and other Rocks from Montana. 293 



lies, while the alumina is higher. The low percentage of CaO 

 and MgO and the high amount of FeO show, however, that it 

 is very closely related to it. In the absence of any crystallo- 

 graphic orientation the only way of distinguishing it from 

 arfvedsonite is by its color and streak. I believe this is the 

 first time that barkevikite has been identified in the United 

 States. The hornblende in the pulaskite* described by J. 

 Francis Williams, also a syenitic rock, is probably closely 

 related to barkevikite, but no analysis was made of it. 



The socialite forms irregular grains and partly developed 

 crystals ; it is allotriomorphic against the feldspar, but usually 

 idiomorphic whenever bordering on the analcite. It some- 

 times fills triangular interstices between the lath-shaped feld- 

 spars. The period of its consolidation seems here to be 

 decidedly later than that of the feldspar. The cleavage paral- 

 lel to a is well indicated by the arrangement of very numer- 

 ous inclusions in part of gas, in part of liquid with very large 

 air bubbles ; frequently these inclusions have the form of the 

 enclosing minerals. Small moving bubbles do not often occur. 

 The sodalite is perfectly isotropic and very fresh. Only very 

 locally may a corrosion and decomposition into analcite be 

 observed, such as shown on the large sodalite grain in figure 2. 



Fig. 2. 



Sodalite-syenite. Magnified 25 diam. A hornblende, B orthoclase, soda- 

 lite, D analcite. 



A little chlorite or serpentine is sometimes infiltrated from the 

 hornblende. The sodalite is somewhat unequally distributed 

 through the rock ; the figures are taken from places where it 



* Geol. Survey of Arkansas, Ann. Eep., 1890, vol. ii, p. 64. 



