34 GEOLOGY AND PALEONT

basin and the five species from the Pacific coastal area are typical Caimito species and typical upper Oligocene species. The age of the pyroclastic-clay member of Madden basin is based principally on an early species of Nodipecten found also in the middle member of the Caimito in the Gatun Lake area.
 The upper part of the Caimito in Madden basin, consisting of the Chilibrillo limestone member, the calcareous sandstone member, and the Alhajuela sandstone member, is assigned to the early Miocene on the basis of mollusks. The lower two members would be referred to the late Oligocene by those who claim that the Aquitanian and its essential American equivalents are of late Oligocene age. The Alhajuela sandstone member, however, is late early Miocene; that is, younger than the disputed Oligocene or Miocene. That it may include early middle Miocene, as suggested in a preliminary account (Woodring and Thompson, 1949, p. 236), appears to be unlikely.
 Only the Oligocene part of the Caimito is recognized in the Pacific coastal area. It is overlain and perhaps partly overlapped by the Panamd formation, which is correlated with the lower part of the Miocene strata in the Caimito of Madden basin. No fossiliferous strata as young as the Alhajuela sandstone member have so far been found in the Pacific coastal or Gaillard Cut areas.
 MIOCENE SERIES
CULEBRA FORMATION, INCLUDING EMPERADOR LIMESTONE MEMBER
 The Culebra formation is recognized along and near the canal in the Gaillard Cut area and immediately to the southeast in the region straddling Pedro Miguel Locks. (For a large-scale map of the Gaillard Cut area see plate 2.) To the southwest presumably it merges into undifferentiated and unmapped volcanic rocks, like other formations in the Gaillard Cut area. The Culebra itself contains volcanic debris, but not nearly so much as the underlying and the overlying formations. The name for the formation, in the form "Culebra clays," was first used by Hill (1898, p. 192195). The type region is in the central Gaillard Cut area, where the town of Culebra was located on the west side of the canal before and during the construetion period. The Culebra formation unconformably overlies the Las Cascadas agglomerate. The maximum thickness of the formation is about 150 meters. The thickness decreases northward, evidently as a result of overlap of successively younger parts of the formation on the Las Cascadas agglomerate.
 Coralliferous limestone exposed in a quarry near Empire attracted attention at an early date and was named the Empire limestone by Hill (1898, p. 195-196). MacDonald (1913, p. 569) changed the name to Em-


'C


LOGY OF CANAL ZONE

perador limestone, presumably because of the possibility of confusion with the Empire formation of Oregon. Empire was the American name for a tow; near Culebra, whereas the French used the Spanish, name Emperador for the same town. The town was located on the pre-construction alinement of the Panama Railroad near Culebra, approximately ati' locality 117 as plotted on plate 2. The quarry near Empire (locality 118 of plate 2), which is to be regarded,. as the type locality of the Emperador limestone member, is overgrown and unrecognizable, and so is the similar limestone formerly exposed on a street in Empire. Limestone agreeing with descriptions of the Emperador is still exposed along the canal. These beds of relatively pure coralliferous limestone probably are at different horizons in the upper part of the CulebraJ formation and probably grade southeastward into calcareous sandstone (Woodring and Thompson, 1949, p. 237). Should it be demonstrated that the name is being used for limestone at different horizons, the name should be abandoned, except for the limestone at the' type locality. In that event, however, a formal name would hardly be needed for a single locality, even if the locality were again found. In the meantime no serious errors should result from usage of the name. The coralliferous limestone has a maximum thickness of 15 meters. It therefore is a minor constituent and is given member rank in the Culebra formation.
 The Emperador limestone member of the Culebra formation is known to occur only in the northern part of the outcrop area of the Culebra. MacDonald's representation of the Emperador as widespread and resting unconformably on formations of different age was based on misidentification of limestone in several. formations (MacDonald, 1913, pl. 68). According to present interpretations, coralliferous limestone in the La Boca marine member of the Panama formation on Rio Masambi, in the Gaillard Cut area, was recently misidentified as Emperador (Woodring and Thompson, 1949, p 237).
 STRATIGRAPHY AND LITHOLOGY
 Stratigraphic secLions of the Culebra formation in Gaillard Cut have been published by MacDonald (1919, p. 535-539) and he also published structure sections of part of the cut (Natl. Acad. Sci., 1924 figs. 4, 5, op. p. 52). He divided the Culebra into lower and upper parts. The lower part consists chiefly of dark-colored, thin-bedded or laminated, fine-grained rocks: carbonaceous or lignitic shale, carbonaceous silty mudstone, tuffaceous siltstone. It includes, however, minor beds of tuffaceous and calcareous sandstone and conglomerate. The upper part is characterized by calcareous and sandy strata ranging in thickness from 0.3 to 3.5