[Sand]
Sand is a mass of small rock particles, from the size of a pea down to ¹/₅₀₀ of an inch in diameter. The material may be any sort of rock, or a mixture of two or more kinds. Sand may be the result of the disintegration of older rocks at the point where it is now found, in which case the grains have the shapes they had in the original rock; but more often the sand grains have been transported greater or lesser distances, and in the process have been more or less rounded.
Those sands, which lie where they were formed are called residual, and such sand is usually composed of a mixture of angular grains, some harder and others softer, such as quartz, feldspar, mica and hornblende, all mixed together. Where the sand has been transported, only the more resistant minerals have remained, such as quartz, magnetite, cassiderite, etc.; with which there are at times rarer minerals, such as gold, platinum, garnets or topaz. Such sands are known as gold-bearing, topaz-bearing, etc.
The sands from different localities differ greatly. The streams gather the rock particles, and sort them according to the size, which the water flowing at any given rate can carry. When the water is slowed down, it drops all the particles above the size which the new rate of speed can handle. The grains of sand from the bed of a stream are usually more or less angular. The further they are carried, the more they are knocked together and rounded; so that after being carried to the sea, and then thrown up on the beaches, they have been well rounded, especially the larger grains. As the air is less viscid than the water, sand which is transported by the wind, is even more rounded; so that desert sands show the most complete rounding, indeed are even polished; and this is true even of the smaller grains. It is the wind-blown, or desert sands, which flow so evenly in an hourglass. Between the angular residual sands and the polished desert sands, there are of course all grades. Glacial sands are angular or “sharp” almost to the degree characteristic of residual sands; and lake-shore sands are between river sands and sea sands in the degree of rounding.
Sands made of particles of lime, calcareous sands, are less resistant to wear than are those of quartz. In regions where the water is “soft” (free from lime), they do not last long, as they are dissolved; but in a limestone region where the water is “hard” (saturated with lime), the grains are not so quickly dissolved and may accumulate into beds of great thickness, as in Florida. Along some shores of the ocean, there occur “green sands,” which are ordinary quartz sands mixed with the dark green mineral glauconite, which is a potassium iron silicate, forming on the ocean bottom as a result of the action of decaying animal matter on iron-bearing clays and potassium-bearing silicates, like feldspar. This is particularly characteristic of some of the sands along the coast of New Jersey.
In places, especially in the beds of rivers, there occur “quicksands.” This is a deposit of fine sand, mixed with a considerable amount of clay, and saturated with water; so that it will not support the weight of a man or an animal. Much that goes under the name of quicksand is a fluid mud, covered with a thin layer of sand.
Sand is used for a wide variety of commercial purposes, and under these conditions gets various trade names; for instance “glass sand” is a pure, colorless to white, quartz sand, which is used as one of the components in making glass. It must be free from impurities, as these color the glass, and much of the sand used for this purpose is quartz, crushed to a fine sand-like condition. “Moulding sand” is a rather fine-grained quartz sand, with a small but very definite admixture of clay, and this is used to make the moulds for castings in foundries. “Polishing sand” is one composed of angular fragments of quartz, usually from stream beds or glacial deposits, or even crushed quartz, and is used for cutting and polishing marble, for sandpaper, and for polishing wood and softer stones. There are many other special uses, like building, ballast, filters, furnaces, etc., in which quartz sand is used, being screened if necessary to get the right sizes.
[Sandstone]
When sand of any sort is cemented so as to make a solid rock, it is termed sandstone, which varies widely according to the size, color and composition of the grains, and also with the sort and amount of the cement. When the size of the grains is larger than that of a pea, sandstone grades into conglomerate; when smaller than ¹/₅₀₀th of an inch, especially if mixed with clay, it grades into shale. There are all grades of firmness, due to the amount and kind of cement, ranging from those which have little or no cement, but are compact as a result of the pressure of the overlying rocks, to those in which the cement has filled all the pore spaces. In general there is a considerable amount of space between the grains of sand; so that a sandstone will absorb large amounts of water, up to 25% of its bulk. In moist climates where it freezes, this makes many sandstones unsuitable for use as building stones, as they are likely to spale, or chip off, as is seen in the “brown stone” so much used in New York City.
Sandstones are usually bedded rocks and are relatively easy to quarry, and most of them are not so firmly cemented, but that they can be readily worked or cut into shape by the stone cutter; and so, certain sandstones are very popular for building stone or for trimming on buildings, where they are not too much exposed to the weather.