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

GENERAL GEOLOGY OF CANAL ZONE. 29 aggregates of orthoclase and plagioclase, with some quartz and feld- spar intergrowths. A small amount of free quartz is present. The chief accessory minerals are grains of magnetite scattered through the rock, some ilmenite, a little apatite, and some light-yellowish grains of secondary mineral showing locally in the groundmass. This rock is strong, fairly tough, and on the whole is well suited for concrete work. BASALT. Basalt, a dark, basic, fine-grained lava rock, is rather plentiful on the Isthmus. As dikes it shows in Culebra Cut near Empire, Culebra (fig. 1 and Pls. VIII and XV), and Pedro Miguel. As moro massive intrusions it forms Office Hill at Empire, the top part of Gold Hill ; also hills near Paraiso, Pedro Miguel, Rio Grande, near the Panama Railroad track 2 miles north of Monte Lirio, and in many other places. In fact, most of the steep hills and ridges within the Canal Zone (Pls. I and II), except Ancon Hill, are formed of hard basaltic rock that formerly came up through the softer rock as dikes and cores of lava. The basalt near Culebra Cut contains practically no olivine, whereas that found in many other places is rich in that mineral. This rock is hard and tough and has locally been used in building roads. If crushed, it would serve well for concrete work. Its use to armor the projected new east breakwater at Colon was proposed, but the rock is so cut with joints that it would not break in large enough pieces to resist the abrasion of sea waves. However, it was to be used as the first coat of armor over the softer and more easily abraded heart or core material, the whole to be faced with large blocks of the Porto Bello rock. Basalt was also used to a considerable extent for facing the water-level zone of the Gatun Dam. Because of its engi- neering importance it is described in some detail. Gold Hill is capped by about 250 feet or more of basalt, and a curved basalt dike 75 to 100 feet thick forms the westerly and northerly periphery of the hill mass. The dike and the capping are fairly typical of the variety of basaltic rock occurring within the Canal Zone that does not bear olivine. Locally the basalt shows some columnar structure (Pl. X) and is considerably jointed. It is tough and strong and resistant to weathering, so that it generally forms hills and ridges which have only shallow coverings of soil. It cuts sedimentary beds of Oligocène age and was perhaps erupted in Miocene (?) time. It shows relatively little contact metamorphism. Hand specimens of this rock show it to bo dark, compact, fine grained, and oven textured, and it is comparatively little weathered. Microscopically it shows fluxional or flow structure, markedly in speci- mens from the capping on the hill and to a much less extent in speci- mens from the dike. In the flow capping the feldspars are abundant, labradorite lathlike crystals, ranging up to 1.2 by 2 mm. in size, pre-