MICA DEPOSITS OF THE UNITED STATES 501 



The next paper was 



AUaiTE-SYENITE GNEISS NEAR LOON LAKE, NEW YORK 

 BY H. P. GUSHING 



Remarks were made by H. S. Washington, J. F. Kemp, M. E. Wads- 

 worth, and N. H. Winchell. The paper is printed as pages 177-192 o^ 

 this volume. It was followed by 



PHENOCRYSTS OF INTRUSIVE IGNEOUS ROCKS 

 BY L. V. PIKSSON 



Remarks were made by J. P. Iddings and Whitman Cross. The last 

 paper was 



MICA DEPOSITS OF THE UNITED STA TES 

 BY J. A. HOLMES 



[Abstract'] 



The deposits of commercial mica in the United States, though widely distrib- 

 uted, are limited to a few districts. They have been worked to some extent along 

 the Appalachian system of mountains in New Hampshire, Virginia, North Caro- 

 lina, the Black Hills region of South Dakota, the Cribbensville district of northern 

 New Mexico, and western Idaho. 



Additional deposits of promise have been found and developed on a small scale 

 in the Appalachian region in Maine, Maryland, South Carolina, Georgia, and Ala- 

 bama, and in California, Wyoming, Nevada, and other portions of New Mexico. 

 As to the geologic occurrence of these deposits it may be said that as far as exam- 

 ined in the United States they are all found in pegmatite "veins" or dikes; and 

 these pegmatite dikes occur in schistose and gneissic rocks which are usually classed 

 as Archean in age, but some of them are evidently more recent. The dikes yield- 

 ing the best and largest quantities of mica are found in the hornblende and micar 

 ceous gneisses and schists, and are in places parallel to but generally breaking 

 across the schistosity of these rocks at varying angles. 



Pegmatite dikes, as is well known, vary in thickness from a few inches to more 

 than 250 feet, and can be traced for distances varying from a few feet in the smaller 

 ones to sometimes several miles in the larger ones. They are generally quite irreg- 

 ular and have arms branching out in almost every direction. Many are vertical, 

 while others are horizontal ; most of them vary considerably in this respect. 



In character the pegmatite may be called an exceedingly coarse granite, consist- 

 ing mainly of quartz and feldspar, in equal or variable proportions, and mus- 

 covite mica. In some places the quartz and feldspar are somewhat uniformly 

 distributed through the pegmatite mass, while in other cases the two are fairly 

 well separated, the feldspar sometimes crystallized out into masses more than a 

 ton in weight. In addition to these three common minerals, there occur in these 

 dikes a large number of accessory minerals with varying degrees of rarity. The 

 dikes in certain localities sometimes contain a considerable number of these min- 



