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

GENERAL GEOLOGY OF CANAL ZONE. 19 size to which the material should be broken ordinarily has to be determined by experiment. The resistance of largo masses of relatively hard rock depends not nearly so much on the tenacity of the solid rock as on the extent to which it is cut by natural joint, bedding, and shear planes. If it were desired to have the maximum size of the fragments not more, say, than 1 cubic yard, and if the joints happened to be well developed more or less at right angles to each other and so spaced that they divided the rock mass into fragments of not more than a cubic yard each., then the holes would not need to be so closely spaced and the charges could be much lighter to give the desired effect than if the joint planes were 2 or 3 yards apart or absent. If tho material excavated is to be crushed and used for road making or concrete work, a high amount of jointing and fissuring lessens tho amount of crushing necessary and greatly cheapens the cost of the product. On the other hand, if great coarse fragments are needed for armoring a breakwater against sea waves or an embankment against river cur- rents, much jointing spoils any rock for such purposes, however suitable it may otherwise bo. Soft argillaceous rocks with little tenacity are easily and cheaply drilled, and some such rocks on the Canal Zone were locally bored with augers. The holes in rock of this kind may bo much farther apart and the explosive used may bo a cheaper, lower grade and a slower acting one than would be required in hard rock. ROCK FORMATIONS AND SPECIFIC ENGINEERING RELATIONS. BAS OBISPO FORMATION. The Las Obispo formation is tho oldest isthmian formation, so far as is known, and is probably of pre-Tertiary age. It was formed of rock fragments and ash blown from old volcanic vents. The débris settled over the surrounding region and was subsequently cemented into 1 airly hard rock by the slow-acting processes of rock comentation. Locally it shows some rough bedding and some rounded water-worn fragments. In composition it belongs to the group of rocks called andesites, and technically would bo classed as an andesitic breccia. It outcrops extensively at Bas Obispo and near old Panama, and small outcrops rise above the alluvium near Miraflores and Diablo Ridge. About 7,000 feet of tho north end of Culebra Cut has been exca- vated in this formation. It is relatively hard and tenacious, except locally where it has been sheared by faulting. These locally faulted places have lot down some masses and fragments of loose rock; but these were relatively unimportant as slides. On the whole, this