GRANITE. This is a massive, unstratified, granular rock composed essentially of quartz and feldspar, but almost always containing other components, such as mica, hornblende, and tour maline in varying proportions. The quartz and the feldspar are called essential ingredients, since their presence is necessary to form a granite; while the other constituents are called accessories. since they merely determine the variety of the granite. The term granite is popularly applied to any feldspathic granular rock, and includes gneiss, syenite, and porphyry, or any crystalline rock whose uses are the same as granite. Gneiss is a rock of granitic composition that has decided banding or parallel arrangement of its mineral constituents. Syenite is a granitic rock containing no quartz. Porphyry is popularly any fine-grained compact rock having large crystals scattered throughout its mass.
Granite varies in texture from very fine and homogeneous to coarse porphyritic rocks in which the individual grains are an inch or more in length. The color may be red, dark mottled, light to dark gray or almost black. The durability is closely related to the accessory minerals present; and although granite is popularly regarded as the hardest and most durable stone, there are some notable exceptions. A quartoze granite, one in which quartz predominates, is too brittle for paving purposes; a feldspathic granite, one containing an excess of feldspar, is too easily decomposed; and a micaceous granite, one containing considerable mica in parallel lamina, is too easily split for use in paving blocks. Gneiss is usually too much stratified to make a good paving material. Syenite is one of the best materials for paving blocks, and usually the darker the color the better the stone.
The average specific gravity of granite is 2.66, and therefore the stone weighs 1661 pounds per cubic foot, or practically 2 tons per cubic yard. Granites ordinarily contain about 0.8 per cent of water, and are capable of absorbing about 0.2 per cent more. The crushing strength is quite variable, but usually lies between 15,000 and 20,000 pounds per square inch.
A most important property possessed by all granitic rocks is that of splitting in three planes at right angles to each other, so that paving blocks may readily be formed with at least nearly plane faces and square corners. So far as discovered, this valuable
property is possessed only by the granitic and trappean rocks. This property is called rift or cleavage, and was caused by pressure before the rock was consolidated. The principal rift or plane of cleavage is always perpendicular to the line of pressure; and the character of the rift depends upon the amount of pressure and the grain of the stone. The fine-grained granites possess the most perfect rift, and it decreases as the size of the grains increase, so that a coarse-grained variety is likely to require considerable dressing to bring the faces of the blocks to a plane surface.
Granite paving blocks are produced in large quantities in Maine, California, Massachusetts, Wisconsin, Missouri, New Jersey, Pennsylvania, South Dakota, New Hampshire, and Georgia. The order in the above list is that of the number of blocks produced in 1889, the first two states producing more than all the others.* In the last few years the production of granite paving blocks has greatly fallen off, apparently more than one half, probably owing to the substitution of asphalt and brick for stone blocks for paving purposes.
Maine, New Hampshire, Massachusetts and Connecticut abound in granite deposits suitable for the manufacture of paving blocks. " Rocks of a similar nature occur in the Blue Ridge section of the Appalachians as far south as Georgia, though in the more southern portions of the region the process of decay has extended so deeply as in general much to reduce their value as sources of paving blocks. Still, blocks of granite of good quality are quarried near Atlanta. In the Cordilleran district, there are many granite rocks which are likely in time to serve as sources of paving stone. It is probable that some of the granite materials in the Ozark district of Arkansas may also serve this need." Granite is employed for paving blocks much more than any other variety of stone; and because of this fact, the term granite paving is generally used as being synonymous with stone-block paving.