GEOLOGY


region in 1947, the boiler fire-box door (now missing) bore the name of a French manufacturer and the date 1883. The country rock is olivine-rich basalt, a typical exposure of which is readily accessible at the Transisthmian Highway bridge across Quebrada L6pez, a small stream 1% kilometers in a direct line northwest of the junction with the road to Nueva Providencia. The ore occurs in sulphide-bearing small quartz veins. No data are available on the mineralogy and tenor of the ore or on the tonnage that has been mined.
 Gold-mining operations at a stock of dacite porphyry southeast of Gamboa were carried on near the head of Quebrada de Oro, a small northwestward-flowing tributary of Rio Casaya. Locality 38 is located on Quebrada de Oro downstream from the adits (pl. 1). Remnants of mine and mill machinery and stretches of tram track are strewn along the stream. Mr. Adrian Bouche, of Pedro Miguel, the present owner of the property, orally reported that an English Company installed the mine and mill in the late 1870's or early 1880's, but that there is no evidence any gold was produced. The adits are located in a narrow aureole of mildly contactmetamorphosed sedimentary rocks of the Gatuncillo formation at the border of the porphyry stock. No attempt was made to enter the caved adits.
 Other adits and pits are located in the same stock of dacite porphyry, across the divide and 700 meters south of locality 38, near the head of a tributary of Rio Sardanilla, which flows southward and westward toward the Panama Canal. Boutan (1880, p. 31-32) mentioned a road built to haul machinery to a gold mine on Rio Sardanilla. The last French map, in the report of a Commission of the second French canal company that includes an account of the geology by Bertrand and Znrcher (1899), shows near Rio Sardanilla a "mine de quartz aurifere en exploitation."
 In his unpublished manuscript, written about 1918 and mentioned on page 4 of the introduction of the present report, MacDonald reported that none of some 40 samples from the most promising veinlets, "about 2% miles east of the canal opposite Las Cascadas," showed gold values of more than $1.00 to the tol. That statement evidently refers to the Quebrada de Oro area. He also reported on samples collected "a mile more or less in a southeasterly direction from these [those of preceding two sentences] old workings"; that is, in the Rio Sardanilla area. The gold value of his samples is as follows:


59


Gold value (as of about 1912) of samples collected in Rio Sardanilla
 area
 [Extracted from manuscript by D lacDonaldj
 Value of gold
 Sample per too Float from quartz vein-- - $6. 80 End of open cut__.---__-_-_---------------------- 1. 04
Open cout ----------- 41
Outerop, 'op of ridge -------- .20 Lower Cart of cot- -------T race
 Manganese.-Plate 1 was extended far enough to the north to show the location of the southern of two manganese prospects near Rio Boquerdn. An abandoned tram line extends from the prospects to the coast near Nombre de Di6s. The country rock consists of strongly deformed, low-grade metamorphic rocks-quartzite, siliceous limestone, micaceous seloist (probably metamorphosed tuff), and greenstone (highly altered alglomerate)-all representing the Cretaceous(?) basement complex. The ore deposits are manganese oxides associated with red jasper. Boulder-like masses of ore form great trains down the slopes and streams. These deposits were described briefly by Sears (1919) and are described in greater detail by Simons in a recent publication (Roberts, R. J., and Irving, E. M., 1917, p. 119128), fromwhich the preceding sentences were abstracted.
 In the publication just cited Simons mentions a minor manganese prospect in Madden basin, south of Rio Chilibrillo and about 2 kilometers south of Casa Larga. According to a written communication from T. F. Thompson, shallow pits and trenches scattered over an area of about a hectare show aggregations of manganiferous concretions and lenses in the Bohio formation.
 NONMETALLIC MINERAL DEPOSITS
 Limestone for cement.-Limestone in the Quebrancha limestone member of the Coimito formation is at present the most important nonmetallic mineral deposit. The limestone is quarried by the Cia. Cement Panama', S.A., immediately east of the Transisthmian Highway on the east limb of the Quebrancha syncline (locality 62, pl. 1) and is processed as an ingredient for cement in the company's adjoining plant. The thickness and properties of three grades of limestone and one of calcareous siltstone are described in Thompson's (1944) detailed report. The reserves are enormous.
 Other limestones in the Caimito formation of Madden basin and the Gatun Lake area, the Emperadorlimestone member of the Culebra formation, and at the base of the La Boca marine member of the Panamd formation