34:4: L. V. Pirsson — Phonolitic Rocks from Black Hills. 



parallel extinction and by their very low single and double 

 refraction. There appear also small hexagonal sections of a 

 mineral which is full of dusty inclusions and always isotropic. 

 It is supposed to be of the sodalite group which is also indi- 

 cated by the chlorine shown in the analysis. Patches of a 

 zeolite in radial structure sometimes occur. 



The powdered rock gelatinized readily with hydrochloric 

 acid and no effervescence of C0 2 could be seen. A chemical 

 analysis of it yielded the following results. 



Sp. G. 2-582. Si0 2 _ 61-08 



Ti0 2 0-18 



A1 2 3 18-71 



Fe 2 3 ... 1-91 



FeO 0-63 



MnO trace 



CaO 1-58 



BaO 0-05 



MgO 0-08 



Na 2 8-68 



K 2 4-63 



H s O(ign.)-- - 2-21 



CI 0-12 



SG 3 trace 



Total 99-86 



= C1 0-03 



99-83 



To be noted is the very small amount of bivalent metallic 

 oxides present and the great excess of soda over potash. 



A phonolite occurring in El Paso County, Colorado, has 

 been described by Cross.* It is the only one thus far in North 

 America which has been investigated. f The analysis of it is 

 almost identical with that just given and shows the same excess 

 of soda over potash. So also do the acmite trachytes from the 

 Crazy Mts. described by Wolff and Tarr4 In this respect 

 these rocks are noteworthy among American occurrences and 



* Proc. Colorado Sci. Soc, 1887, p. 167. 



f Recently A. Osann (Geol. Surv. Texas, Ann. Rep. 1892, p. 130) has briefly 

 mentioned phonolites as occurring in the Trans Pecos district, Texas. This 

 makes the third American locality. E. Goldsmith has described so-called phono- 

 lites from several localities in eastern Pennsylvania, but since he states that these 

 rocks contain plagioclase, hypersthene and quartz and does not mention nephe- 

 lite it is not likely that the majority of petrographers will agree with his deter- 

 minations. His chief reason for calling them phonolites seems to be due to the 

 fact that the rocks split into thin platss and ring when struck with the hammer. 

 He calls them gabbro phonolites ! and seems to have been unaware of the previous 

 work of Cross and Caswell. (Proc. Acad. Nat. Sci. Philadelphia, 1893. p. 176.) 



JOp. cit, p. 232. 



