Section of basal part of Gatuncillo formation on south side of Rio
 Pequeni near head of Madden Lake (locality 1)
Gatuncillo formation: Meters
 Limestone, thin-bedded nodular-weathering; lowest 1 meter more granular and somewhat sandy.
 Scattered angular pieces and few pebbles of basement rocks (diameter of largest 8 centimeters) throughout lower half, more numerous in some layers. Lepidocyclina abundant; many Yaberinella in one layer. Collection (locality 1a) 3 to 4.5
 meters above base--_--- ---- 10. 6
 Unexposed (possibly sandy or silty strata) _---- 1. 5
 Limestone, thin-bedded, in thicker beds than overlying limestone. Contains Lepidocyclina and Yaberinella. Collection (locality 1) 0.5 meter
 above base -- ............-----.--------- 1. 5
 Unexposed (possibly sandy or silty strata) 1. 5 Limestone, few boulders of altered volcanics at base 5 Basement of altered volcanic rock.

 Thickness of section.--.... ---.--... --------_-.. 15. 6

 Fine-grained and sandy rocks presumably make up the bulk of the formation. Fine-grained rocks are not readily accessible, but they doubtless could be found by traversing streams. Sandy strata form a treeless area surrounding locality 11 and sandy strata in the upper part of the formation near locality 13 include practically pure quartz sand.
 Rio Agua Salud area.-The Rio Agua Salud area is the long, narrow strip of the Gatuncillo formation on the upthrown side of the Chinilla fault, in the eastern part of the Canal Zone. Coring along the alinement for a diversion channel for Rio Chagres-part of the Sea-level Canal project-showed the presence of the Gatuncillo formation in this area, the first record of Eocene deposits in the Canal Zone. R. H. Stewart and T. F. Thompson recognized larger Foraminifera of the Gatuncillo formation in cores from core hole SL84 (locality 29), drilled in 1947, and in float limestone at the core-hole locality. Their age identification was confirmed by W. S. Cole, who described the Foraminifera (Cole, 1949). The three localities in the Rio Agua Salud area are core holes (localities 29-31), wbich started and bottomed iis the Gatuncillo formation. The strata penetrated consist of mudstone, siltstone, silty sandstone, calcareous siltstone, silty limestone, and limestone. The depth of penetration ranged from 30.7 to 54.1 meters.
 Rio Frijol area.-The Rio Frijol area includes the outcrop area of the Gatuncillo formation in the drainage


basin of Rio Frijol and the smaller area farther east. The thickness of the formation along Rio Frijol is estimated to be between 300 and 400 meters. The lithologic types are similar to those elsewhere: mudstone, siltstone, sandstone, and thin beds of limestone. Some of the sandstone contains carbonaceous debris. For a discussion of the Gatuncillo formation in the Gatun Lake area seaward from the Rio Frijol area see page 61.
 Gamboa area.-A narrow band of Gatuncillo rocks lies in the lowland north of the high ridge north of Gamboa. The strata dip northeastward and appear to be overlain in normal succession by the Bohio formation. If the high ridge is underlain by basement rocks, as shown on plate 1, and if the Gatuncillo is not bounded by faults to the south or north, the thickness of the Gatuncillo is not more than 150 meters. The only outcrop of the Gatuncillo seen in this area consists of calcareous mudstone containing a thin layer of limestone (locality 37).
 Rio Casaya area.-Outcrops of fossiliferous rocks of the Gatuncillo formation were found by R. H. Stewart along Quebrada de Oro, a northwestward-flowing tributary of Rio Casaya about 4 kilometers southeast of Gamboa (locality 38). The stream may be recognized by mine-machinery debris and caved adits. Limestone, sandstone, and siltstone of the Gatuncillo formation are intruded by dacite porphyry. These sedimentary rocks are partly silicified. They probably are faulted against the Caimito formation, although no fault is shown on plate 1.
 No outcrop areas of the Gatuncillo formation are known southwest of the Rio Casaya area. Though the formation seems to be thinning southwestward, it may extend farther in that direction beneath the Oligocene and lower Miocene strata penetrated by the canal and the undifferentiated volcanic rocks farther southwest.
 FOSSILS AND AGE
 Soaller Foraminifera.-In most of the areas smaller Foraminifera are abundant in mudstone and siltstone. Coryell and Embich described species collected at Tranquilla, a village on Rio Chagres flooded by Madden Lake, and assigned the fauna to the upper Eocene (Coryell and Embich, 1937). H. H. Renz and P. J. Bermudez kindly identified the following species in five samples:


GEOLOGY


15