Building and ornamental stones. 407 



E.— TnE GRANITES AND GNEISSES. 



(1) COMPOSITION AND ORIGIN. 



By the term "granite" is understood a crystalline granular mixture 

 of the minerals quartz, orthoclase, and plagioclase, which, in varying 

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

 nearly always present one or more of the minerals biotite, muscovite, 

 or hornblende, and more rarely augite, chlorite, tourmaline, graphite, and 

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

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

 cannite, and microcline. These last, although of scientific interest, are of 

 little practical 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 plagioclase 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 

 laminae produced by twin formation. 



The quartz does not occur in the form of crystals, but rather in that 

 of angular crystalline grains. It appears always fresh and glassy, but 

 on 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 filled 

 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. 

 Carbonic 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 been estimated from one to ten 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 interstratified 

 with metamorphic rocks or forming the central portion of mountain 

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



* Judd on Volcanoes, p. 64. 



