Soft soils are composed of detrital materials, having so little cohesion that they may be excavated without the use of explosives. Tunnels excavated through these soils must be strongly timbered during excavation to support the vertical pressure and prevent caving; and they also always require a strong lining. Gravel, sand, shale, clay, quicksand, and peat are the soft soils generally encountered in the excavation of tunnels. Gravels and dry sand are the strongest and firmest; shales are very firm, but they possess the great defect of being liable to swell in the presence of water or merely by exposure to the air, to such an extent that they have been known to crush the timbering built to support them. Quicksand and peat are proverbially treacherous materials. Clays are sometimes firm and tenacious, but when laminated and in the presence of water are among the most treacherous soils. Laminated clays may be described as ordinary clays altered by chemical and mechanical agencies, and several modifications of the same structure are often found in the same locality. They are composed of laminæ of lenticular form separated by smooth surfaces and easily detached from each other. Laminated clays generally have a dark color, red, ocher or greenish blue, and are very often found alternating with strata of stiatites or calcareous material. For purposes of construction they have been divided into three varieties. Laminated clays of the first variety are those which alternate with calcareous strata and are not so greatly altered as to lose their original stratification. Laminated clays of the second variety are those in which the calcareous strata are broken and reduced to small pieces, but in which the former structure is not completely destroyed; the clay is not reduced to a humid state. Laminated clays of the third variety are those in which the clay by the force of continued disturbance, and in the presence of water, has become plastic. Laminated clays are very treacherous soils; quicksand and peat may be classed, as regards their treacherous nature, among the laminated clays of the third variety.