CHAPTER IX BROKEN STONE ROADS Broken stone roads are extensively built in all countries where stone is available. Although the water-bound type can no longer be built and maintained at a reasonable expense when subjected to certain types of traffic, there are conditions where it will continue to prove the most economical type of surfacing. The conclusion of the Second International Road Congress with regard to the water-bound type was as follows: "Macadam constructed according to the methods of Tresaguet and McAdam causes dust and mud, is expensive to maintain, and is only suitable in large cities for streets where the traffic is not very great or heavy." Some of the methods of constructing broken stone roads in combination with a bituminous material, up to a certain point, are practically the same as the construction of the water-bound type except for the addition of the bituminous material. The mileage of broken stone roads in the United States, according to statistics obtained by the U. S. Office of Public Roads, in 1909, was approximately 59,000. Rocks ROCK CLASSIFICATION. Rocks may be separated into the following classes: 1. Igneous rocks, or those which have flowed upwards in a molten condition and cooled. 2. Aqueous rocks or those formed through the agency of water, including all sedimentary rocks. 3. Metamorphic rocks, or those rocks changed by dynamic or chemical agencies from their original condition which may have been anyone or a combination of the above classes. Some rocks may be identified by the unaided eye by noting the color, the structure, the weight and the hardness. By preparing microscopic slides and examining them under a petrographical microscope, it is possible to identify those which cannot be determined by the eye alone. Definitions. A few terms used in describing the structure of rocks will be given. Amorphous, wholly without crystalline structure. Cellular, due to the weathering out of some constituent. Clastic or fragmental, a reconsolidation without crystallization of rocks previously broken down. Colloidal, a jelly or glue-like structure. Compact, granular, but too fine to be determined without microscope. Crystalline, such as ordinary granite. Foliated or schistose, a tendency to split along line of stratification. Granitoid, a descriptive term for igneous rocks composed of recognizable minerals of approximately the same size. Granular, made up of distinct grains. Holocrystalline, made up wholly of a crystalline structure. Laminated, a banded structure common in sedimentary rocks. Massive, igneous rocks that show no stratification Plutonic, rocks which never reached the surface when cooling. Porphyritic, a compact mass throughout which are attached larger crystals. Slaggy, a ropy structure assumed on cooling. Vitreous or glassy, only found in igneous rocks. Volcanic, rocks which were erupted in a molten condition and cooled on the surface. Mineral Constituents. A rock is a mineral or combination of minerals. There are a few rocks which are composed entirely of one mineral, but the majority are made up of a combination of two or more different minerals. Among the most important chemical compounds occurring in rocks are silicates, oxides, carbonates, sulphates, chlorides, phosphates, sulphides, and one native element graphite. Sedimentary rocks are spoken of as calcareous, siliceous, ferrugineous or argillaceous according as lime, silica, iron oxides or clayey matter are predominant. Eruptive rocks are spoken of as acidic or basic, the former being those showing more than 65 percent silica, and the latter those which show less than 55 percent, but are rich in iron, lime and magnesian constituents. Among the more common minerals found in rocks are the following: quartz, feldspar, orthoclase, plagioclase, amphibole, pyroxene, mica, olivine, epidote, calcite, dolomite, magnetite, hematite, limonite, pyrite, chlorite, serpentine, galconite and zeolites. The mineral constituents which make up a rock can be determined by means of the crystalline formation, by chemical analysis, by blow-pipe analysis, and microscopical examination. Table No. 5* gives the mineral constituents and the physical characteristics of the principal road-making rocks. Group I to 8 comprises the plutonic igneous rocks, 9 to 14 the volcanic igneous, 15 to 20 the sedimentary, and 21 to 34 the metamorphic rocks or crystalline schists. Trap. Andesites are generally gray, greenish or reddish in color. Their structure when examined under a microscope is found to vary from a glassy to a holocrystalline. Their principal constituent is plagioclase. Basalts are a homogeneous rock, generally of a dark gray or black color, although red and brown colors are also common. In structure they vary from a glassy to holocrystalline. Diabases are holocrystalline in structure and vary in color from greenish to a dark gray or nearly black. Peridotite is greenish or black in color with a variable structure that may be either crystalline, granular or porphyritic. Diorite. The diorites are a granitoid rock whose essential constituents are plagioclase, feldspar, either labradorite or oligoclase, and hornblende or black mica. They are green, dark gray or black in color. Gabbros are crystalline granular in structure, the prevailing color being black and sometimes * See "Rocks for Road Making," by E. C. E. Lord. U. S. Office of Public Roads Bulletin, No. 31. greenish. As is seen from the table its main constituents are plagioclase, augite and hornblende, quartz being rarely present. Granite. The essential constituents of granite are quartz, feldspar and plagioclase, combined usually with mica, hornblende and pyroxene. It is holocrystalline granular in structure. It may be gray, green, yellow, or red in color, the lighter colors being due to the feldspar and the darker colors due to the mica and hornblende. Quartz porphyries have the same constituents as granite but are porphyritic in structure, having quartz crystals scattered through a ground mass which may be red, gray, yellow, green, black or white in color. Syenite. Syenites are the same in composition as granites except that there is no quartz. In structure and color they are the same as granite. Gneiss. Gneisses are of a holocrystalline granular structure arranged in parallel bands. At one time this foliated structure was supposed to have been due to original stratification. The theory now, however, is that it is due in a large part to pressure. Gneisses are practically the same in composition and color as the granites, the principal difference being the structure. The different varieties, as in granites, are called by the name of the predominating mineral. Limestone. Limestones are extremely variable in both color and structure. All shades from white through a gray to a black are common and sometimes even red and yellow are found. They are stratified in structure and vary from a soft variety to a rock with a dense structure that breaks with a distinct fracture. Chats is a term used in the West to denote the tailings of lead mines. It is a dolomite limestone. Sandstone. Sandstones, like limestones, are variable both as to color and structure. In color they are red, brown, green and yellow. Pudding stone or conglomerate is a coarse sandstone. Breccia is similar to conglomerate except the stones are more angular. Sandstones consisting almost entirely of quartz are sometimes called grit. Flints, so-called in Great Britain, are found in the upper layers of chalk pits or in gravel deposits near |