GEOLOGY AND PALEONTOLOGY OF CANAL ZONE


at which fossils were collected. A decision was reached to compile a geologic map, based on the material in the publications and files of the Geological Section of the Special Engineering Division, supplemented by scattered personal observations. The resulting map on the scale of 1:75,000, issued as a separate publication in 1955, is reproduced with minor alterations as plate 1 of the present report. The quality of the map is very uneven. Parts of it show the geology in considerable detail; other parts are greatly generalized and represent rapid reconnaissance. Despite its defects, however, it shows the geologic setting of the Canal Zone and adjoining parts of Panama. The only comparable map was published by MacDonald (1915, pl. 4; 1919, pl. 153) on a scale of about 1:260,000. The base used for the 1955 map does not show the recent suburban expansion of the city of Panamai.
 At first the present report was planned to consist of a discussion of the stratigraphy, summaries of the occurrence of fossils other than mollusks, and description of the Tertiary mollusks. While the work was in progress, an incomplete carbon copy of a manuscript by MacDonald on the geology of Panami was found among Dall's effects at the U. S. National Museum. This report, prepared soon after MacDonald's tour of duty as resident geologist in the Canal Zone, was mentioned by MacDonald (1913, p. 579) and Vaughan (1919, p. v). It was not finished because Dall's work on the fossils was not completed. One of MacDonald's duties as resident geologist was to recommend rock for use as concrete aggregate, as armoring for breakwaters and earth dams, and for other construction purposes. In carrying out this assignment he examined outcrops of igneous rocks, studied thin sections of them, and arranged for chemical analyses of some of the rocks to be made in the chemical laboratory of the U. S. Geological Survey. His description of the rocks, which is more complete than his published notes, and the chemical analyses are included in his manuscript. They have been incorporated in the present report, although the analyses have already been published.
 Though the present report includes more than stratigraphy and paleontology, the title-chosen for brevity-is too comprehensive. Many aspects of the geology are omitted or are only briefly considered. This report is, in fact, a progress report so far as the geology is concerned. Much of the area covered by plate 1 has not yet been studied and it may be a long time before the entire region is adequately studied.
 The systematic paleontology deals with the mollusks in about 260 collections from all the fossiliferous Tertiary formations, which range in age from middle and late Eocene to early Pliocene. In chapter A, 78 species and subspecies of gastropods are described and 13 others


are recorded. These 91 forms are estimated to repre r sent about a seventh of the total available mollusca fauna to be described. Fifty of the 91 are from th Gatun formation, an indication of the size of the Gatun fauna.
 Fuller discussion of the age and correlation of th formations is planned for the final part of the report That part also is to contain a discussion of the broade) aspects of the succession of faunas, including their bear ing on the history of the Panam6 land bridge and th light they shed on paleoecology.
 ORTHOGRAPHY OF GEOGRAPHIC NAMES
 Spanish orthography, including accent marks, is used for geographic names in Panam6. In the Canal Zone, however, many names of Spanish origin are anglicized and for such names accent marks are omitted. The major streams cross the boundary, and therefore "rio" or "quebrada" is used for all the streams that are named, regardless of location. The plan just outlined results in "Panamd" for the name of the country, the capital city, and a geologic formation, but "Panama Railroad" and "Panama Canal" for two features in the Canal Zone.
 ACKNOWLEDGMENTS
 Brig. General J. H. Stratton, (retired, then Col.), Supervising Engineer in charge of the Special Engineering Division, and T. F. Thompson, Chief of the Geological Section of the Division, placed every facility at my disposal during the field work in 1947, and Mr. Thompson again in 1949, when the Division had practically completed its work and its staff was greatly reduced. Mr. Thompson has a wide familiarity with the geology of the Canal Zone and Panama, which lie freely shared, and he guided me to many localities where' fossils are available. Other geologists of the Geological Section were very helpful. Special acknowledgment should be made to S. K. Bartholomew, L. H. Henderson, S. M. Jones, J. M. Matthews, T. G. Moran, J. R. Schultz, R. H. Stewart, J. A. Tavelli, and L. C. Woolfe.
 The general geologic map (pl. 1) is based for the most part on material gathered by geologists of the Geological Section: principally a published map of the Gatun Lake area by S. M. Jones (1950, pl. 2); a map of the Quebrancha syncline by T. F. Thompson, a small part of which was published (Thompson, 1944); a map of an area east of Gamboa, between Rio Chagres and Madden Highway, by L. C. Woolfe; strip maps along the proposed sea-level canal (Thompson, 1947a, figs. 29-32); strip maps of the Chorerra route by J. R. Schultz. Mr. Thompson and Mr. Stewart offered valuable suggestions for filling in gaps. I, however, must assume responsibility for the map's shortcomings,


4