GEOLOGY AND PALEONTOLOGY OF CANAL ZONE


west it grades into undifferentiated volcanic rocks and west of the border of plate 1 volcanism continued much later.
 The marine strata constituting the La Boca marine member and the agglomerate making up the Pedro Miguel agglomerate member interfinger with each other and with the lower part of the tuff and generally finegrained agglomerate forming the Panama formation proper. All three were formerly given formation rank (Thompson, 1947a, p. 18-19, 20; Woodring and Thompson, 1949, p. 241-242). For the time being, however, member rank appears to be preferable for the La Boca and the Pedro Miguel.
 The Panama formation was named by Hill (1898, p. 200-202). That the name was casual is indicated by the expression "so-called Panama formation" on page 206 in his publication. The formation was named for exposures along the water front in the city of Panama, which is considered the type area. The names La Boca formation and Pedro Miguel agglomerate were proposed by Thompson (1943, p. 16-18). The Miraflores Locks area has been designated the type region of the La Boca marine member (Woodring and Thompson, 1949, p. 241) and the Pedro Miguel area is the type region of the Pedro Miguel agglomerate member. The thickness of the formation is estimated to be at least 300 meters.
 STRATIGRAPHY AND LITHOLOGY
 The La Boca marine member extends farther inland than any other part of the Panama formation. It overlies the Cucaracha formation or interfingers with the upper part of that formation. Nevertheless, if the La Boca is correctly identified, it also overlaps the Cucaracha and Culebra formations and rests directly on the Bas Obispo formation. The member consists principally of silty or sandy tuffaceous mudstone, flaggy tuffaceous sandstone, calcareous tuffaceous sandstone, conglomerate, and coralliferous limestone. Agglomerate and tuff, presumed to represent tongues from the Pedro Miguel agglomerate member and the main part of the formation, respectively, are other constituents. The stratigraphic relations of these strata, most of which contain marine fossils, were not understood until the subsurface explorations of the Geological Section of the Special Engineering Division revealed evidence that they overlie the Cucaracha formation. The fine-grained. strata formerly were assigned to the Culebra formation, sandstone to the Caimito formation, and limestone to the Emperador limestone member of the Culebra formation. No satisfactory outcrop section showing both a considerable part of the member and its stratigraphic relations is known. For that reason the Miraflores Locks area has been designated the type region. In that area there are outcrops, and relations to the Cuca-


racha formation are shown by subsurface sections. The town of La Boca, which furnished the name, was located near the entrance to Balboa Harbor, but was abandoned in 1954. Though the La Boca member overlies the Cucaracha formation, the lower part of the La Boca evidently is the southward marine equivalent of the upper part of the Cucaracha formation in the area of maximum thickness of that formation. The maximum thickness of the La Boca member is about 185 meters.
 Along and near the canal the La Boca member is represented principally by mudstone in both outcrop and subsurface sections. The mudstone is similar to that in the Culebra formation, but may be distinguished by the lower content of carbonaceous matter and the richer foraminiferal fauna of the La Boca. Locality 124, on the east side of the canal at canal station 1702, is the northernmost locality where such mudstone is now known to crop out along the canal. Core drilling, however, penetrated the La Boca farther north in a syncline on the west side of the canal.
 Mudstone of the La Boca exposed in the canal excavation between Paraiso and Pedro Miguel Locks (locality 130), at the north end of Mariflores Locks, and south of those locks (Geological Survey locality 6009) was described by MacDonald as part of the Culebra formation (MacDonald, 1919, p. 533-534).
 Fossiliferous calcareous tuffaceous massive sandstone of the La Boca is exposed in an abandoned quarry off old Gaillard Highway near Summit (locality 128). When the cuts along the present alinement of the Panama Railroad were fresh, MacDonald found fossils in similar but less massive sandstone in cuts north and south of Summit (localities 126 to 127b.) At locality 127b the sandstone is overlain by tuff that MacDonald identified as representing the Panama' formation. (See his data in description of locality 127b, p. 124.) Finegrained fossiliferous tuff and tuffaceous siltstone crop out at locality 132 near Red Tank, a village that has been abandoned since plate 1 was drafted. Flaggy tuffaceous strata, ranging in grain size from sandy siltstone to poorly sorted gritty sandstone, are exposed in a cliff at the mouth of Rio Masambi on the east side of the canal. These strata are considered part of the La Boca member. They are unlike any strata in the Culebra formation and, like the La Boca member elsewhere, contain molds of Acila cf. A. isthmica.
 Cream-colored and gray coralliferous limestone of Emperador type at the base of the La Boca member overlies, and partly interfingers with, the Cucaracha formation on Gaillard Highway 400 meters northwest of the junction with Madden Highway (locality 129). Similar limestone on Rio Masambi, on the east side of the canal (locality 123), lies directly on the Bas Obispo formation and has a thickness of 35 meters, the greatest


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