GEOLOGY AND PALEONTOLOGY OF CANAL ZONE


therefore have different properties in excavations. Both crop out in the northern part of Gaillard Cut. and in'the Gamboa area northwest of the cut. The Las Cascadas agglomerate extends farther west along the canal than the Bas Obispo formation. To the southwest both merge into undifferentiated and unmapped volcanic rocks.
 According to Howe and MacDonald (in the publications just cited) and to accounts published by geologists of the Geological Section of the Special Engineering Division, both formations consist principally of agglomerate and tuff. The matrix of agglomerate of the Bas Obispo is hard sandy tuff so firmly indurated that the rock breaks through the larger constituents. Crude bedding is apparent in local thin deposits of the Bas Obispo made up of imperfectly rounded pebbles and cobbles. Such rock is not as well indurated as the agglomerate. The matrix of agglomerate of the Las Cascadas consists of soft fine-grained altered tuff and bentonitic clay. Beds of tuff in the Las Cascadas also are softer than those of the Bas Obispo. Both formations include andesitic and dacitic flow breccias and both are cut by a few andesitic dikes and by numerous basaltic dikes.
 AGE
 Fossils have not been found in either the Bas Obispo or Las Cascadas. They are doubtfully referred to the Oligocene because of their inferred relations to the Bohio and Caimito formations (p. 25, 28). The Bas Obispo and Las Cascadas presumably represent most of the Oligocene, not only early Oligocene as previously suggested (Woodring and Tiiompson, 1949, p. 228).

 OLIGOCENE AND MIOCENE SERIES
CAIMITO FORMATION OF MADDEN BASIN AND PACIFIC COASTAL AREA

 The Caimito formation of Madden basin, unlike that of other areas, includes both upper Oligocene and lower Miocene deposits. As shown on plate 1, in the northern part of the basin the Bohio formation is overlapped by the Caimito. The thickness of the Caimito in the basin appears to be about 450 meters. The deposits now referred to the Caimito formation were designated the Culebra formation, Emperador limestone, Caimito(?) formation, and Gatun(?) formation by Reeves and Ross (1930, p. 14-17).
 The strata in the Pacific coastal area assigned to the lower part of the Caimito appear to be a direct extension of that part in Madden basin. These strata in the Pacific coastal area have an estimated thickness of not more than 250 meters.


 STRATIGRAPHY AND LITHOLOGY
 Madden basin.-Five members are tentatively recognized in the Caimito formation of Madden basin. The only formal member names that are used areI those that have already been proposed, for detailed work may show that some arrangement other than that adopted in the present report is preferable. Two members are grouped as the lower part of the formation and the upper three as the upper part. In the follow-ing paragraphs the members are described in upward stratigraphic sequence.
 The calcareous sand stone-siltstone member overlies the Bohio formation or overlaps it and rests on the Gatuncillo formation. Sandstone of this member, ranging from very fine-grained to very coarse-grained and conglomeratic, is well exposed on Rio Chilibrillo upstream and downstream from the bridge on the road from Buenos Aires to Casa Larga. The sandstone is variably tuffaceous, and at least on Rio Chilibrillo the member includes massive coarse-grained tuff. The exposures on Rio Chilibrillo indicate a thickness of" at least 200 meters.
 The pyroclastic-clay member includes agglomerate, tuff, bentonitic clay, conglomerate, and limestone. Agglomerate may be seen on the Transisthmian Highway near Rio Chilibre on the east side of the basin. The strata on the east side of the basin also include three lenses of limestone, two of which are exposed on the Transisthmian Highway. The thickness of the member on the east side of the basin is about 110 meters. On the west side of the basin, where the member is represented by steeply dipping clay immediately north of Rio Chagres and just west of the Transisthmian Highway, the thickness is probably not more than 50 meters.
 The Chilibrillo limestone member-the lowest member in the upper part of the formation -consists of lenticular limestone that has a maximum thickness of about 30 meters. Detailed mapping may show that limestone of that thickness lies at more than one horizon. The name Chilibrillo was casually used by Olsson (1942, p. 234). The type region is on the east side of the basin near Rio Chilibrillo. Entrances to caves in the limestone are located at locality 81, about 150 meters west of the Transisthmian Highway, and nearby.
 The calcareous sandstone member overlies the Chilibrillo limestone member, or in its absence overlies the pyroclastic-clay member and in that event is at the base of the upper part of the formation. The most accessible exposures of the medium-grained


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