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

GENERAL GEOLOGY OF CANAL ZONE. 27 IGNEOUS ROCKS. The igneous rocks within the Canal Zone belong to six distinct groups or families as described below. GRANODIORITE. Granodiorite is a quartz-bearing, locally porphyritic rock, a mem- ber of the. broad group of granitic rocks. It outcrops in a few places in the Chagres River Basin, for many pebbles of it may be found among the gravels of that river. Another rock that may be classed as granodiorite but in reality is a dacite (or quartz-diorite-porphyry), is that forming Cocovi Island in Panama Bay. It was planned to use this rock in some of the smaller parts of the dry dock, hence it is here described in some detail. It is light colored, weathers almost white, and shows in the hand specimen only occasional shreds of dark ferromagnesian mineral. Under the microscope it is markedly porphyritic, with phenocrysts (large crystals in a finer groundmass) of andesine-labradorite greatly predominating over quartz, orthoclase, shredded and altered remnants of hornblende, and what probably was biotite. Many of the pheno- crysts are cracked and broken and show zonal inclusions of glass. The largest are 6 mm. in diameter and from this size they range down- ward into fine groundmass crystals. The groundmass seems to bo of about the same composition as the coarser crystals and shows a sort of pepper-and-salt effect. The accessory minerals arc magnetite and apatite; also some secondary chlorite is present. The rock is strong, fine grained, and will make a good building stone. DIORITE. Diorite, a rock much like granite, but with little or no free quartz and with much dark hornblende, is found as rounded fragments among the gravel of the Chagres River, and in the Bohio conglomerate. Both diorite and granodiorite form some of the core masses of the mountains in the interior of Panama. The diorite is a tough well- crystallized rock and will serve well for building purposes. ANDESITE. Andesite, a rock of about the same chemical composition as diorite, is known at a few places in the Canal Zone. It differs from diorite in having reached the surface as a lava-magma, whereas diorite cooled below the surface and is therefore generally much more coarsely crystalline. Volcanic necks and dikes of andesitic rock cut the Las Cascadas agglomerate in various places. Similar dikes seem