This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
40. If a man stands on soft mud, marshy ground, or quicksand, he sinks to a greater or less degree, proportional to his weight. If, however, he stands on a plank or wooden platform, or on a post, or posts, driven through the mud or marsh to firmer ground, his weight is distributed over a larger area in the first case and carried down to a better foundation in the second.
It is similar with the footings of buildings. By spreading the load, or weight of the structure, over a larger area or bearing surface, the weight of the building is more evenly distributed, and the likelihood of a vertical settlement, due to the compression of the ground, is much diminished. For this reason, the higher and heavier the building is to be, the wider and deeper the supports or footings for the foundation must be; and if extremely soft or yielding ground is encountered, piling should be resorted to in order to carry the weight of the building to a more solid base.
41. Footings may be of iron, timber, large flat building stones, laid directly on the ground or on a bed of concrete, or they may be of concrete alone, or concrete and "stepped-up" brickwork. Where piling is used, heavy capping timbers are often placed on the heads of the piles, with either stone or concrete footings; or large footing stones may be laid directly on the piles.