48 NEW YORK STATE MUSEUM 



ing distance from the contacts, giving way to normal crystalline 

 limestones. There is little regularity in the shape of such contact 

 zones ; in fact a highly irregular form may be said to be the pre- 

 vailing one. Their nature and mineral content are even more vari- 

 able than in the case of pegmatites. 



The working of mica deposits in this State, as well as in most 

 sections of the country, has been a rather uncertain business.. Of 

 necessity, it involves small-scale individual operations. The techni- 

 cal difficulties surrounding the industry are such that they do not 

 admit of the methods employed in other branches of mining and the 

 adoption of labor-saving devices that might tend to reduce costs. 

 Labor conditions, therefore, exercise a great influence upon the 

 course of mining operations. The principal mining activity in this 

 country at present is in North Carolina, where according to the arti- 

 cle by Sterrett in Mineral Resources for 19 12, volume 2, there are 

 probably as many as a hundred separate mines and prospects, many 

 worked by farmers in the off-season of their labors. As the entire 

 output of the country in recent years has been well under $400,000 

 annually, the average outturn from the individual mines evidently 

 is very small. India contributes most of the mica consumed in the 

 United States, though Canada is an important source of the electri- 

 cal grades. It has been stated (Mineral Industry for 191 2) that 

 the cost of labor in the production of mica in India is about one- 

 sixth of the labor cost in the domestic mines. 



The quality of mica depends upon a great many factors which 

 can be estimated accurately only by the expert. Of the three vari- 

 eties, biotite has more limited use than the others and the market 

 does not warrant mining operations for its production alone. It 

 is obtained mainly as a by-product in the working of pegmatites for 

 feldspar. Owing to its iron content it is not much used for electri- 

 cal insulation which is the prinicpal application for the colored 

 micas. Muscovite, having the greatest transparency, is preferred 

 for glazing and for lamp chimneys, shades and similar purposes. 

 The larger sizes only can thus be used. For electrical insulation in 

 motors and dynamos, phlogopite seems to find the most favor on 

 account of the softer nature of that variety, it is said. Much of 

 the electrical mica is used in the form of " micanite " which con- 

 sists of small sheets cemented and compressed into boards. The 

 presence of inclusions of iron oxid, usually magnetite, which is 

 quite common in muscovite and phlogopite, is stated by^ Sterrett to 

 have no injurious effect upon the quality of the electrical mica. 



