It is also well to remember that the line of water-flow is not necessarily a horizontal plane, for it frequently follows the contour of the chalk, and that the causes of surface irregularities of subterranean water are unknown; but rain-water accumulating in chalk principally rises and issues most rapidly along the bed lines; consequently the flow along these must be gently discharged, or slips will occur; but chalk uniform in character and of solid and close texture, without flints and fissures, usually is not water-bearing, and will stand almost vertically. As a rule the cohesion of the upper beds, if they are homogeneous, is greater than the lower beds, although the mass may be softer. Should the drainage or natural outlet of the land waters of a chalk district be obstructed or dammed back, from the quantity of water being in excess of that the fissures or water seams in a chalk hill can discharge, and the pent up waters be unable to escape, hydrostatic pressure, in addition to a weakened condition of the chalk through excess of moisture, will be caused, and extensive slips may be expected along the escarpment, the displacement being gradual, the ground separating and fissuring until at length it is pushed out by hydrostatic pressure. Such a slip usually occurs in large masses, resembling a fallen cliff, for the disturbing agent is all-powerful, and the area affected very considerable, and particularly so if the chalk is superimposed upon different soil, or harder ground, as then the whole mass will probably move forward. Although some approximate inclinations have been previously given, the varieties of chalk are so numerous that no absolute slopes of repose can be named, for chalk or calcareous earth may be:— Marble or crystalline limestone. Ordinary limestone rock. Hard, compact chalk rock. Lower white chalk. Upper white chalk. Hard grey chalk. Ordinary grey chalk. Pure white chalk. Friable white chalk. Yellow, light and dark blue, soft chalk becoming of marly character. Hard chalk marl. Grey marly chalk. Grey clayey chalk. NOTE.—The preceding chalk marls contain so large a proportion of argillaceous matter as to become almost clays so far as regards treatment in earthworks. Many serious slips have occurred in chalk soils, and their history indicates that the chief disturbing element was water, whether held back over a large surface until the hydrostatic pressure became too great for the slopes to withstand it, or from its bursting out in springs, and so separating and disintegrating masses of the earth.