the one described by Howe: from Gatun to Mount Hope (Monkey Hill of Howe's time). As a result of faulty paleontological information, Howe excluded the oldest strata near Gatun from the Gatun formation and grouped them with the Bohio formation (1907, p. 113). It is now known that the oldest outcropping part of the formation is not represented in the type region.
 The outcrop area of the Gatun extends from Maria Chiquita, 20 kilometers northeast of Colon (pl. 1), to Rio Miguel, 50 kilometers southwest of Col6n (fig. 11), but much of that area has not yet been examined. The relations of the Gatun to the next older formation in the Gatun Lake and Caribbean coastal districts-the Caimito formation-are unknown. In the Canal Zone the contact between the two formations is covered by the waters of Gatun Lake and even before the flooding of the lake perhaps all of the contact was concealed by swamps. East and west of the Canal Zone, however, the Gatun presumably rests on the Caimito formation at outcrop localities, as shown on plate 1. So far as now known, no deposits of early Miocene age are included in the Caimito formation of the Gatun Lake area. The boundary between the two formations therefore is presumed to represent a discontinuity representing early Miocene time. Still farther east the Gatun formation overlaps the Caimito and directly overlies the Cretaceous(?) basement. At the west end of the outcrop area the upper part of the formation is interpreted as overlapping on the Caimito formation, not on the basement complex as previously surmised (Woodring and Thompson, 1949, p. 243).
 The dip of the Gatun is low, between 5' and 10', and flattens out northwestward toward the coast. Nevertheless a water well at Mount Hope penetrated a thickness of 425 meters of Gatun strata without reaching the base of formation (Thompson, 1947a, p. 20). The total thickness is estimated to be at least 500 meters and perhaps a considerable thickness is concealed by overlap.
 STRATIGRAPHY AND LITHOLOGY
 Massive medium- to very fine-grained sandstone and siltstone are the chief constituents of the Gatun formation. They are somewhat calcareous, or marly, somewhat tuffaceous, and have a clay-like matrix. The sandstone contains numerous grains of black and greenish volcanic rocks and is practically a subgraywacke, as indicated by Boutan's (1880, p. 13) early account, the only description of the microscopic petrology so far published. Conglomerate and hard brittle very fine-grained tuff make up a small part of the formation. Basalt intrudes older formations in the


Gatun Lake area, but is not known to penetrate the Gatun.
 The Gatun formation has been subdivided in various ways on faunal grounds (Woodring, 1928, p. 76-77; Olsson, 1942, p. 244-247; Thompson and Keen, 1946). The subdivisions adopted for the present report correspond to the three faunal zones proposed by Thompson and Keen. Though the subdivisions are based on faunal grounds, they are simply designated lower, middle, and upper parts, at least until the study of the fossils is completed. Both fossil collections and observations on the lithology, however, are scattered and eventually some other nomenclature may be found to be more satisfactory.
 The lower part consists principally of medium- to very fine-grained sandstone. This part of the formation was unknown before the explorations of the Geological Section of the Special Engineering Division. In some exposures along the Transisthmian Highway and the road from the highway to Maria Chiquita, a basal conglomerate of variable thickness is present. It is most conspicuous along the Transisthmian Highway ihmediately south of Sabanita and is thin or absent along the road to Maria Chiquita. At locality 135 molds and impressions of marine mollusks were found in sandstone partings in the conglomerate: in the sandstone itself and in ferrugineous concretions. At some localities along the road to Maria Chiquita, sandstone is at the base of the formation and at others, where the base itself is not exposed, carbonaceous siltstone or mudstone, containing molds of marine mollusks, is close to the base. Fine-grained sandstone is exposed in cuts on the Transisthmian Highway between Sabanita and Cativa. Much of the sandstone, as at localities 136-138, contains numerous well-preserved fossils.
 The middle part includes the best known strata: those at and near Gatun, including the strata excavated for the Gatun Locks and the uncompleted Gatun Third Locks. The three members recognized by MacDonald (1913, p. 570) and the strata he described later (1919, p. 542-543) are in the middle part. Though sandstone is the chief constituent, the middle part includes conglomerate, siltstone, and tuff. When dry the tuff is almost white and forms conspicuous outcrops in excavations. It was designated fullers earth by MacDonald.
 The following section, described in a report by the Geological Section of the Special Engineering Division (Thompson, 1943a, p. 10-19, figs. 5-13 to 5-22) and by Jones (1950, p. 916-917, table 3), is exposed in the Gatun Third Locks excavation east of Gatun. The numbering of the units is that used by Thompson and Jones.


GEOLOGY


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