Vol. 5] Louderback.—Bc nitoite . 359 



individuals are large — two to four centimeters long — with prism 

 faces 6 to 10 mm. broad, and show a combination of the unit 

 prism m(110) and unit pyramid o(lll). The faces are somewhat 

 dull, but they show a very perfect cleavage, yielding lustrous 

 surfaces. A distinct zonal structure is present, appearing to the 

 eye as layers of different degrees of translueency lying parallel 

 to the faces of the external form. These prisms show straight 

 extinction and in a section perpendicular to prism edges an acute 

 positive bisectrix, the axial plane bisecting the cleavage at the 

 obtuse angle, that is, ||(010). It does not seem probable that 

 these crystals formed under the same conditions as the natrolite 

 matrix of the titano-silicates. The peculiarity of the prevailing 

 habit may be considered as dependent on the unusual composi- 

 tion of the solutions giving rise to the benitoite and neptunite. 

 No crystals of benitoite or neptunite were found with or near 

 the group showing this prismatic habit. Plate 35, figure 1, is 

 from a photograph of one of these prismatic groups. 



A chemical analysis of the pure white benitoite matrix was 

 made by Professor Blasdale showing it to be pure natrolite. 



Matrix. Natrolite calculated. 



SiO, 47.69 47.49 



ALA 27.14 26.79 



Na 2 15.74 16.2S 



H,0 9.56 9.44 



100.13 100.00 



A somewhat careful search failed to show the presence of even 

 traces of titanium, barium, manganese, or potassium. 



COPPER MINERALS. 



In some parts of the deposit the copper sulphide chalcocite is 

 not infrequent. It occurs enclosed in the natrolite in dark gray 

 or black anhedral grains several millimeters in diameter. Occa- 

 sionally on fractured surface it shows distinct and characteristic 

 crystal outline, occurring then as a flat table with the trace of a 

 hexagon elongated parallel to one pair of sides. It is to be con- 

 sidered an original mineral of the veins. 



In the same part of the deposit is found the copper silicate 

 chrysocolla. It occurs in small quantity as a stain or thin coating 



