Mollusks from Caimito formation of Gatun Lake area, Rio in the Gatun Lake area, the age of which is unknown, Mandinga area, and Quebrancha syncline (Calyptraeidae to Turritellidae) i tentatveoy groped .ith tht middle and tpper nootboro ao iato Oligocono. Locaite Rio Que. X50 e branGatan Lake area dinga banNo branMiddle member 0 loe recogmSton otood meN7er 56 57 57a 60 62 Trochita cf. T. trochiformis (Born) -- Netica (Naticarius?) spSinum sp - -_Globularia (Globularia) aff. G. fiocheri (D a ll) .... - _ _- .- _- ._ ._ Pachycrommium? cf. P.? trinitatensis (M ansfield) -- _------- __ ------Turritella meroensis Olsson _-_- _(Torcula) altilira Conrad, subsp-- ? X X X Collections of mollusks from Barro Colorado Island and Pato Horqueto Island are not included in the systematic descriptions of chapter A of the present report. (Pato Horqueto Island is one of the Brujas Islands northwest of Barro Colorado Island.) The Barro Colorado collections, which represent a moderate-depth facies, contain unidentified species of Solariella? (locality 541), Calyptraea? (locality 54m), Natica (Natica?) (locality 54k), Polinices (locality 54h and probably localities 54j and 54k), and Neverita? (locality 54m). Conglomerate on Pato Horqueto Island yielded mollusks of shallow-water facies, including Calyptraea sp., Sinum sp., Ampullinopsis spenceri, and Turritella sp. Ampullinopsis spenceri, a representative of a genus mainly of Oligocene age not found heretofore in the Canal Zone, is to be described in chapter B. Echinoids.-Fragmentary remains of Clypeaster are fairly common in some areas. C. W. Cooke identified a complete specimen from locality 60 as C. concars. Age.-A late Oligocene age for the fossiliferous parts of the Caimito in the areas described in the preceding pages is indicated by larger Foraminifera, corals, and mollusks. Most of the mollusks just listed would not be out of place in either upper Oligocene or lower Miocene deposits. Ampullinopsis spenceri [late Oligocene of Antiguo, Puerto Rico(?), western Panoima(?), Ecuador(?), and Perd(?)] and Turritella meroensis (late Oligocene of western Panam6, Ecuador, and Perd) however, indicate late Oligocene. The lower member TUFFACEOUS STRATA IN CHORRERA AREA Tuffaceous strata in the Chorrera area, west of the Canal Zone and south of Gatun Lake, are shown by a separate pattern on the geologic map (pl. 1) in a region of undifferentiated volcanic rocks. Though the unnamed tuffaceous strata consist principally of tuffaceous siltstone, tuffaceous sandstone, and tuffl, they include bentonitie clay, conglomerate, and agglomerate. Leaf bnprints are the only fossils that have been found. The unnamed strata, w which may be the equivalent of part of the Caimito formation of the Gatun Lake area, are doubtfully referred to the late Oligocene. BAS OBISPO FORMATION AND LAS CASCADAS AGGLOMERATE The Oligocene formations so far described contain more or less volcanic material, mostly in the form of tuffareous debris. The Bas Obispo formation and Las Cascadas agglomerate are entirely volcanic. They are interpreted to represent pyroclastic rocks and minor flows that accumulated at the periphery of a volcanic pile. The center of the pile evidently was west of the Canal Zone south of the continental divide, but presumably is concealed by later flows. At all events that region is characterized by thick volcanic rocks. Still farther out from the center of the volcanic pile the Bas Obispo formation and Las Cascadas agglomerate are thought to grade, respectively, into the Bobio and Caimito formations. The volcanic rocks now included in the Bas Obispo formation and Las Cascadas agglomerate were described as massive igneous rocks by Hill (1898, p. 189-191), and as "roche de Gamboa' by Bertrand and Zrcher (1899, p. 89). They were named the Obispo formation or Obispo breccia by Howe (1907, p. 110-111). The emendation to Bas Obispo formation and the splitting off of the younger part as the Las Cascadas agglomerate were proposed by MacDonald (1913, p. 568). The type region of both formations is in the northern part of Gaillard Cut, where they are the oldest exposed formations. Their thickness is unknown, but the combined thickness is presumably several hundred meters. According to plate 1, near Gamboa the Las Cascadas agglomerate rests on the Bohio formation, the Gatuncillo formation, or the basement complex. Confirmation of this overlap is needed. STRATIGRAPHY AND LITHOLOGY The Bas Obispo formation and Las Cascadas agglomerate probably would ordinarily be combined as one formation. They differ, however, in induration and GEOLOGY 31