294 



As these are all fully described in the numerous works on mineralogy 

 it is not deemed necessary to enter into any elaborate discussion of their 

 properties here, excepting in the case of those few which from their abun- 

 dance, or from other causes, have a pronounced effect upon the rocks in 

 which they occur. 



QUARTZ. — Chemical composition : Pare silica, Si0 2 . Hardness, 7.* 



This is one of the commonest minerals of the earth's crust, and is an 

 essential constituent of granite, gneiss, mica schist, quartz porphyry, 

 liparite, quartzite, and ordinary sandstone, occurring in the form of 

 crystals, crystalline grains, and fragments of crystals. It is usually 

 easily recognized by its clear, colorless appearance, irregular, glass-like 

 fracture, hardness, and entire insolubility in acids. Its hardness is 

 such that it scratches glass, and in this respect alone it differs from any 

 other of the essential constituents. It is, however, brittle, and hence, 

 though the hardest mineral, is by no means the most refractory; stones 

 like granite, which are rich in quartz, working more easily than the 

 trap-rocks, in which it is, as a rule, entirely lacking. 



Although ordinarily one of the most indestructible of minerals, and in- 

 fusible in the hottest flame of the blow-pipe, yet highly quartzose rocks 

 like granite are by no means fire-proof, but scale badly when subjected to 

 the heat of. a burning building. This peculiar susceptibility of the rock 

 to heat is thought by some to be due to the microscopic fluidal cavities 

 which exist in the quartz, and which are at times exceeding abundant. 



THE FELDSPARS. Hardness, 5 to 7. 



The feldspars are essentially silicates containing alumina together 

 with potash, soda, or lime. There are six varieties that are common 

 -constituents of building stones, viz, orthoclase, microcline, albite, oli- 

 goclase, labradorite, and anorthite. Of these, albite, oligoclase, labra- 

 dorite, and anorthite are usually indistinguishable from one another by 

 the eye alone, especially in fine-grained rocks, and are therefore desig- 

 nated by the convenient term plagioclase feldspars or simply plagio- 

 clase. Orthoclase is the pre vailing feldspar and most important con- 

 stituent in granites and gneisses, and is usually accompanied by albite 



IV _ , 



*For convenience in determining minerals the "scale of hardness" given below 

 has been adopted by mineralogists. By means of it one is enabled to designate the 

 comparative hardness of minerals with ease and definiteness. Thus, in saying that 

 serpentine has a hardness equal to 4 is meant that it is of the same hardness as the 

 mineral fluorite, and can therefore be cut with a knife or other tool, but less readily 

 than calcite or marble. 



1. Talc. — Easily scratched by the thumb-nail. 



2. Gypsum. — Can be scratched by the thumb-nail. 



!?. Calcite. — Not readily scratched by the thumb-nail, but easily cut with a kuife. 



4. Fluorite. — Can be cut with a knife, but less easily than calcite. 



5. Apatite. — Can be cut with a knife, but only with difficulty. 



6. Orthoclase feldspar. — Can be cut with a knife only with great difficulty and on 

 thin edges. 



7. Quartz. — Can not be cut with a knife; scratches glass. 



