BUILDINd AND ORNAMENTAL STONES. 407 



E.~TIIE (HIAXITES A^TD GNEISSES. 



(1) COMPOSITION AND OKKJIN. 



By tbe term "granite'' is understood a crystalline granular mixture 

 of the minerals quart/, orthoclaso, and plagioclase, which, in varying 

 proportions, malce up tlie chief bulk of the rock. Besides these, there is 

 lu^'irly always present one or more of the minerals biotite, muscovite, 

 or hornl)lende, and more rarely aagitc, chlorite, tourmaline, graphite, and 

 lienuitite. By the aid of the microscope may frequently be detected other 

 accessory minerals such as apatite, epidote, zircon, magnetite, menac- 

 eannite, and microcline. Tliese last, although of scdentilic interest, are of 

 little i>ractical importance. 



Microscopic study of properly prepared thin sections of granite have 

 shown that there are at least two varieties of feldspar and that they 

 are radically different. The one is orthoclase, which is usually the pre- 

 dominating constituent, while the other is a triclinic variety, usually 

 albite or oligoclase, called for convenience j.t^r((7to(;/^f.s'e when the exact 

 variety can not be definitely ascertained. It is easily distinguished 

 from the orthoclase by its beautiful banded structure as seen in polar- 

 ized light. A third variety, identical in chemical composition with or- 

 thoclase, but crystallizing in the triclinic system, is also frequently pres- 

 ent. This is microeline. Under the microscope it shows a peculiar 

 basket-work structure, due to the nearly rectangular intersection of its 

 lamina' produced by twin formation. 



The quartz does not oc(nir in the form of crystals, but rather in that 

 of angidar crystalline grains. It appears always fresh an<l glassy, but 

 oil microscopic examination is found to contain numerous inclosures, 

 such as rutile needles and little prisms of apatite. A most interesting 

 fact is the presence of minute cavities within the quartz, usually tilled 

 wholly or in part with a liquid, though sometimes empty. This liquid 

 is commonly water containing various salts, as the chloride of sodium or 

 potassium, which at times separates out in the form of minute crystals, 

 ('arbonic acid is frequently present, giving rise to a minute bubble like 

 that of a spirit-level, and which moves from side to side of its small 

 chamber as though endowed with life. So minute are these cavities 

 that it has l)een estimated from oiu' to t(Mi thousand millions could be 

 contained in a single cubic inch of space.* 



Granites are massive rocks, occurring most frequently associated with 

 the older and lower rocks of the earth's crust, sometimes interstratiiied 

 with m(;tamorphic rocks or forming the central portion of nu)untain 

 chains. They are not in all cases, as was once supposed, the oldest of 



* J 11(1(1 (III Volcauoets, p. ()4. 



