of the marine type Aquitanian. An early Miocene age for the Culebra formation agrees with the age assignment adopted by the Geological Survey for the Tampa limestone. That assignment for the Culebra, including the Emperador member, is adopted for the present report, instead of the late Oligocene(?) and early Mioeene age assignment recently used (Woodring and Thompson, 1949, p. 239).
 CUCARACHA FORMATION
 The Cucaracha formation crops out along and near the canal in the Gaillard Cut area and southeastward to Miraflores Lake. Its distribution is much like that of the underlying Culebra formation, but it is recognized at a greater distance from the canal than the Culebra. The name was proposed by MacDonald (1913, p. 569). Up to that time the strata constituting the Cucaraicha formation had been included in the Culebra. The type region is in the southern Gaillard Cut area. The site of the village of Cucaracha was on the east side of the canal between the continental divide and Paraiso. The maximum thickness of the formation is about 190 meters. The discontinuity at the base of the Cucaracha generally is sharp and marked by conglomerate, but in central Gaillard Cut a transition zone lies between the two formations. (See p. 35.)
 STRATIGRAPHY AND LITHOLOGY
 The Cucaracha formation is the most distinctive and the most uniform formation in the Canal Zone. It was involved. in the extensive slides in Gaillard Cut during excavation of the cut and during t period of several years after the canal was opened. Its physical properties were exhaustively investigated during the studies of the Third Locks project, carried out by the Special Engineering Division.
 The principal constituent of the formation is massive generally grayish yellow green waxy highly slickensided bentonitic clay. Carbonaceous and lignitic clay, clayey siltstone containing yellowish gray calcareous concretions, tuffaceous clayey sandstone, and small-pebble conglomerate that has a tuffaceous matrix are minor constituents (Thompson, 1947a, p. 16-17). A bed of dacitic welded tuff is a useful and exact datumi plane. Its thickness ranges from 0.3 to 10 meters. Ii the type region of the Cucaracha it lies 85 meters above the base of the formation and 60 meters below the top (MacDonald, 1947, p. 9; Thompson, 1947a, p. 17). It is the only hard rock in the formation and looks much like a lava flow. In fact, it was described as a sill by Howe (1908, p. 231), as a flow by MacDonald (1913, p. 569), and was shown as a flow in MacDonald's structure sections (Natl. Acad. Sci., 1924, figs. 4, 5, op. p.


52). Later, however, it was found to be an aggloiieratic tuff (MacDonald, 1947, p. 9-10; Thompson, 1947a, p. 16). Hand specimens show feldspar crystals, flattened little lentils of dark clay, and greenish angular rock fragments. During the investigations of the Geological Section of the Special Engineering Division this bed of tuff was known as the ash flow. Description of thin sections of the tuff and a chemical analysis are presented on pages 54, 55. Chemical analyses of six samples of clay from the Cucaracha were published in the National Acadeiy of Sciences report on slides (Nat]. Acad. Sci., 1924, p. 54) and were reproduced by MacDonald (1947, p. 10). Both reports just cited also include descriptions of microscopic and other features of the clay (Natl. Acad. Sci., 1924, p. 54-66; MacDonald, 1947, p. 12-19, 65-70). No mineralogical study of the clay by tmodirn techniques has beeii undertaken.
 FOSSILS AND ACE
 The absence of marine fossils and the presence of plant debris in carbonaceous clay suggest that the bulk of the Cucaracha formation is noii1narine. The only plant remains recorded consist of wood (Berry, 1918). A few marine and brackish-water fossils have been found in the lower part of the formation: Anadara and Crassostrea in conglomerate and poorly preserved molds and impressions of Anadara, Lucina?, and Tellina? in carbonaceous clay (locality 122). The collection from locality 122 is the only collection now available.
 In November, 1956, R. H. Stewart found the distal end of a femur in the Cucaracha formation, about 10 meters above the top of the welded tuff, at Contractors Hill, in the Gaillard Cut area at the continental divide. It was examined by R. A. Stirton, of the University of California, who reports (in a personal communication) that it may represent a North American rhinocerotid or a South American notoungulate. In other words, the uncertainty is the same as that for the metapodial from the trasition zone between the Culebra and Cucaraehia formations (p. 37).
 The few fossils found thus far furiiish no reliable evidence concerning the age of the Cucarachia. It is assigned to the early 'Miocene because both the inderlying Culebra foriation and the overlying Panam formation are considered to be of that age. PANAMA FORMATION, INCLUDING 1A BOCA MARINE MEMBER AND
 PEDRO MIGUEL AGGLOMERATE MEMBER
 The Panama' formation is the youngest Tertiary forimation in the Gaillard Cut and Pacific coastal areas. It crops out in scattered areas in the central and southern Gaillard Cut area and more extensively farther southeast and east. It consists mostly of volcanic rocks, the youngest volcanic rocks in the Canal Zone. To the


GEOLOGY


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