Fragmental rocks, including sandstones and slates.
Granite is a crystalline siliceous rock of igneous origin. It con sists essentially of quartz, with some feldspar and usually mica. It is readily quarried into blocks of regular shape, but is very hard and tough and is expensive to cut for ornamental work. It is the strongest and most durable of our building stones in common use, and is very generally employed in important work where these qualities are of special importance. Ileavy foundations, base courses, water tables, and columns in important buildings are very commonly of granite.
The color of granite is usually gray, but pink, red, and black granites are found. It is largely used in monumental work and in architecture for exterior work where the most beautiful and durable results are desired. The use of machinery for working the stone has made this use economically feasible.
Syenitc is a rock similar to granite, but composed mainly of feld spar instead of quartz. It has much the same qualities as granite and is usually classed as granite when used.
Diorite and Gabbro are rock of the same general character as granite but differing in mineral composition. They are usually classed commercially as granites.
Gneiss is a metamorphic rock of the same composition as granite. It is metamorphosed granite or sqenite, and usually classed as granite, being often called stratified or bastard granite. Gneiss differs from granite in having a somewhat laminated structure which causes it to split in parallel layers. It is often used for flagging and paving blocks on this account.
Granites are found quite widely distributed in the mountain regions of the United States. The main supply comes from the New England States, where large quarries are in operation and have gained wide reputation. Commercial granites of good quality are found in the South Atlantic States, in Wisconsin and Missouri, while Montana, Wyoming, Colorado, California, and Washington are plentifully supplied with granite which is comparatively unde veloped.
Limestones are sedimentary calcareous rocks, consisting mainly of the mineral calcite, which is composed of calcium carbonate (CaCO3). They also usually contain small amounts of iron oxide,
silica, and clay. Magnesia is also commonly present in the pure limestones in very small amounts, and varying—through the mag nesian limestones, in which 10 per cent or more of magnesia is pres ent—to dolomite, which consists mainly of the mineral dolomite (CaMg)CO3.
Limestone varies from stone soft enough to cut with a saw to hard material which works with difficulty. Some of the soft stones harden on exposure and are durable in use, the Topeka stone used in Kansas being of this character. Many of the fine-grained, light colored limestones form excellent building material; they are hard and tough and show good durability in use, although inferior in this respect to the best sandstone and granite. Some of them are used in ornamental work and take good polish. Stone containing pyrite is apt to show poor weathering qualities, while spots of flint found in many of these stones are objectionable on account of weathering unevenly and sometimes causing the stone to split under frost action.
The following analysis of typical limestones are given by Ries r to show the range of chemical composition: Limestones exist in large quantities through the States of the Middle West, and are locally developed in many places. The well-known Bedford, Indiana; stone is extensively used and shipped for considerable distances.
Marbles are limestones which have been subjected to metamorphic action. In composition they are identical with limestones, or dolo mites, but are crystalline in texture and may be polished. The term marble is commonly used to designate any limestone capable of taking a polish.
Marbles are commonly employed for interior finish in buildings and for monumental work. The scarcity and cost of the. best marbles have prevented their extensive use for ordinary building construc tion. 'Their weathering properties are similar to those of limestone, although some of the more ornamental ones arc suitable for interior work only. For structural work the more dense fine-grained stone is to be preferred.