108 ilOW (HOPS FEEt). 



In nature alumina is found in tlie form of emery. The 

 sappliire and ruby are tinely colored crystallized varieties 

 of aluuiin:i, highly j^rized as gems. 



Hydrated alumina dissolves in acids, yielding a numer- 

 ous class of salts, of which the sulj^hate and acetate are 

 largely employed in dyeing and calico-printing. The sul- 

 phate of alumina and potash is familiarly known under 

 the name of alum, with which all are ac^quainted. Other 

 compounds of alumina will be noticed presently. 



§2. 

 MINERALOGICAL ELEMENTS OF ROCKS. 



The mineralogical elements or minerals * which compose 

 rocks are very numerous. 



But little conception can be gained of the appearance 

 of a mineral from a description alone. Actual inspection 

 of the different varieties is necessary to enable one to rec- 

 ognize them. The teacher should be provided with a 

 collection to illustrate this subject. The true idea of their 

 composition and use in forming rocks and soils may be 

 gathered quite well, however, from the written page. For 

 minute information concerning them, see Dana's Manual 

 of Mineralogy. We shall notice the most important. 



Quartz. — Chemically speaking, this mineral is anhy- 

 drous silica — silicic acid — a compound of silicon and ox- 

 ygen, Si O^. It is one of the most abundant substances 

 met with on the earth's surface. It is found in nature in 

 six-sided crystals, and in irregular masses. It is usually 

 colorless, or white, irregular in fracture, glassy in luster. 

 It is very hard, readily scratching glass. (See H. C. G., 

 p. 120.) 



Feldspar (field-spar) is, next to quartz, the most abund- 



* The word mineral, or mineral "species," here implies a definite chemical 

 comoouiid of natural occurrence. 



