Gypsum.-Gypsum, the hydrous sulphate of ime, is the chief constituent of those stratified rocks known as rock gypsum, and is not found in many building stones. It is soft enough to be scratched with the thumbnail. Alabaster, the fine grained, white massive variety, is translucent in thin plates and has been used for ornamental purposes.

Serpentine.-Serpentine, a hydrous silicate of magnesia, is a green or yellow material of soapy feel, no cleavage, and soft enough to be easily scratched with a knife. In massive form it is a common and important constituent of the serpentine or verd antique marbles, but occurs sometimes as specks or lumps in ordinary marble. Its resistance to weather is low.

Pyrite.-Pyrite, or iron pyrite, the iron disulphide, may occur in all kinds of rocks. Its yellow color and metallic lustre make it easily recognizable. It is an undesirable constituent of building stones since it weathers easily to limonite, producing a rusty stain.

Limonite.-Limonite, a hydrous iron oxide, is common in the cementing material of some sedimentary rocks, especially sandstones, but in others may be formed by the weathering of iron-bearing silicates.

16. Rocks Used for Building Stones.-A rock may be defined as an aggregation of minerals forming a portion of the earth's crust. Rocks can be divided into the three following groups: igneous rocks, stratified rocks, and metamorphic rocks.

16a. Igneous Rocks.-Igneous rocks are formed by the cooling of a molten mass. With few exceptions they agree in being of massive structure, more or less crystalline texture, and free from statification planes. The following are the characters of the common types:

Granites.-Composed essentially of quartz and orthoclase feldspar, but usually containing some mica, amphibole, or pyroxene. Texture coarse to fine, usually even, sometimes porphyritic. Color-variable, pink, gray, or white common. Valuable as a building stone because of usually high durability, variety of color and texture, and susceptibility of taking a high polish. The darker granites often give excellent contrast between hammered and polished surface.

Pegmatite. Usually of very coarse grain, occurring commonly in the form of dikes. It is of no value as a building stone and its occurrence in some granite quarries causes serious waste.

Syenite. An even granular rock, resembling granite in texture, composed chiefly of orthoclase feldspar, but usually having some hornblende, mica, or pyroxene. Color-usually white, pink, or gray. Takes a good polish but of little importance as a building stone because of its restricted occurrence.

Diorite. Similar to granite in texture, composed of hornblende and plagioclase feldspar, often much biotite. Color-dark gray or greenish. Not as common as granite. Takes good polish and sometimes of very ornamental character.

Gabbro.-Like granite in texture. Consists chiefly of pyroxene and plagioclase feldspar; the former sometimes predominating to such an extent as to give the stone a very dark color. Color-dark gray or greenish to black. A common rock in the United States, being known in New England, the Adirondacks, Maryland, Minnesota, Rocky Mountains, and California.

Diabase or Trap.-Dark, fine grained, containing plagioclase and pyroxene as essential constituents. Differe from gabbro in manner of distribution of minerals, which gives it characteristic appearance. Sometimes almost black on rock face and polished surfaces. Very hard.

Basalt.-Agrees with gabbro mineralogically, but is finer grained, gray to black, and sometimes cellular. Occurs as lava flows. Not important. Much jointed.

166. Stratified Rocks. Stratified rocks are derived from the weathering products of pre-existing rocks and laid down by water or sometimes by wind. They show a stratified or layered structure with texture varying from coarse to fine. The hardness is due to cement (usually iron oxide, silica, or lime carbonate) deposited between the grains, and hence the degree of hardness depends on amount of cementing material. Beds are of variable thickness, hence only the thick bedded ones are of value for dimension stone. Thin bedded ones may be useful for flagging or curbing. Important types are:

cement.

Sandstone.-Grains are chiefly quartz of varying size and regularity, bound together by some kind of mineral Color is variable. Varieties are micaceous sandstone, with abundant mica scales; argillaceous sandstone, containing considerable clay; and arkose, containing angular feldspar fragments, usually in abundance. Conglomerate.-Stratified rock composed of pebbles of rounded character and more or less cemented together. Pebbles may be of different kinds of rocks but quartz ones are common. Conglomerates may grade into sandstones.

Not much used for building stone.

Shale. A thin layered clay rock, formed by consolidation of clay. No value as a building stone. of value formaking brick, tile, and other burned clay products.

Sometimes

Limestone. As used in its broadest sense, the term includes rocks which are made up largely of calcite or dolomite, or botn. Properly speaking, limestone is applied to the calcite rocks, and dolomite to the dolomitic ones. There is no sharp line of separation between the two. Colors of limestone and dolomite are variable, but white, gray, or black are common; hardness is also variable. Sand and clay are sometimes common impurities. Texture

coarse to fine. present in some.

Some varieties contain large quantities of shells and other fossils.

Flint and pyrite may be

Varieties of limestone proper are: chalk, a soft limestone, of earthy texture and usually white color; travertine, a lime carbonate deposit from springs; coquina, a loosely cemented shell aggregate; and onyx, a dense crystalline form of lime carbonate, deposited usually on the floor of caves by percolating water carrying lime carbonate.

16c. Metamorphic Rocks.-Metamorphic rocks are those formed by a reorganization of preexisting rocks, through the action of pressure, heat, and water. They usually show a crystalline or grained texture, foliated or banded structure, and low porosity. Important types are:

Quartzite.-Hard siliceous rock differing from sandstone in being denser and harder.

Slate. A clay rock, harder than shale, and possessing a well developed cleavage. Color-gray, black, green, red, and purple. Texture-very fine.

Marble. A crystalline limestone or dolomite. Texture and color, variable. May contain silicate minerals, graphite, or other mineral matter scattered through it in grains, streaks, or blotches. Pure marble is white.

Gneiss. A banded metamorphic rock often having the same mineral composition as granite or sometimes of other igneous rocks. Color is variable.

Schist. More thinly foliated than gneiss, due usually to an excess of bladed or scaly minerals, such as mica. Is of little value as a building stone.

17. Properties and Testing of Building Stones.-The properties of a stone exert a distinct influence on its uses and durability. There are standarized tests for some.

Texture. The texture, or the grain of the stone, varies from coarse to fine, or regular to irregular. Uniformity of grain is desirable.

The grains of sandstone are usually rounded but may be angular. The grains of igneous and metamorphic rocks are angular. Limestones are usually fine grained, but may be very coarse if made up largely of shells, or have an oolitic structure. Marbles are apt to show uniform texture though some may have the grains more elongated in one direction than another. Greater elongation in one direction affects the splitting qualities of the stone and should not be overlooked. Texture of marble is affected by mineral impurities and folding. White Alabama marble and Italian Carrara are fine grained, while Georgia marble is coarse grained. Granites show a wide range of texture but do not reach as great a degree of fineness as marbles. Dale has graded Vermont marbles as follows:

It can be

Hardness.-The hardness of a rock depends on the state of aggregation of its component mineral grains. A sandstone may be made up entirely of hard quartz grains very loosely cemented, so that the rock is soft. There is no standard test for hardness of stone. tested by measuring its abrasive resistance, or the rate of penetration of a drill. Color.-Building stones show a variety of colors including white, brown, red, yellow, gray, buff, black, etc. These are sometimes a blend of the colors of the component minerals.

Sandstones owe their color to the character of the cementing material. Limestones, if pure, are white; but carbonaceous matter frequently colors them gray or black. Pure marbles are white. Graphitic material colors them gray or black, while silicate mineral impurities give

them various colors, the latter often being disposed in bands or cloudings. In igneous rocks and gneisses, the color is commonly due to that of the prevailing minerals, as the pink color of orthoclase in many granites. Change of color is common in some rocks but it does not necessarily indicate decay of the stone. Green slates especially may fade; dark limestones may become lighter colored on the surface; some pink granites also fade; sandstones occasionally develop a rusty hue, the Berea grit of Ohio being a well-known case. Some bluish limestones may turn buff on exposure due to the change of the iron in them from the carbonate to the oxide. Atmospheric dust may speedily discolor light stones, such as white marble. A white scum seen on the surface of sandstones is an efflorescence derived from soluble salts. These may come from the stone itself or from the mortar.

one.

Permanence of color can often be gaged by comparison of a fresh surface with a weathered

Lighter colored stones are more in demand for general work than dark colored ones, and an otherwise perfectly good stone is sometimes rejected because its color is undesirable.

Polish. This is affected by the density and character of mineral constituents. Dense stones take a better polish than porous ones. An aggregation of different minerals gives a less continuous polish than a mass of the same minerals. Quartz, feldspar, calcite, and dolomite polish well, hornblende and augite less so, while mica is difficult to polish.

Porosity. The percentage of pore space varies with different rocks. Granitic rocks, marbles, gneisses, quartzites, and most limestones have low porosity. Sandstone is usually fairly porous; volcanic tuffs, some lavas, and soft limestones may show high porosity. Porosity may be determined by the following formula

in which P

=

per cent. porosity, W

=

saturated weight, D

=

dry weight, and S

=

suspended weight of saturated stone. Saturation may be obtained by soaking the stone in water for 24 hr. and then boiling for at least 1 hr.

Straight

A fact often overlooked is that the pores of a stone may vary in size and shape. open pores permit ready drainage of absorbed water. Narrow tortuous pores impede it. Two stones of equal porosity might therefore retain absorbed water quite differently because of difference in size and shape of pores.

Stones of high porosity usually show high absorption, but not necessarily low frost resistance. Some of high porosity may show low absorption because of low permeability, the latter being influenced by ease with which the water can pass from one pore to another. The following figures give the range of porosity for several kinds of building stones from different regions.

Absorption. This refers to the amount of water that a stone will absorb when immersed under atmospheric pressure and should not be confused with porosity. While stones of low porosity can absorb little water and others with high porosity may absorb much water, the aborption does not stand in a constant direct relation to the porosity. Moreover, the water absorbed by cold immersion under atmospheric pressure does not usually completely fill the pores.

Dense rocks like granites, gneisses, slates, marbles, quartzites, and many limestones, usually show a low absorption, often under 1 %. Others including many sandstones, some mestones, and many lava or tuff rocks may absorb from 2 % up to perhaps 15% of water.

To test the absorption, a piece of the stone is thoroughly dried at 110 deg. C., weighed, and then immersed in water for 48 hr., after which it is weighed again. The increase in weight represents the weight of water absorbed, and the percentage of absorption is calculated in terms of the original dry weight. Immersing the stone under a vacuum is sometimes advocated, but this gives misleading results as it causes the stone to absorb more water than it normally would.

The quantity absorbed under different conditions does not always show great variation in dense rocks, but may vary considerably in porous ones.

The following tests by Parks give the average ratios of absorption for different rocks under different conditions.

As a building stone in use absorbs water under atmospheric pressure, and usually only from one side, even the ordinary form of testing is liable to make the stone absorb more water than it would in use.

Permeability. Water will permeate stone even though of low porosity, resulting sometimes in discoloration by the liquid carrying coloring materials. It has been found that the permeability does not stand in any direct relation to the porosity, as shown by the following table:

Stone may be tested for permeability by cutting slices of the stone 3 mm. thick in a direction at right angles to the bedding. Water is then forced through these under a pressure of 15 lb. per sq. in., and the amount of flow in 1 hr. recorded.

Quarry Water.-Rocks in the ground contain more or less water in their pores. Quarrymen call this quarry water and it is necessary on removal of stone from quarry to let this water dry out, or allow the stone to "season." Drying out of the quarry water, especially that from the smallest pores, is accompanied by the deposition of the dissolved mineral matter which it contains. This makes the rock often appreciably harder. Its effect is most pronounced in the case of porous sandstones and limestones.

A stone should not be allowed to freeze during seasoning as the quarry water freezing within its pores may cause it to split. Slate must be split before the quarry water dries out, and many stones carve much easier before seasoning, which in some cases makes the stone appreciably harder.

The working faces in some quarries are sometimes protected during the freezing season either by covering the face, or filling the quarry with water.

Since quarry water travels more readily along the rift or bedding, placing a block on edge with one side in sand, often prevents disintegration by freezing.

Strength. The strength of a stone is governed: (1) by the rock structure, such as rift, cleavage, and bedding; (2) by the hardness of the grains; and (3) by the state of aggregations, as whether interlocked or cemented, and in the latter case by the nature of the cement.

Many tests have shown that most building stones have 2 to 10 times the crushing strength required in any structure where they will be used.

The minimum strength permissible. can be determined only by a trained engineer or architect, and while few cases of yielding due to crushing are observed, failures due to stresses applied in other ways are not uncommon.

Crushing Strength.-Buckley states that the stone at the base of the Washington monument sustains a pressure of 314.6 lb. per sq. in., and that, in the tallest buildings the maximum pressure at the base is not more than half of this. Assuming, as a wide factor of safety, that a stone should have 20 times this strength, a resistance of 3146 lb. per sq. in. will answer all ordinary requirements.

Almost any reputable stone may be used in the construction of ordinary walls, but all may not be suitable for special architectural elements, as pillars, or railroad piers which may not only have to sustain a very great weight, but also be subject to jars.

Stones of high crushing strength are in general denser and heavier than those of low crushing strength, and hence more durable, but there are not a few exceptions to the last.

Most building stones show a loss in crushing strength after freezing. Tests for crushing stones are made upon cubes of the stone, preferably of 2-in. size.

The following table gives the crushing strength of a number of different kinds: