proceedings: geological society 47 



have been forced upward into the sedimentary beds, in some cases in a 

 fluid^ondition but more generally after having solidified. The intrusion 

 was accompanied by tilting and crushing of the once horizontal sedi- 

 mentary beds. The andesiteis generally intersected by numerous joints, 

 but does not disintegrate readily on exposure, and where supported 

 below will stand in vertical slopes. Gold Hill and Contractor Hill are 

 examples of these andesitic intrusive masses. All the other high hills in 

 the Culebra section of the Zone are apparently due to the presence of 

 similar rocks. (3) Lava flows: A flow of light gray trachytic lava, 20 

 to 30 feet thick, is conformabhy interbedded with the Culebra clays 

 throughout the central portion of the cut. The trachyte does not disin- 

 tegrate readily, but by reason of its abundant joints it affords no support 

 to the clays above. (4) Residual clays: The rocks in this region are 

 deeply weathered and the original surface is covered with residual 

 clay, derived directly from the rocks on which it rests. It is thinnest 

 over the intrusive andesites, which decay less rapidly than the sedimen- 

 tary rocks, and is thickest over those sedimentary rocks that have been 

 so fractured as to permit the easy percolation of water. 



The original "three on two" slopes have failed to stand at numerous 

 points. Varying conditions have given rise to three classes of failures 

 which may be designated (1) surface flows; (2) structural breaks; (3) 

 surface disintegration. 



Wherever the residual clay rests on an inclined rock surface and the 

 lateral support is removed, as when a trench is excavated, a portion of 

 the mass tends to flow. The material has no cohesion and very slight 

 adhesion to the underlying rock. It acts like a viscous fluid and may 

 flow on slopes as low as one on seven or eight. Having started it may 

 gather sufficient momentum to carry down portions of the underlying 

 unweathered rock, particularly if the latter is much fractured. The 

 Cucaracha and Paraiso slides are of this type. 



Every variety of rock has a limiting strength beyond which it crushes 

 or flows. The crushing strength varies between wide limits and is 

 lowered when one or more sides of the block are unsupported. The 

 weakest member in the sedimentary series in the Culebra Cut is the 

 lignite or lignitic clay. When lateral support is removed from a 

 lignite bed it tends to give way under the overlying weight and a slide 

 results. This may occur if the weak bed is merely unloaded by excavat- 

 ing, in which case it bulges up in the excavation. This type has occurred 

 at various points between Empire and Cold Hill and in nearly every case 

 the immediate cause appears to be the failure of a lignite bed in conse- 

 quence of excavation. These structural breaks offer more serious diffi- 

 culties than the surface flows, and the obvious prevention is to make 

 the slopes sufficiently low so that the weakest rock is not loaded beyond 

 its crushing strength. In the section of the canal threatened by slides 

 of this type less material will have to be removed if the slopes are lowered 

 before the break occurs. 



As previously stated, all the sedimentary rocks of this region disin- 

 tegrate rapidly when exposed to the air. They first crumble into small 

 angular fragments and eventually into the finest powder. Slopes cut 



