Marly soils are of various hues, red, blue, grey, and yellow, and consist principally of clay and lime, and are usually called clay-marls when clay preponderates, and marl-clays or chalk-marls when chalk is in excess. They are dry to the touch and will effervesce with acids, the presence of lime being thus made evident by the ebullition produced, and some idea of the relative quantity may be judged by comparing results with different lumps: they vary in character according as lime or clay predominates. Chalk-marl may act as an almost impermeable stratum and arrest the percolation of water from more permeable soil, but it is liable to slip because of fissures being present in it, which are common to most calcareous earths; and when these crevices are bared in cuttings by the ground being excavated, the flow from the previously-confined or diverted springs is facilitated, and perhaps the harmless underground passage of water is prevented, and consequently the earth may become loosened and unstable owing to the changed condition. Although marl may be so hard that it cannot be excavated by picks and bars, but requires blasting, some varieties crumble and become soft under weather influences, and the slopes need protection, or a constant trickling and wearing away of the surface will result. Grey marl generally weathers quickly, and if there is much clay in it, it often falls to pieces upon exposure to the air, and becomes broken and so split into disjointed pieces as to admit water, and as it sometimes contains a considerable quantity of sand it is soon reduced to an unstable state, for marl-clay and sand beds are always likely to slip. The variegated marls are treacherous, but red marl is usually stable, still it must be lined in tunnels; and so are those that are impermeable or closely approach that condition, but any marl in which the lime may separate from the mass is unstable. All clay or marl soils that soon work into “slurry” may be classed as treacherous, or any earth that quickly becomes in a liquid state; for instance, some of the hard “pan” soil met with in Canada melts away by the action of air, rain, snow and frost, and becomes unmanageable and like blue paint, and will quickly fill drains and run down the slopes and cover the formation of a cutting. Also the top black cotton soil found in India expands by aqueous action, and contracts in drying as clay, and is of doubtful stability.