100 JOUKNAL OF THE MiTCHELL SoCIETY [AugUSt 



4 feet in diameter and from 2 1-2 to 3 feet in tliickness. All 

 of tlie large blocks of mica are not of sufficient quality to cut 

 into sheets as large as would be expected from tbe size of the 

 crystal, on account of much of the mica having been converted 

 into v^^hat is called "ruled mica", the mica being divided into 

 narrow strips whose edges are parallel to the intersection of the 

 prism and base edges of the crystal, or into "A" mica, in which 

 the sheets are cut or striated parallel to two adjacent edges. 



PERCENTAGE OF MICA IN THE DIKES. 



There is considerable variation to be noted in the per- 

 centage of mica that occurs in these dikes and in different parts 

 of the same dike. It is usually found, however, that in any 

 considerable distance in the same dike the mica will average 

 for that distance approximately the same per cent. It is sel- 

 dom that the mica in a dike will average over 10 per cent, 

 of the contents of the dike for any considerable distance, and 

 it will sometimes average as low as 1 per cent. Portions of 

 certain dikes have been observed that had the appearance of 

 containing a very large percentage of mica on account of a 

 number of blocks of mica being clustered together in bunches 

 almost touching, one another, while in other portions of the 

 same dike there would be almost a complete absence of mica 

 for a distance of from 5 to 20 feet; but even in such a dike 

 the general average of the m,ica to the other minerals corres- 

 ponded to about 10 per cent. The general limits of the per- 

 centage of mica in various dikes is probably, therefore, from 

 1 to 10 per cent. 



Of the mica 'that is obtained in these dikes, there is prob- 

 ably an average of not over 10 to 15 per cent, that can be 

 cut into sheet mica, the rest being waste or scrap mica. Select- 

 ed masses or blocks of mica, however, have been mined that 

 have averaged from 30 to 40 per cent, and occasionally as 

 high as 75 per cent, of sheet or plate mica. On the other hand, 

 there will be certain portions of the dike in which none of the 

 mica mined can be cut into sheets or plates, and is all of value 

 only as scrap mica. These variations have been observed where 



