54f, doubtless corresponds to the foraminiferal marl of publications issued before the flooding of Gatun Lake. Foraminiferal marl was exposed at localities on Rio Clagres, including Penia Blanca, the type of locality of Lepidocyclia canellei. The approximate location of Penia Blanca is shown as locality 55 on plate 1. Hill claimed that foraminiferal marl could be seen to rest unconformably on conglomerate (of the Bobio ,formation) near Bohio (Hill, 1898, p. 178-179), but Howe (1907, p. 113) was unable to recognize the locality Hill described. The rhyolitic tuff on Rio Chagres at Barbacoas, where the original line of the Panama Railroad crossed the river about 9 kilometers west of Gansboa, presumably is to be included in the Caimito. The tuff was found to be so similar to tuff in the Panami formation that Hill (1898, p. 201), Bertrand and Ztlrcher (1899, p. 91), and Howe (1907, p. 117) did not hesitate to correlate them. Though no data are available on the comparative volcanic constituents of the different formations, the correlation 'is not accepted (p. 41). Hill casually used the expression "Barbacoes formation" for the tuff and "San Pablo phase of the Barbacoas formation" or "San Pablo formation" for underlying rock lie described as ,conglomerate of volcanic material (Hill, 1898, p. 184-185, 187).
 Tuff, agglomeratic tuff, tuffaceous siltstose, and discontinuous sandy tuffaceous limestone are the principal constituents of the upper member, the thickest and most widespread part of the formation (Jones, 1950, p. 901).
 The thickness of the Caimito in the Gatun Lake area is estimated to be at least 100 meters and may be considerably more.
 Rio Mandinga area.-Along a tributary of Rio Mandinga, west of the canal and south of Gamboa, the Caimito formation is characterized by a unit of conglomerate and conglomeratic sandstone that has a thickness of between 75 and 100 meters. Conglomerate is not rare in the Caimito of other areas, but it consists of thin scattered beds. The unusual thickness of conglomerate prompted Jones to propose the name "Caraba facies of the Caimito formation" (Jones, 1950, p. 901). The conglomerate is overlain by fossiliferous silty calcareous sandstone (localities 59 and 60) and limestone. Locality 61 represents coralliferous limestone in this area.
 The distribution of the conglomerate is unknown. The extensive area farther west along the south border of Gatun Lake, shown on plate 1 as doubtfully underlain by the Caimito formation, has not been examined.
 Quebranche syncline.-The Caimito formation of the Quebrancha syncline consists of two members: in ascending order, the Quebrancha limestone member
 413788-57---3


and the calcareous siltstone member. They save been mapped and described by Thompson (1944, p. 17-21). The economically important Quebrancha limestone member, which is quarried for the manufacture of cement, was named by Thompson (1944, p. 17) as a separate formation. The type region is on the east limb of the syncline and includes the quarry of the Panama Cement Company. The Quebrancha member has a thickness of 110 to 135 meters. Subsurface explorations and outcrops reveal that it is made up, in ascending order, of three parts: calcareous siltstone and calcareous medium-grained sandstone, limestone and thin partings of calcareous siltstone, and somewhat marly foraminiferal limestone consisting for the most part of closely packed specimens of Lepidocyclina. The foraminiferal limestone is by far the thickest part.
 The calcareous siltstone member, which gradationally overlies the Quebrancha limestone member, includes calcareous siltstone, tuffaceous pumice-bearing siltstone, and calcareous medium-grained sandstone. This member has an estimated thickness of 150 meters and is the youngest unit in the Quebrancha syncline.
 Rio Chagres area.-The lowland along Rio Chagres north-northeast of Gamboa probably is underlain by the Caimito formation, but only a small part of it southwest of Nuevo San Juan was examined. Calcareous coarse-grained pebbly sandstone at locality 93 and limestone and siltstone farther west contain Foraminifera, including orbitoids identified in the field as Lepidocyclina canellei and L. saughani. Limestone exposed at and near Las Cruces before the flooding of Gatun Lake yielded a few mollusks (localities 94, 94e).

 FOSSILS AND AGE
 Larger Ioraainifera.-Larger Foraminifera are widespread and locally abundant in the Caimito formation of the areas just described, particularly in limestone and calcareous siltstone. No fossils of any kind, however, are known in the lower member in the Gatun Lake area and larger Foraminifera from one locality are the only fossils available for the upper member in that area. Douvill6 recorded larger Foraminifera and calcareous algae in collections from localities near Pefia Blanca and Bohio Soldado, and expressed the opinion that they are Oligocene (Douvilli, 1891, p. 498, 499). Later he identified the small orbitoid from Peia Blanca as Lepidocyclina and reaffirmed the Oligocene age (Douvil6, 1898, p. 598-599). This small species, one of the most common in the Caimito, was still later named L. canellei for the collector, an engineer of the first French company (Lemoine and R. Douvill6, 1904, p. 20). L. vasghani, another common species, is extraordinarily abundant in the Quebrancha limestone


29


GEOLOGY