GRANITE, in mineralogy, is a particu lar mountain rock, composed of felspar, quartz, and mica. In general the felspar is the predominating substance, and mica the least considerable. In some varieties the quartz is wanting, and in others the mica. The constituent parts differ like. wise considerably in their magnitude : they alternate from large to small, and even very fine granular. The large and coarse usually belong to the oldest, and the small and fine granular to the newer ' granite formation. It differs also in co lour, and this difference depends chiefly on the felspar, the quartz and mica being usually of a grey colour. The felspar passes from the white to the red. The felspar in granite has usually a vitreous lustre, and perfectly foliated fracture ; in some varieties it passes into the earthy, with the loss of its lustre and hardness, even into porcelain earth. This is owing to decomposition, effected, according to Mr. Davy, by eletro-chemical agencies. Sometimes the constituent parts of gra nite are regularly chrystallized, but prin cipally the felspar and quartz. The mica sometimes occurs in nests unmixed with the other parts. Sometimes the consti tuent parts are so arranged, that when a specimen is cut, its surface has a kind of resemblance to written characters. Hence it has been denominated GRAPHIC stone.
Besides felspar, quartz, and mica, the essential constituent parts of granite, other fossils occur in it : of these, schorl is the most frequent, and next is garnet and tin-stone. There are three forma tions of granite ; the first, or oldest, serves as the basis for all the other clas ses of rocks. The second occurs only in the first; and the third, or newest, ap pear to be among the newest of the pri. mitive rocks. In the oldest granite for. mation, when it rises to a height above the surface of the earth, and is surround ed by other primitive rocks, these are al ways wrapped around it, or the strata are mantle-shaped. This is one of the most widely-extended and abundant formations with which we are acquainted. The se
cond granite formation occurs only in veins which traverse the oldest forma tion, but never reaches any of the newer rock. The newest granite formation al ways rests on some of the older primitive rocks, and usually in an overlying posi tion. It never occurs in globular dis tinct concretions : its structure is very irregular, sometimes contains grains of precious garnet, and has a deep red co lour. it often occurs in veins that shoot from the rock, or in veins that are not connected with any rock beyond the stra ta which they traverse.
When granite is exposed, it frequently occurs in high and steep cliffs,which form vast mural precipices : often also in lofty summits, denominated peaks. It is found in almost every country, and in many places the stones are of an immense size. The largest, as an unconnected stone, has been described in the sixty-eighth volume of the Philosophical Transactions. It is found near the Cape of Good Hope. Granite rocks are frequently traversed by rents, which widen by the action of the elements : the mass separates into frag ments of greater or lesser magnitude, and they remain long piled on each other, in the most fanciful manner, appearing like vast artificial tumuli, or masses brought together by an immense flood. The hard white granite, with black spots, is a very valuable kind : it consists of congeries of variously constructed and differently co. loured particles, not diffused among nor running into one another, but each pure and distinct, though firmly adhering to which ever of the others it comes in con. tact with, and forming a very firm mass. It is much used in London for the steps of public buildings, and in other situations where great strength and hardness are required. The hard red granite, varie gated with black and white, is common in Egypt and Arabia. The stones used in paving the streets are another species of granite. Granite, though not abounding in metal, contains occasionally some of the most important Iron and tin occur most frequently.