10. Virgin soil is either clay, loam, gravel, marshy ground, or sand, in its natural condition.

11. Clay is the most uncertain of soils, owing to its elasticity, due to being mixed with marl, etc.; its tendency to absorb moisture; and, in many-cases, the position of its bed or strata. In dry seasons it is very firm, while in wet seasons it is elastic and unreliable. When- the layers of clay are inclined, the foundation has a tendency to slide, producing results threatening to the stability of the superstructure.

12. Loam, or clay mixed with sand and other earthy substances, when compact and of considerable depth, is a good material to build on, providing the structure is not an extremely lofty or heavy one.

13. Compact gravel, united with sharp sand, makes the best foundation (except bed rock), and, on account of its being more easily leveled, is much less expensive to build on.

14. Marshy soils are formed by the decay of plants, weeds, and other vegetable matter in sluggish water, which, having no current, allows the plants, etc. to take root in the bed. When these plants die, others take their place each year. These successive beds of decayed matter are formed under slight pressure, and have innumerable cavities between them, as would a heap of decayed hay. Sometimes these deposits reach to such a depth that their bottoms have not been reached. Large areas of marshy lands are formed in this way, by the periodical overflowing of rivers, and the rise and fall of the tide along the coast.

15. Sand is formed from the decomposition of the older rocks, either by the effects of the weather, the action of heavy rains, the wearing away by running water, or the spontaneous decomposition of the rocks themselves. The particles are carried down to the rivers and there deposited, either in their beds or borne out to the ocean.

The sand usually found in excavations has its origin either as the formations in the beds of ancient rivers that have long ceased to flow, called river sand; or by the attrition or grinding of the rocks themselves during the geological upheavals in past ages. The latter is called virgin, or pit, sand, and has never known the action of water.

Quicksand is a very fine sand, often mixed with clay or loamy material in such proportion that it will retain water until it is perfectly saturated. But by confining quicksand and keeping it dry, or as nearly dry as possible, it may be excavated or built upon with little more difficulty than common sand. In many cases, quicksands are mixed with a bluish or leaden colored silt or soapstone slime. It is often the case in excavating through quicksand, that beds of this blue marl are found; when wet it is tough and hard, but when dry, crumbles to a powder, and is utterly unfit for foundations. An attempt to excavate in quicksand without previously getting rid of the water contained therein, is almost as useless as to dig in water itself, for the saturated sand will flow into the excavation faster than it can be removed.

16. During freshets, rivers bring down large quantities of soil held in suspension, which is deposited when the waters subside. This formation is called alluvial, from the Latin word alluvius, meaning a washing upon. The term alluvial is often used to designate deposits that are of yearly recurrence, as the Nile and the Mississippi deltas, although the river bottoms of many streams are originally of alluvial origin. The value of alluvial soil for foundation purposes varies much. In many cases, it consists of a clay formation that is hard on top, especially during dry weather, but soft and unreliable underneath. Heavy buildings should not be erected on alluvial ground without a careful investigation of the subsoil by means of borings or trial pits.