ANALYSIS AND CHARACTER 287 



devoid of optical properties. The biotite occurs in large bunches and 

 tufts. It discloses various stages of chloritization. The accessory min- 

 erals are few and unimportant. 



Analyses 



Two analyses of the granite made by Miss Zaumbrecher and two by 

 Mr J. N. Pearce, of Northwestern University, failed to show molybdenum 

 in the granite. This is somewhat surprising, since this element is notably 

 at home in acid igneous rocks.f This would seem to indicate that the 

 molybdenite in the quartz veins was not derived by lateral secretion. 



Character op the Molybdenite 



The molybdenite occurs in crystals and flakes of varying size (plate 

 13, figure 2), from minute specks to irregular masses, sometimes 20 mil- 

 limeters in diameter. The crystals show the characteristic rifts parallel 

 to the side of a hexagonal prism. The prism faces, however, are rarely 

 developed, but the usual form is that of flat pyramids which are built 

 up as shown in plate 13, figure 2. The majority of these pyramids are 

 very beautifully and strongly striated, the striations being parallel to 

 the union of the prism face with the base. Cleavage surfaces are not 

 smooth and unmarked, but are divided bylines elevated at right angles 

 to edge of intersection of pyramid with base, explained by Mugge J not 

 as " glide planes," but rather as the result of bending with " translation " 

 parallel to base (0001). Bent planes are common, and, in addition, evi- 

 dence of torsion due to molecular stress. 



These crystals furnish no new facts upon the crystallography of 

 molybdenite. Being opaque, it is not possible to use optical methods 

 in their study, and hence one of the best means of determination is 

 wanting. It is generally agreed, however, that the mineral is hexagonal, 

 according to the measurements of Brown. § Repetition twinning in 

 some of the crystals examined seems to accord with this determination. 

 The crystals show fine luster and are very pure. 



Paragenesis 



The molybdenite crystallizes sometimes before and sometimes after the 

 inclosing quartz. For example, in localities, as in Silesia, it is found 



♦Mineral Resources of the United States, 1901, p. 266. 



f Hillebrand ; Bulletin U. S. Geol. Survey, no. 167, p. 53. 



X Mugge : N. Jahrbuch d. Min., 1898, i, p. 109. 



§ A. P. Brown : Proc. Acad. Nat. Sci., Philadelphia, 1896, p. 210. 



