Sandstones are mechanical in their origin, being formed by the consolidation into rock masses of beds of sand and small gravel, normally high in quartz because of its resistance to weathering. Usually the constituent grains are rounded and water-worn, but at times they may be more or less angular in shape. Typical sandstones are composed of grains of quartz held together by some substance acting as a cement. The size of grain may vary from that of peas down to that of fine seeds; as they become finer the rocks pass into shales, just as on the other hand they graduate upward into conglomerates, and thus no sharp line can be drawn between the three kinds. While some sandstones are very pure, consisting of quartz grains alone, others contain intermingled particles of feldspar, garnet, iron ore, tourmaline, flakes of mica and fragments of other minerals. It can generally be observed with a lens that the grains tend to be spheroidal, and that the larger they are, the more perfect the rounding is apt to be.
It is to be noted that when a sandstone breaks it is usually the cement that is fractured, while the individual grains remain unbroken, so that the fresh surfaces of the rock have a granular appearance and feeling. This give the general appearance of many sandstones, with respect to their granular texture, is much like that of loaf sugar. As described under quartzite, to which reference should be made, the fracture, in breaking sandstone, takes place chiefly in the cement, leaving the grains outstanding, and this gives the rock its sugary appearance and feeling. Sandstones differ very much in regard to the cementing material which holds the grains together, and thus different varieties are produced. Sometimes it is deposited silica, sometimes a carbonate commonly calcite, but on occasion dolomite or siderite, sometimes extremely fine argillaceous material or clay, and at other times deposited oxides of iron, either reddish (hematite, turgite), or yellowish (limonite).
The colors are very variable, white to gray, buff to dark yellow, and brick-red to reddish brown and brown, are common; green, purple and black are rare. These colors depend largely on the nature of the cement; in the yellow, red and brown sandstones oxides of iron predominate, with the other, lighter colors, it is apt to be calcareous (calcite) or argillaceous. In addition, the calcareous sandstones are readily detected by their effervescing when touched with acid, while the argillaceous ones give the characteristic odor of clay, when breathed upon. The green color is due to glauconite, or in some cases admixed chlorite. Some varieties appear to be almost devoid of any cement. Sandstones are usually very porous rocks, and this appears to depend to a large extent upon the amount and character of the interstitial cement. Thus the ratio of the volume of pore space to that of the rock has been found to vary from 5 to almost 30 per cent, the latter being about the greatest amount theoretically possible in deposited sand grains.
The same characters also condition to a large degree other physical properties and also explain their variations: thus the weight per cubic foot varies from 125-150 pounds, the crushing strength from 1500 to 15,000 pounds per square inch. The specific gravity is about 2.6 (2.5-2.7), with the rock pores filled with water, when weighed in it. The chemical composition of sandstone varies considerably; the chief element is silica, but the proportions of the other elements depend on the nature of the associated minerals and cement. The presence of the alkalies points to that of feldspar (or mica) in the rock; in the amount of feldspar must be large, and such a rock is to be classed as an arkose rock rather than sandstone. The structure of sandstones is essentially that of the stratified rocks. They are sometimes thinly laminated and fissile, and sometimes very thick bedded and within the individual bed may show a very even texture and be practically free from any evidence of stratification. Sandstones such as the latter are valuable for structural purposes on account of their homogeneous character and capability for cleaving or working equally in all directions; they are often called freestones.
These rocks are frequently distinguished according to the character of the cement or admixed material as described above; thus there are calcareous sandstones, argillaceous sandstones, ferrugineous sandstones and siliceous sandstones. Micaceous sandstones contain considerable muscovite; the tabular flakes are parallel to the bedding and induce a more or less ready cleavage in the rock, giving it a fissile character; the cleavage faces are apt to be somewhat silvery in appearance from the mica films coating them. Grit is a term applied to coarse-grained sandstones whose particles are in general more or less sharply angular, and whose cementing material is, as a rule, quite siliceous. They have been considerably used for grindstones and millstones, hence the term "millstone grit." In siliceous sandstones it may happen that the deposited silica is precipitated upon the rounded or angular quartz grains in crystalline position, thus reconverting them outwardly into crystals; examination with the lens shows the crystal forms and faces of the little regenerated quartzes; these are known as crystal sandstones.
Green sandstone is a variety full of grains of glauconite which impart a general greenish color to the rock. Sometimes these rocks are very friable, indeed scarcely coherent, as in the Cretaceous formations of the Atlantic border, especially in New Jersey. They are then called greensand or, inappropriately, greensand-marl. They are apt to contain, in addition to the sand and glauconite, iron oxides and fossil shells, either whole or fragmentary. These deposits have been considerably used as fertilizers.
Arkose Sandstone. This is a special variety of sandstone in which a notable quantity of feldspar grains is mingled with those of quartz. Often there is considerable mica present and, if the material is firmly cemented, the rock to a casual glance may bear no small resemblance to granite. The particles are generally sharply angular, and the feldspar is apt to be soft and more or less changed to kaolin. Under a lens the irregular, clastic, angular shape of the particles readily distinguishes it from a granite. The mineral composition and the shape of the grains show that the material has been derived from quickly disintegrating granite and has suffered but a very short transport before being deposited. Arkoses often grade into conglomerates and breccias by increasing size of some of the particles. They occur in all of the different geological formations. The red-brown Triassic sandstones of New England are in large part arkose and conglomerate or breccia.
Graywacke is a sandstone-like rocks of a prevailing gray color, sometimes brown to blackish, which, in addition to the quartz and feldspar of an arkose, contain rounded or angular bits of other rocks, such as fragments of shale, slate, quartzite, granite, felsite, basalt, etc., or of varied minerals, hornblende, garnet, tourmaline, etc. They are in reality fine-grained conglomerates and readily pass into them by increase in size of some of the component particles. The amount of cement, as in sandstones, is usually small and it is generally argillaceous, but sometimes siliceous or calcareous. Such rocks, when fine grained and compact and largely composed of feldspathic material, may be difficult in the hand specimen to distinguish from some felsites, but close examination with a good lens will generally show their non-homogeneous character. The name has been rather loosely used and has never had the vogue in America that it has in Europe.
As is well known, sandstone is everywhere used for constructional purposes. The ease with which it is worked, and the large size of the blocks which may be quarried, make it particularly valuable for this purpose. Thus in the United States a very considerable portion of the buildings of the eastern cities are wholly or in part of the red-brown sandstone, generally called "brownstone," of the Triassic areas of the Atlantic border, while for instance in Great Britain the city of Edinburgh is largely built of the Carboniferous sandstones of that region. On account of the insoluble nature of the iron oxide forming their cement, the red and brown sandstones in moist climates retain much better the details of fine cutting and carving for architectural effects, than do the lighter colored gray or buff stones. The latter are liable to have a calcareous cement, which dissolves under the action of atmospheric agencies and water, allowing the stone to crumble, and thus in the course of years the fine details of carving are spoiled. Many examples of this may be seen in the older cities where expensive and beautiful buildings have been much injured. If possible, in building, a sandstone should always be laid upon the quarry bed as it is then much less liable to flake or spall. Sandstones are of such wide and general distribution in all parts of the world where stratified rocks are found, that it is unnecessary to give any detailed account of their occurrence.
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