Weed and Ph'sson — Bearpaw Mountains, Montana. 145 



usually the case in the larger leucites of leucitic rocks, and 

 they show the cross bands of twinning, best seen with a sensi- 

 ble violet tone. Only the faintest traces of the beginnings of 

 zeolitization can be seen along occasional cracks. They are of 

 perfectly normal character as seen in rocks of this class, and 

 offer nothing of especial interest. 



The case is, however, quite otherwise with the little leucites 

 that compose the grotmdmass. They vary in size from -05 to 

 •10°'"' and are thickly crowded together. They exhibit differ- 

 ent stages of growth that the mineral has passed through, and 

 by study and comparison these may be easily traced out, and 

 arranged as shown in the adjoining fig. 1. In c^ we have the 



Fig. 1. Forms of leucite growth in leucitite 

 of Bearpaw Peak. 



first stage, a simple growth along the cubic axes. In h these 

 are thickened by club-shaped ends which by further growth 

 begin to define the leucitohedron as in g ; here for the first 

 time the included glass base is shown by the dotted areas. 

 In the next stage, as seen in d, growths along the dodecahe- 

 dral axes start from the center, finally advancing and complet- 

 ing the outline, when a wheel-shaped figure is produced as 

 seen in e. The final stage consists in the production of mate- 

 rial along the octahedral axes and the completion of the outer 

 surface of the leucitohedron, when the process of growth 

 ceases and the crystal then has a club-shaped inclusion of glass 

 projecting from the center of each icositetrahedral face inw^ard 

 toward the center. A median section then gives the appear- 

 ance shown in/."^ By far the greater number of sections 

 appear like/", though c, d, and e are very common, with a and 

 h less so. The sections often exhibit the figures given with 

 almost ideal symmetry, and more interesting sections of leucite 

 for purposes of study have rarely, if ever, been seen. 



It is evident if a crystal like/ were cut through near the 



* Somewhat similar skeleton leucites from Vesuvius have been described by 

 Sinigallia, Jahrb. Min. Beil., vii, 1891, p. 417. 



