But, if non-weather-resisting, ½ to 1 TO 1 to 1, The slope becoming flatter as the rock becomes softer and more easily disintegrated. Friable rock, consisting of hard particles, ½ to 1 TO ¾ to 1. Loose rock, ¼ to 1 TO 1 to 1. Soft shaly limestone and the argillaceous rocks may not be permanently stable until the slope is, 1½ to 1 TO 2 to 1. Schistose rock is troublesome in earthwork, being fissile in structure and deleteriously affected by rain and the atmosphere. On the Panama canal works in the Culebra cutting, maximum depth 333 feet 6 inches, numerous slips occurred, displaced the roads, and overturned the excavators. Mica-schist is variable and frequently has numerous water-bearing fissures. Its hardness depends upon the quantity of quartz it contains; when the proportion of mica is greater than the quartz it is soft and very fissured and contains veins, sometimes of clay, often yielding a considerable flow of water. Should the percentage of quartz be large it becomes hard, holds little water, and is of a gneissose character. As water is the chief disintegrating agent and cause of instability, it may be well to mention that Professor Prestwich has stated that “hard quartzites, slates and grits (Silurian), purple and grey shales, schists and fissile sandstones with hard compact limestones and dolomites (Devonian), rarely contain any _levels_ of water, and that it is only encountered in fissures”; hence the importance of knowing the position of the fissures, and taking the necessary precautions to promote stability. Where rocks, especially if generally known as water-bearing, crop out at a high angle, and are in well-defined beds, water may be expected, as although it may not percolate vertically, it will along the inclined beds. Sandstone and limestone bands in rock usually cause small springs.