150 NEW YORK STATE MUSEUM 



varieties. The commercial sources of the two minerals, however, 

 are limited to pegmatites, those modifications of granite in which 

 the minerals are coarsely crystallized and irregularly distributed. 

 Pegmatite is found in rather limited bodies, usually in dikes or 

 lenses, which have intrusive relations with the country formations, 

 more rarely as irregular masses within normal granites. The dikes 

 or lenses range from very small examples, a few inches or a foot or 

 two thick, up to bodies several hundred feet in diameter and of much 

 greater length. They afford feldspar, quartz and other commercial 

 materials in addition to mica. 



Phlogopite is seldom if ever found in granite pegmatites, but its 

 occurrence in New York is practically restricted to crystalline lime- 

 stones where it appears to represent a secondary product of meta- 

 morphism, probably in most cases as a result of contact influences 

 exerted by igneous intrusions. It is associated with such other 

 minerals as amphibole, pyroxene, wernerite, tourmaline, fiuorite, 

 titanite and apatite. The mineral association varies from place to 

 place and the occurrence of phlogopite is quite irregular or bunchy, 

 or else restricted to a definite part of the contact zone. According 

 to the report of Cirkel,^ the commercial phlogopite deposits of 

 Canada are associated with pyroxene which penetrates country 

 gneisses and limestones in the form of dikes, the pyroxene being 

 regarded as an igneous rock. There is no resemblance to such 

 conditions in the Adirondack occurrences, though pyroxene is a 

 frequent accompaniment of the mica. The minerals rather have 

 resulted from a conversion of the limestones by mineralizing solu- 

 tions and vapors given off by granite intrusions, and they gradually 

 disappear with increasing 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 prevailing one. Their nature and mineral content are 

 even more variable 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 tech- 

 nical 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 



1 " Mica, Its Occurrence and Uses." Mines Branch, Department of the 

 Interior^ Ottawa, 1905. 



