120 IF. Cross — Igneous Rocks in Wyoming. 



procured for analysis. It proves to be soda-nitre containing a 

 little potash, as shown by the analysis below, made by L. G-. 

 Eakins : 



Analysis of soda-nitre. 



£\° 8 f°9 ) 87-98 NaN0 3 . 



so? :::::::::::". ::::::: tt I °-il sy r m - 



H,6'.. -68 ) ' 54 Water - 



Cf - - trace 



99-89 



The discovery of soda-nitre in anything resembling this 

 occurrence has not been announced before, as far as I can 

 ascertain. The similarity in the conditions of occurrence of 

 the two nitrates described above adds to the strength of the 

 hypothesis that both are intimately related in origin to the 

 peculiar magna of this region. Yet it is possible that organic 

 matter from clefts above inhabited by birds or small animals 

 might have furnished the nitrogen for this nitre. 



The rock of the Boar's Tusk is nearly identical with the 

 massive variety of the Leucite Hills, and the conclusion seems 

 unavoidable that the Tusk is a plug occupying one of the con- 

 duits through which the potash-rich magna rose to the surface. 



Wyomingite. 



From the Leucite Hills proper. —The only rock type from 

 this locality described by Zirkel or Emmons is, according to 

 the experience of both Kemp and myself, much less abundant 

 than the next variety to be discussed. From reasons to be 

 more fully presented later on, I believe that this rock should 

 receive a special name, and hence it is proposed to call it 

 wyomingite, from the State in which it occurs. Although 

 this original type has been described in some detail by others, 

 it seems best to discuss it still more fully in this place in con- 

 nection with the allied rocks, particularly since this special 

 name is proposed for it. It is a massive rock, of a peculiar, 

 dull reddish-gray tone, exhibiting a marked schistosity through 

 the nearly parallel arrangement of the abundant, small, reddish 

 mica flakes. This mica, which is the only megascopically 

 recognizable constituent, is developed in very thin hexagonal 

 or rhombic platesf which, under a hand lens often show a 



* Calculated for Na 2 + K 2 0. 



f Zirkel says that the mica does not occur "in six-sided or rounded plates, but 

 in the form of remarkably long stripes and dashes, such as have seldom been 

 observed." In all the rocks of the Hills which I have examined the mica presents 

 a more or less distinct crystal form. 



