BLACK HILLS GEOLOGY. 275 



XA'. ^ Phonolite from the East Slope of Bald Mountain. The 

 exposure from which this was taken is probably a dike. An- 

 alysis by Professor Flintermann, of Deadwood, South Dakota. 



If we run over these an^yses, it will at once appear that the 

 phonolites are of a rema. ably uniform composition. 



The Rock from Ragged Top Mountain, which is a typical 

 phonolite with much nepheline, shows little i any difference, 

 from the more trachytoid varieties, in which very little o that 

 mineral is present. The inference is that since the ratio of soda 

 to potash remains unaltered, much of the ormer has contrib- 

 uted to the formation of anorthoclase feldspar. That this is true 

 in the case of the rock from the Mato Tepee has been shown by 

 L. V. Pirrson.- 



The rocks shovv'ing tinguaitic texture do not differ in chem- 

 ical composition from the rest of the phonolites. 



The rocks in which leucite is supposed to occur show little 

 if any difference from the rest of the sci..'- In one instance, 

 however, No. XVII, the amount o potash rises to six per cen . 

 and the soda s proportionately diminished. This rock has 

 more the appearance of a leucite rock than any of the others. 



The complete investigation of these supposed leucite rocks is 

 reserved for a f paper. 



3. Rhyolite Family. 



The rocks belonging to this family show a considerable vari- 

 ation in texture and appearance, but are easily separated from 

 the phonolites. They are usually very much more decomposed 

 than the latter, and seldom show dark silicates in an undecom- 

 posed condition. They are very extensively developed, and 

 nearly equal the phonolites in the extent of the area they cover. 



They will be described under the following types : 



1. Portland Type. 



2. War Eagle Hill Type. 



'Rocks Nos. XVIir, A'/Jfand JfA' were not examined by the writer but de- 

 termined by Mr. D. H. Newland in the laboratory of Cohimbia University. 

 '^ American Journal of Science, May, 1894, p. 344. 



