656 THE POPULAR SCIENCE MONTHLY. 



tions, as in the processes of glass-making, but in the simple pres- 

 ence of air and water at ordinary temperatures, it remains un- 

 affected through long ages. "When the silica is united with a 

 metal, such as aluminum, in kaolin and the ordinary clays, the 

 compounds are still very stable, but they are less so than the sim- 

 ple oxide. When, further, there are several metals included in 

 the compound, as in the mica minerals and their allies, the silicate 

 decreases in stability as it increases in complexity, and we have, 

 as with carbon, a readily decomposable compound. 



The world has chosen rock as the symbol of. stability, but it 

 has not chosen very wisely, for the majority of rocks are any- 

 thing but stable. 



In the case of the mica family the readiness with which the 

 minerals take up water and part with the more soluble of their 

 components is shown in the many gradations by which they pass 

 through the hydrous micas to the clays and soapstones. It is very 

 noticeable in the mica regions themselves. A mica mine is, in- 

 deed, an instructive object-lesson in soil formations. One can 

 almost see the decay of the crystalline rocks going on before 

 one's eyes. 



Were the micas only important as a rock constituent, they 

 would doubtless receive very careful study by reason of the 

 many interesting problems which their occurrence and alteration 

 bring up, but in addition to this, their characteristic physical 

 qualities, their transparency, elasticity, laminar structure, luster, 

 comparative infusibility, and electrical non-conducting power, 

 give them a number of applications in the arts, and make them 

 the object of industrial mining. The mica of the market is in 

 nearly all cases the common white mica or muscovite. From its 

 chemical composition it is sometimes known as potash mica, to 

 distinguish it from lithia and other micas, but these names are 

 more common in the laboratory than in trade. Although mica is 

 so widely distributed in Nature, it is only in a few localities and 

 under well-defined conditions that it occurs in large enough plates 

 to be profitably mined. Granite and gneiss both consist of a 

 mixture of the three minerals, mica, quartz, and feldspar (an- 

 other silicate of potash and alumina), but as ordinarily found, 

 the mica is too thoroughly mixed with the other ingredients, 

 and is in too small masses, to be available. It is only when fis- 

 sures in the rock have been filled with very coarsely crystallized 

 granite that the mica can be mined with profit. 



Such fissure veins occur in a number of localties, notably in 

 Siberia and Norway on the other side of the water ; and in our 

 own country, in New Hampshire, in North Carolina, in Wyo- 

 ming, in New Mexico, in the Black Hills of Dakota, and probably 

 in paying quantities in Alaska. Of late years the importation of 



