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

GENERAL GEOLOGY OF CANAL ZONE. 21 intrusive masses of igneous rock now found at intervals along the central part of the Isthmus. The formation must be at least 1,000 feet thick, and it outcrops extensively in the vicinity of Bohio and near Caimito Junction. Though not outcropping in the Culebra Cut, it was encountered in many of the cuts near Bohio on the new lino of tho Panama Rail- road. It is somewhat difficult to drill, on account of the many hard cobbles and bowlders that it contains. Lack of jointing makes it rather more difficult to blast than might bo expected from the fact that tho matrix filling its interbowlder spaces is relatively soft and friable. CULEBRA FORMATION. The Culebra formation contains an upper and a lower member. The lower member (fig. 1 and Pl. XV) consists of dark, well-lami- nated beds of soft shales, marls, and carbonaceous clays, with some pebbly, sandy, and tufaceous layers. There are a few thin beds of lignitic shale, but the whole upper part of the formation contains considerable organic matter. It outcrops in the Culebra Cut, near Culebra, and near Pedro Miguel. The upper member consists of calcareous beds and lenses ranging in character from sandy limestone to calcareous sandstone, 3 to 10 feet thick, separated by partings of dark carbonaceous clays and fine-bedded tuffs (fig. 1 and Pls. VI and XV). Locally this formation, gives off a little natural gas and in some small areas it shows slightly bituminous shales. The lower member of this formation, because of its soft, friable nature, could be drilled and blasted easily and was economically handled by the steam shovels. In spite of its weak and friable char- acter it was relatively loss given to sliding than, say, the Cucuraclia formation, owing to the following facts: (1) It is fairly well laminated or bedded, and the thin beds are relatively horizontal, except locally; hence there is practically no tendency toward sliding along the bedding planes. (2) It is slightly sandy to granular and both its granularity and its bedding pianos cause it to be better drained than tho more clayey formation (tho Cucuracha) in which maximum sliding developed. (3) It seems to be relatively free from particles of chlorite and other greasy ferro-magnesian minerals which contribute greatly to the instability and mobility of the Cucuracha rocks. 1 he upper member of tho formation contains many thin beds of sandy limestone and limy sandstone that are fairly resistant to drill- ing and blasting. They also strengthen the slopes and thus locally tend to prevent slides. ( ertain local areas of the Culebra formation became heated, gen- erally soon after having boon exposed to the atmosphere by drilling