GEOLOGY


calcareous and tuffaceous sandstone are on the east side of the basin along the Transisthmian Highway. Highly calcareous sandstone is exposed at the north abutment of the Transisthmian Highway bridge across Rio Chagres. The thickness of this member is about 30 meters.
 The Alhajuela sandstone member is confined to a small area in the central part of the basin at and near Madden Dam. Before the construction of the dam the village of Alhajuela was located on Rio Chagres opposite locality 85. (For location of Alhajuala see Reeves and Ross, 1930, pl. 5.) The massive fine- to coarsegrained tuffaceous sandstone forms the foundation of Madden Dam and the gorge of Rio Chagres below the dam. Plate 7 is a view at the dam site. Fossil shells are more conspicuous in this member than in any other part of the Caimito formation in Madden basin. The thickness of the Alhajuela is 85 meters. The name, in the form "Alajuela sandstones", was proposed by Olsson (1942, p. 234, 243). The restricted usage suggested by his chart (Olsson, 1942, p. 234) is adopted in the present report.
 Pacific coastal area.-The lower part of the Caimito formation appears to extend continuously from Madden basin to the Pacific coastal area. Agglomerate, tuffaceous sandstone, tuffaceous conglomeratic sandstone, and conglomerate in the region between the basin and the coastal area are thought to represent the lower part of the Caimito. The geology of this intermediate region, however, is complicated by numerous intrusive stocks, and the succession of sedimentary strata and their relations to those in adjoining areas have not been worked out. Limestone at locality 97, just east of Madden Highway, contains a small Lepidocyclina suggesting L. canellei on the basis of field identification.
 In the Pacific coastal area the lower part of the Caimito is made up mainly of tuffaceous siltstone, tuffaceous sandstone, and conglomerate. Algal limestone, like that at localities 95 and 96, is a minor constituent.
 FOSSILS AND AGE
 Smaller Foraminfera.-Smaller Foraminifera were found in the calcareous sandstone-siltstone member of Madden basin on Rio Chilibrillo: in silty very finegrained sandstone at locality 68 and in sandy siltstone at locality 70. These collections have not been identified.
 Larger Foraminifera.--Lepidocyclina vaughani is widespread and abundant in the calcareous sandstone-siltstone member on the west side of Madden basin, ranges throughout that member in the exposures on Rio Chilibrillo, and occurs in the lower part of the formation in the Pacific coastal area. Despite an apparently favorable depositional environment, no orbitoids were


33


observed in limestone of the pyroclastic-clay member in Madden basin. In fact, an Archaias-like species is the only larger foraminifer noticed in limestone of that member. The species in the table that follows were identified by Cole.

Larger Foraminifera from lower part of Cainito formiation of
 Madden basin and Pacific coastal area atole, 1952 (193), p. 7]
 Locaities


Ileterostegina antillea Cushman _-_ Lepidocyclina (Lepidocyclina) canellei Lemoine and R. Doutttd -parvula Cushman _--- ___o
 yurnagunensis morganopsis Vaughan
 (Nephrolepidina) vasghani Cushman-Miogypsina (Aliogypsina) antillea Cushm a n _ -- - - - -- - - --_ - - _


 maddenn cifc
 basin coast64 67 69 95


--- ------_ _X
X X X X
 - - - - X


 Mollusks.-Mollosks occur in the Caimito of Madden basin, but none was found in the Pacific coastal area. Limestone in the pyroclastic-clay member (localities 71-73) and submerged calcareoos strata of the lower part of the Caimito (localities 65, 66) contain mollusks, but none of tle families covered by chapter A of the present report is represented in the collections. The typical form of Turritella altilira occurs in the Alhajuela sandstone member at locality 89. Specimens of '. altilira from the Alhajuela at localities 88 and 92, and from the underlying calcareous sandstone member at localities 77 (T. cf. T. altilira) and 80, are not sufficiently well preserved to determine whether they represent the typical form. Turritella gatunensis was found in the calcareous sandstone member (locality 82).
 Echinoids.-According to identifications by C. W. Cooke, Clypeaster lanceolatus occurs in limestone of the pyroclastic-clay member (locality 71) and in submer-ed calcareous strata of the lower part of the Caimito (locality 66), and Clypeaster cf. C. pinarensis in the calcareous sandstone member (locality 84a).
 Age.-The lower part of the Caimito formation in Madden basin, consisting of the calcareous sandstonesiltstone member and the pyroclastic-clay member, and the formation in the Pacific coastal area are considered of late Oligocene age, like the entire Caimito of other areas. The two species of larger Foraminifera from the calcareous sandstone-siltstone member in Madden