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

30 ENGINEERING PROBLEMS OF PANAMA CANAL. dominating. In the dike specimens the crystals are a little stouter and larger, and many of them arc cracked and bent. Augite is fairly abundant and occurs mostly as pale grains and granular aggregates. The accessory minerals are small black specks and irregular aggregates of magnetite plentifully peppered through the mass, a little ilmenite, and some apatite. Epidote in light and dark yellow irregular patches is present as an alteration product from some of the feldspar crystals, for it fills the little cracks of those that are crushed and broken and occurs as cloudy masses and aggregates in the interior parts of others. Three thin sections were examined, but no olivine was noted in them. Basaltic rock from a little hill near the railroad track and 1| miles north of Monte Lirio was used to face tho water-level zone of the Gatun Dam. This rock is somewhat coarser in texture than that just described, and its feldspars are more calcareous. Otherwise tho rock is much like that of Gold Hill. META-BRECCIA. Under meta-broccia aro classed tho metamorphosed tuff, sandy and argillaceous bods, agglomerate, and breccia masses that form Gold Hill, Contractors Hill, Office HiH (Culebra), and the breccias at Pairaiso, at Empire, and at other places (see Pl. I, II, and XV). Around at least a part of tho periphery of several of these masses, and projecting through each of them in one or in. several places, are basalt dikes. All of these masses, so far as is known, are separated from the rocks surrounding them by a contact along which faulting has occurred. Both ’ the Gold Hill and the Contractors Hill masses have been faulted downward some hundreds of feet (see Pls. XV, XIX, and XX), These mcta-breccias are similar to certain phases of the Bas Obispo breccia. Disturbed bedding is found in their upper parts, so that a vertical section through them would look somewhat like a vertical sec- tion through tho upper part of the Bas Obispo, and a part of the over- lying Las Cascadas agglomerate, with some higher-bedded tuffs in- cluded. Tho evidence indicates that these meta-breccia masses wore pushed upward as somewhat metamorphosed and toughened caps on top of basalt plugs or cores. On cooling, shrinkage and outlets for various dikes and apophyses caused a gradual settling back of these plugs to about their present condition. These breccia masses have acted as strengthening pillars to buttress some of tho sliding areas in Culebra Cut. This function is discussed in a subsequent considera- tion of slides. LAVA-MUD FLOWS. In addition to the igneous rocks enumerated, there are several cooled and hardened mud-lava flows which are found in the Las Cascadas agglomerate. Many of these local masses show columnar structure (see Pl. XXVIII) and are rather hard, but on exposure to tlio air for a few years they crumble considerably.