INTRODUCTION. 9 



try, Clarke " has made the following calculations of the relative 



abundance of various rock- forming minerals: 



Per cent. 



Apatite (phosphate of lime) 0.6 



Titanium 'minerals (oxids and silicates) 1.5 



Quartz, (silica) 12.5 



Feldspar (silicates of aluminum, potash, soda, and lime) 59.0 



Biotite mica (silicates of aluminum, magnesium, potash, etc.) .S. 8 



Hornblende and pyroxene (complex silicates) 16. S 



Miscellaneous 5. 8 



Total ; 100. 



It is seen from this that probably the majority of all the original 

 rocks contain more than 50 per cent of feldspar. There are, however, 

 a great many different varieties of feldspar. The feldspars belong to 

 a type of minerals known as silicates, in which aluminum and other 

 elements, such as potash, soda, and lime, are combined with silica. 

 Pure orthoclase feldspar is a silicate of aluminum and potash, which 

 theoretically should contain 16.8 per cent of potash (K.O). As a mat- 

 ter of fact it is extremely difficult to find an example of pure ortho- 

 clase, and in most of the feldspars, which occur in large deposits, the 

 potash is to a greater or less extent replaced by soda or lime, or both. 

 For this reason it is impossible to tell without a chemical analysis 

 how much potash any given feldspar will contain. The granite rocks 

 consist of feldspar, quartz, and a little mica in varying proportions; 

 many of the other well-known rocks, such as the slates, sandstones, 

 gneisses, and traps, also frequently contain more or less feldspar. 

 Some of these rocks carry little or no potash, although many of them 

 will run as high as 5 per cent ; very few of them run higher than this. 



A cubic foot of granite weighs about 170 pounds; on a 5 per cent 

 basis, 8.5 pounds of this is potash. A quarry 100 feet square and 100 

 feet in depth of such rock would therefore contain 8,500,000 pounds 

 of potash (KoO) . It is at once apparent that nature offers a limitless 

 supply of raw material which only awaits an economical process to 

 make the potash available. 



In many localities there occur large dikes or deposits of more or 

 less pure feldspars, which usually run higher in potash than the true 

 granite rocks. These large deposits, which occur in a great many of 

 the States, have been developed in Maine, Connecticut, Pennsylvania, 

 New York, and Maryland, where they have been mined exten- 

 sively and the feldspar ground to fine powder, principally for use in 

 potteries. As has been said, pure orthoclase feldspar should contain 

 16.8 per cent of potash (KoO), but none of these deposits will run on 

 the average much more than 8 to 10 per cent. For pottery use it is 

 essential that the spar should melt in a kiln fire to a clear white glass, 



a Bul. 228, U. S. Geol. Survey, p. 20. 

 29485— No. 104—07 m 2 



