480 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.58. 



softened completely only 0.93 per cent of water was obtained. The 

 amount of water given above was obtained by using a fire brick and 

 charcoal oven, with the blast as recommended by Penfield. 



Optically this mica differs sharply from muscovite in indices of 

 refraction. The following optical data were obtained: Biaxial, 

 negative; axial angle, 2V; moderately large. 



Indices of refraction : 



a=1.549±.003. 



p=1.579±.003. 



7=1.500±.003. 



0-7= .041±.O03. 



The variation in mean index of refraction doubtless accompanies 

 reciprocal variation of potash and hydroxyl. 



Mica of this foliated type occurs in practically all of the quartz 

 veins of this vicinity, as seen especially in the main pit and in small 

 openings along the tramway. It is probably in all cases derived 

 from topaz. 



MARGARITE. 



Associated with the margarodite and topaz of the limekiln vein 

 were blocks of a micaceous fibrous mineral which in the field was 

 supposed to represent a fibrous phase of the margarodite. This 

 material which apparently came from the wall of the vein, next the 

 marble, forms veinlets from 2 to 4 inches thick. The mineral 

 extends out from either side of the veinlet for an inch or more as a 

 compact layer of straight or very slightly curved fibers perpen- 

 dicular to the wall. These fibers do not meet in the centers of the 

 veinlets which contain a confused granular aggregate of the same 

 substance in which masses of colorless to pale salmon fluorite and 

 clear large plates of margarodite are scattered. There is interposed 

 between the layers of this fibrous material and the adjacent lime- 

 stone a band from 2 to 10 inches thick, largely composed of granular 

 fluorite which varies in color from brownish salmon adjacent to the 

 fibrous vein to purple next the marble. The mass of fibers is com- 

 pact and tough in the aggregate but is readily pulverized in a mortar 

 wherein it differs from margarodite. In color the fibrous mineral is 

 snowy white and the luster is pearly. Upon analysis the results in 

 column 1 of the following table were obtained. In column 2 is 

 given, for comparison, the analysis b}' J. Lawrence Smith of mar- 

 garite from Chester, Massachusetts,^ and in column 3 are given the 

 values calculated to satisfy the formula CaO.2Al2O3.2SiO2.H2O. 



1 Dana, J. D., Syst. Min., ed. 6, p. 637. 



