Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

24 ENGINEERING PROBLEMS OF PANAMA CANAL. covered by 10 to 25 foot of rod soil. They are easily eroded, so that the outcrops of this formation have mostly been worn into flats or valleys. Extending for more than a mile over wliat must have been, an-old land surface, and now forming an interbedded unit of this formation, is a light to dark grayish, or, on fresh fracture, greenish, lava-breccia flow of andesitic composition. Hand specimens of it show a few little shiny faces of feldspar crystals up to 2 mm. in length, set in a groundmass that resembles indurated clay. The brecciated frag- ments are small, somewhat altered, and seem to have been picked up from the formation over which the flow moved. Under the micro- scope the rock is seen to consist of euhedral phenocrysts of andesine ranging in size up to 1 by 2 mm. and some crystals of potash feldspar set in a cloudy claylike groundmass, dark scaly areas resulting from the decomposition of some mineral, considerable chlorite, some cal- cite, and a little secondary quartz. The outlines of the brecciated fragments were recognized but their original composition was ob- scured by alteration. This altered andesite flow is somewhat jointed and weak, so that it adds but little strength to the slopes and is practically no protection against slides. The prevailing grayish-green color of the formation is duo to the fairly high percentage of very finely divided chloritic material that it contains. These greasy mineral particles are a marked source of weakness and mobility and are ono of the factors that have caused maximum sliding in this formation. In contrast with the clays of the Culebra beds, those rocks are massive, largely of terrestrial origin, contain, little organic matter outside of the few lignitic shale beds mentioned, have a greenish, color from a high chlorite content, and are much moro given to sliding than the other rocks. EMPERADOR LIMESTONE. The Emperador is a light-colored, fairly pure limestone. It lies unconformably on several of the older beds. Its outcrops are com- paratively small and weather locally into a pitted and platy condi- tion. Near Las Cascadas a section cut by the canal (Pl. VIII), shows the limestone, about 25 feet thick, overlying the upper part of the Culebra formation. It outcrops northwest of Empire, south of Las Cascadas, on the new lino of the Panama Railroad, near San Pablo, near Frijoles,in tho swamp southeast of Diablo Ridge, and extensively near Alhajuela. Tho formation also has prospective value as a local source of limo and possibly of cement. In places it is somewhat hard and tough, and is almost as resistant to drilling and blasting as is the Bas Obispo formation.