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

28 ENGINEERING PROBLEMS OF PANAMA CANAL. to cut the diorite at Point Parfan, opposite Balboa. Rocks of this general character form the steep headlands at Porto Bello and con- stitute the material quarried and crushed there and transferred to Gatun for use in the concrete work of the locks. Huge pieces of this rock, from the Porto Bello quarry, were transported on lighters and used to armor the breakwater at Toro Point. It is a typical andesite, hard, strong, and dark colored. Shiny little crystal faces appear scattered over the surface of hand specimens of it. Under the micro- scope it shows many large crystals of andesine and andesine lab- radorite, together with largo crystals of augite and some of hyper- sthene, all set in a fine-grained dark to brown groundmass. This groundmass contains minute crystals or microlites of essentially the same, composition as the large crystals. Rock obtained at Point Farfan, opposite Balboa, where the French had an old quarry, shows a groundmass relatively coarse for andesite. In fact, the large crystals or phenocrysts of andesine and augite are not much bigger than groundmass or small crystals in which they are set. Quartz is also present in this rock in irregular masses and in micropegmatitic intergrowths with feldspar. The rock is a quartz andesite in composition. The well-crystallized character of both the Porto Bello and the Point Farfan rock renders them tough and resistant to abrasion by waves; hence they are well suited for breakwater or sea-wall con- struction. RHYOLITE. Rhyolite has the same chemical composition as granite, but is much finer in grain. Crushed rhyolite was used for the concrete work of the Miraflores and Pedro Miguel locks; therefore it is here somewhat fully described. It is the rock that forms Ancon Hill, and Culebra and Naos Islands, in Panama Bay. Ancon Hill is a slightly curved, roughly tabular intrusion, about half a mile long and several hundred feet thick, cutting the softer Panama formation of light-colored bedded tuff. It stands as a much elongated cone or short curved ridge, 654 feet above sea level and nearly 600 Jeot above the lowland that surrounds it (Pl. IX). The rock is much fissured and jointed, so that it is easily crushed. It weathers to a light-buff color, but is bluish gray on fresh fracture. In the hand specimen it shows a fine texture with some small lathlike phenocrysts, the larger ones having faces about 1 by 5 mm. in size. Microscopically the rock shows some flow structure, especially around the phenocrysts, which are largely oligoclase or oligoclase-andesine, with a few small and considerably altered needles of hornblende. The phenocrysts are much in the minority and in size grade into the coarser particles of the groundmass. The groundmass is fine grained and slightly cloudy and decomposed. It consists largely of perthitic