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

20 ENGINEERING PROBLEMS OF PANAMA CANAL. material has stood fairly well at steep angles and will continue to do so. In blasting it broke out in relatively largo fragments and did not readily weather or wash; so the excavated material from this section of the cut was in greater demand for fills and dams, for which maximum stability was necessary, than the softer and more friable rock. LAS CASCADAS AGGLOMERATE. Th© Las Cascadas agglomerate also had its origin in fragmental rock material blown from volcanic vents and later washed down into different beds and masses. It rests uncomformably on the Bas Obispo formation, is much less consolidated and cemented than tlic latter, and is of much moro recent origin. It is a greenish to gray, basic agglomerate, which contains largo and small subangular fragments in a fine-grained groundmass of volcanic clay and tuff (Pl. \ ). rI Iio whole is arranged in massive to roughly bedded deposits, often unconformable with each other. Interbedded with these deposits arc andesitic flow-breccias, some fine-grained grayish and some coarse-grained dark andesitic flows, and a few easily crumbled lava- mud flows which show columnar jointing where exposed in the canal (see PL XXVIII). The whole is cut by largo and small basalt dikes. The formation outcrops extensively along the canal between Empire and Las Cascadas. In degree of hardness and tenacity and in texture the rock of this formation presents groat variety, but, on the whole, it is much softer and more friable and was cheaper to drill and blast than the Bas Obispo rock. Several considerable slides developed in it, due chiefly to local areas of weakness and faulting, and to its somewhat cluyoy character and ground-water content, fliese are described in detail under another heading. The oxtent and relative position of this formation is shown in figure 1. BOIIIO CONGLOMERATE. The Bohio conglomerate consists of beds containing water-worn cobblestones and pebbles. These beds are separated from each other by layers of sandstone and clay rock. The lower part of the formation contains more cobbios and pebbles than the upper part and seems to bo largely a product of running water. It is generally fairly well bedded, though locally massive, and contains hard bowlders, up to a few foot in diameter, of andesitic, dioritic, and other igneous rocks. The upper part of this formation is of the same general compo- sition as the lower part, but contains some beds of dark-gray clay marl rock which have tlio fossil romains of foraminifera—minute shell-bearing animals. The cobbles, bowlders, and gravel are fiom cherts, andesites, and diorites, and were derived from the older