In foundation and general work, rocks are usually not loaded with a greater weight than from 8 to 18 tons per square foot, according to the character of the rock. As the crushing strength has been principally ascertained from cubes, and not from prisms, rectangular blocks, or irregularly shaped pieces, and as the resistance of rocks to transverse strain or breaking across is considerably less than the compressive strength, and varies greatly and not always according to the crushing resistance of the material, from 8 to 20 tons per square foot is a prudent limit for the safe load, and should not be exceeded unless under exceptional circumstances; as unequal bearing may greatly intensify the strain, and irregularity in the texture may reduce the resisting powers to that of the weakest part. Sandstone rock that can be crumbled in the hand should not be loaded with more than 1½ to 1¾ ton per square foot, or it will probably begin to flake and disintegrate. The strength of sandstone varies very greatly, and in experiments it has been found that when fine close grained, it supported before being crushed five times the weight that very coarse gritty sandstone, having a sandy appearance, would sustain; the respective crushing pressures per square foot being 362 and 67 tons. Reference to authorities on the resistance of stones to crushing, tension and transverse strain, will give the approximate safe load per square foot; but in foundations, _i.e._, upon the rock in its natural location, it should not exceed one-tenth of the ultimate resistance, and the compressive strength should not alone be taken as a guide to the safe load, but the resistance of the rock to tensional and transverse strain should be considered. The value given for crumbling sandstone is for the softest material that can be called rock, and is merely stated to show that although some earths may be generally classed as rocks their bearing power may be limited. The safe load upon an artificial rubble mound foundation depends upon its character, and firmness and solidity when deposited, and upon that of the ground on which it is placed. No general values can be named, although it may be classed as clean or compact gravel. In a cutting, by excavation, the normal pressure of the earth, which varies with the depth and weight of the soil, is removed. Let D = the depth in feet of a cutting from the original surface of the ground, W = the weight of a cubic foot of earth in decimals of a ton, P = the normal pressure in tons per square foot at any given depth, Then P = D × W.