26 GEOLOGICAL SURVEY OF ALABAMA 



oclase or microcline, occurs in masses or distributed 

 throughout the aggregate in variable amounts. 



The occurrence or position of mica in the pegmatites 

 also varies widely. In the granular masses, of finer tex- 

 ture, it is often quite uniformly distributed throughout 

 the aggregate, and is therefore of small size although of 

 good quality. 



In the pegmatites of coarser texture, the mica crystals 

 and masses are commonly found in concentration along 

 the contact with either the foot or the hanging walls, 

 partially plated or bedded on or in the wall rock, and col- 

 lected around the edges of inclusions of horses, which 

 are composed mainly of quartz, lying wholly within the 

 pegmatite. Thin, and more than usually persistent bodies 

 of pegmatite, schist-walled, giving them the appearance 

 of veins, frequently carry highly concentrated mica in- 

 clusions, as is notably the case in certain deposits devel- 

 oped near Dadeville in Tallapoosa County, Alabama. 



Horses, or wall-rock intrusions, are a very general 

 and disturbing feature of practically all the pegmatite de- 

 posits of the State, complicating methods of mica mine 

 recovery with attendant increase of mine cost. Their oc- 

 currence has a wide range of irregularity, both as to size 

 or bulk, and in respect to conformity or non-conformity 

 with the foot and hanging walls. 



Mica, of what is commonly called A-form, or A-struc- 

 ture, is of frequent occurrence in Alabama as elsewhere, 

 being usually found in masses, often of large size, but in 

 such irregular shaped and distorted blocks as to be of small 

 commercial value except for grinding, although small 

 amounts of good flat sheet mica is to some extent recov- 

 erable near the centre of some of these blocks by splitting. 

 Among mica miners, opinion varies widely as to whether 

 or not a heavy surface capping of A-form mica may be 

 taken as a reliable indication of an underlying deposit of 

 flat mica, at a depth, and under conditions, favoring more 

 perfect crystallization. As a matter of fact, variable 

 amounts of good flat mica occur in the Alabama field co- 

 existent with the A-form masses, and in some instances 

 observed and studied, there is indicated a decided and 

 possible change from A-form to altogether other and fav- 



