GEOLOGY AND PALEONTOLOGY OF CANAL ZONE


report, are shown in figure 4. Some features cannot be shown properly in figure 4: the age assignment for the Bas Obispo formation and Las Cascadas agglomerate is Oligocene(?), despite the seemingly definite position on the chart; the age assignment for the marine member of the Bohio(?) formation of the Gatun Lake area is late Eocene or early Oligocene, not late Eocene and early Oligocene as the chart suggests.
 The correlations differ only in some minor details from those of Woodring and Thompson in 1949 (fig. 2). The following changes have been made in age assignments: Gatuncillo formation, middle and late Eocene instead of late Eocene; Bohio formation of all five areas, early and late Oligocene instead of only early Oligocene (except in Pacific coastal area, which was not shown in the 1949 chart); Culebra formation, including Emperador limestone member, early Miocene instead of late Oligocene(?) and early Miocene; upper part of Gatun formation in western area (not shown in figure 4), late Miocene, instead of middle or late Miocene; Chagres sandstone, early Pliocene instead of late Miocene or early Pliocene. Though the Bas Obispo and Las Cascadas formations are still considered Oligocene(?), they are given a greater probable time range in the Oligocene.
 Some of the proposed correlations are unsatisfactory. There is no satisfactory faunal or lithologic correlation between Madden basin and the Gaillard Cut area. On the contrary, at least parts of the Caimito formation of the Quebrancha syncline, the Gatun Lake area and the Pacific coastal area can be correlated with the lower part of the Caimito of Madden basin on fairly satisfactory faunal grounds. Plate 1 shows at a glance that Gaillard Cut and the Pacific part of the canal are close to the eastern border of a pile of volcanic rocks. Madden basin, the Quebrancha syncline, and the Gatun Lake area are farther seaward in the marine basin. The coarse pyroclastic rocks and flows of the Bas Obispo formation and Las Cascadas agglomerate, and the coarse pyroclastic rocks in the Panama formation represent seaward extensions of volcanic rocks from the volcanic pile. In Madden basin and the Quebrancha synchne such rocks are much thinner and are found only in the volcanic member of the Bohio formation and the pyroclastic-clay member of the Caimito formation. Pyroclastic deposits in the Gatun Lake area are of much finer grain than in Gaillard Cut. Tuffaceous debris is present in the Tertiary formations of all the area, but is more dominant in the Gaillard Cut area and the Pacific part of the canal than in Madden basin, the Quebrancha syncline, or the Gatun Lake area. The Cucaracha formation, for example, consists almost entirely of altered volcanic ash.
 The lack of studies of the pyroclastic rocks and the


tuffaceous constituents of other rocks is the most serious deficiency in present knowledge of the geology of the Canal Zone and adjoining parts of Panamd. Such studies are likely to yield clues to correlations of the formations that may be more convincing than the faunal correlations.
 IGNEOUS ROCKS
 The following brief account of the igneous rocks is based principally on MacDonald's manuscript on the geology of Panamd mentioned on page 4. Much of the original manuscript, including some pages of the part dealing with the igneous rocks, is not preserved and his rock specimens and thin sections are no longer available. MacDonald's 1915 publication (p. 27-30) contains more data on the igneous rocks than his other publications.
 The igneous rocks may be divided into two age groups: Cretaceous(?) and Tertiary. That classification embodies the chief addition to MacDonald's treatment.
 CRETACEOUS(?) VOLCANIC AND INTRUSIVE ROCKS
 Altered basaltic and andesitic lavas are the most common rocks in the basement complex, at least along the Transisthmian Highway and the pipe-line road in the eastern part of the Canal Zone. Two samples from the eastern part of the Canal Zone were examined by W. S. Burbank, who reports that the rocks are similar in texture and composition to the basaltandesite rocks of the Southern Peninsula of Haiti (Woodring, Brown, and Burbank, 1924, p. 320-330). Chlorite, calcite, and a little epidote are the principal alteration products in the two samples.
 Basalt containing unaltered olivine is exposed on Quebrada L6pez (between Sabanita and Rio Agua Clara) at the Transisthmian Highway bridge. According to R. H. Stewart, similar olivine-rich basalt crops out in an extensive area northeast of the highway.
 Andesite at Porto Bello-the colonial settlement 35 kilometers northeast of Col6n-is presumed to be part of the basement complex. As described by MacDonald, the rock is dark. Under the microscope it is markedly porphyritic and the phenocrysts are andesine, labridorite, bronzite, and some augite. The groundmass is largely glassy, but contains some minute crystals of plagioclase. This rock was used for concrete in the construction of Gatun Locks and great slabs were quarried for armoring the Limon Bay breakwaters.
 The Cretaceous(?) lavas are intruded by dioritic rocks and dacitic porphyry. Though no debris from these intrusive rocks was noticed at the few localities where conglomerate of the Gatuncillo formation was observed, they probably antedate the Gatuncillo


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