BLACK HILLS GEOLOGY. 271 



and not present in great abundance. The specimens selected 

 for study were obtained from the freshest portion of the rock, 

 and showed Httle or no decomposition. Under the microscope 

 the rock is seen to be composed of a fine mesh of aegirine need- 

 les, between which are hexagonal, slightly rounded and irregular 

 masses of an isotropic mineral, which, with subordinate rods 

 of orthoclase, makes up the body of the rock. The hexago- 

 nal form predominates, but occasional square sections are ob- 

 served. Octagonal sections were not observed. Sanidine phen- 

 ocrysts are sparingly distributed, and when seen are zonal ly 

 banded, and contain hexagons and squares of the same iso- 

 tropic mineral. 



The isotropic mineral remains almost completely dark during 

 the rotation of the stage, no light penetrating it except when 

 doubly refracting inclusions are present. 



Not the faintest trace of twin lamellation was observed by 

 means of the gypsum plate, even in the larger crystals. As a 

 rule, innumerable dusty inclusions can be observed, but these 

 may sink in prominence until the mineral is almost clear. The 

 inclusions are almost without exception aegirine. Slides were 

 treated with hydrochloric acid and then stained with fuchsine, 

 the result being a strong gelatinization which seems to be almost 

 wholly confined to the isotropic mineral. 



A glance at the analysis No. XV when compared with those 

 of the other phonolites shows that there is no increase in the 

 proportion of soda as compared with the potash, although such 

 has been shown not to be essential to the formation of leucite. 

 Still, one would hardly expect so high a percentage of soda 

 where so little nepheline and so much leucite are present. Again 

 such marked gelatinization seems to bar out such an interpre- 

 tation, and the fact that the mineral occurs included in the 

 feldspar phenocrysts would seem sufficient to exclude the possi- 

 bility of analcite, which in rocks of this kind is a secondary 

 mineral. We are then forced to consider the sodalite group as 

 the only explanation. 



Nosean occurs with such frequency in these rocks and is so 

 often without the prominent border, that in small crystals a con- 

 fusion with leucite might readily occur. 



