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

22 engineering problems of PANAMA canal. or blasting. This heating was due to the oxidation of finely divided pyrite. It thus was necessary in certain areas to test the drill holes to determine whether they had become hot and therefore dangerous to load with dynamite. This matter of heating ground is discussed more fully in a subsequent section. CUCURACHA FORMATION. The Cucuracha formation (see figs. 1, 3, and 4 and Pl. XV) is liere described in considerable detail, because in it not only the Cucuracha slide but also the big Culebra slides developed. It is so named because of being the site of the Cucuracha slide and because it is typically exposed near Cucuracha village. The formation consists of a dark green, massive and locally bedded, slightly indurated, volcanic clay rock of andesitic composition. It is a land-deposited formation, overlying the marine Culebra beds, from which it is separated by 10 to 20 feet of slightly indurated gravel. It is the upper part of what Hill ° and Howe a b called the Culebra formation. Locally it contains red beds and lenses, but those arc of the same general character as the green clay rock in which they arc inter- bedded, except that they contain slightly more iron and alumina and a little less silica. In certain beds there is a network of small irregular joints contiguous to which the greenish clay rock has turned red; such a change seems to be due to the oxidation of the greenish ferrous iron to the rod ferric condition by surface waters. In some of these red beds, however, there lias been some local concentration of iron and alumina products. In addition to the red bods there are a few local beds and lenses of gravel and of sandy, dark gray, tufaceous material. This gravel, like the gravel at the base of the formation, is fairly fine, loosely cemented, and consists of the rounded fragments of indurated shales, cherts, and concretions from the lower part of this formation and from some of tho older rocks. There arc also four distinct beds of lignitic shalo, 1 to 5 foot thick. They are the fossilized remains of former swamps. The formation is cut by some largo and some small basaltic dikes (Pls.’ VII and XV), but these have caused scarcely any meta- morphism. Faulting has considerably broken the beds and, owing to their soft and brittle character, relatively small faults, where tlio movement seems to have been less than 75 feet, have resulted in shear zones up to several feet wide. These rocks weather readily, and are a Hill, R. T., Geological history of the Isthmus of Panama and portions of Costa Rica: Bull. Museum Comparative Zoology of Harvard College, vol. 28,1898. b Howe, Ernest, Canal Commission Ann. Report 1907, Appendix E, pp. IOS-138; Isthmian geology of the Panama Canal: Econ. Geol., vol. 2,1907, pp. 639-658; Geology of the Isthmus of Panama: Am. Jour. Sei., vol. 26, ser. 4, 1908, pp. 212-237.