone and Alhajuela sandstone members, consists of marine deposits of early Miocene age. Nonmarine and marine lower Miocene deposits in the Gaillard Cut area are subdivided into relatively thin formations: in ascending order, the Culebra formation (including the Emperador limestone member), the Cucaracha formation, and the Panama' formation (including the La Boca marine member and Pedro Miguel agglomerate mornber). These three formations contain more volcanic material than supposedly equivalent deposits in Madden basin. The marine Culebra formation transgresses northward across the Las Cascadas agglomerate. According to present interpretations, the La Boca marine member of the Panama' formation interfingers with the upper part of the Cucaracha, or overlaps the Cucaracha and Culebra formations and rests on the Bas Obispo. All three formations and some of the lower Miocene of Madden basin are assigned to the upper Oligocene by some paleontologists.
 The two Tertiary formations younger than those of early Miocene age are found only in the Gatun Lake and Caribbean coastal areas. These youngest formations are marine and, like the Eocene Gatuncillo formation, are more uniform lithologically than those of Oligocene and early Miocene age. The middle and upper Miocene Gatun formation is famous for its wellpreserved fossils. Though the field relations between the Gatun and Caimito formations are unknown, the two formations evidently are separated by a discontinuity and perhaps by slight discordance. East of the Canal Zone the Gatun overlaps onto the Creteceous(?) basement. The Gatun of the region covered by plate 1 is considered to be middle Miocene. The Gatun at the west end of the outcrop area, about 5G kilometers southwest of Colon, in the western part of the region shown in figure 3, is assigned to the late Miocene.
 The lower Pliocene Chiagres sandstone is the youngest of the Tertiary formations. It overlies and partly overlaps the Gatun formation. The thin Toro limestone member lies at the base of the Chagres in the eastern part of the outcrop area.
 All the Tertiary formations are somewhat tuffaceous but the oldest and youngest contain the least tuffaceous debris, the Oligocene and lower Miocene the most. Moreover, the Oligocene and lower Miocene formations show a progressive southwestward increase in volcanic and intrusive rocks, until in the southwestern part of the region covered by plate 1 there is nothing but volcanic and intrusive rocks. The change is strikingly shown on plate 1; in fact, more so than would be the case if the map were equally detailed throughout. The volcanic rocks themselves and also the intrusive rocks


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