Weed and Pirsson — Bearpaw Mov/ntains of Montcma. 197 



11. Leucite tinguaite, Magnet Cove, Arkansas. J. F. Wil- 

 liams anal. (Igneous Rocks of Ark., Ann. Rep., 1890, vol. ii, p. 

 28V.) 



la. Molecular proportions of No. I. 



The interesting feature of this analysis is the rather low 

 silica and great quantities of alkalies. A consideration of the 

 molecular proportions shows that there is considerably more 

 than enough to furnish the proportion R^O : ^fi^ = l : 1, and 

 this excess is, of course, due to the presence of the sodalite, 

 which forms 10 per cent, of the total weight of the rock. 

 The sulphuric anhydride proves that the nosean molecule is 

 certainly present and forms about 4:'3 per cent of the rock. 

 The minerals being fresh, clear, and colorless, it is impossible 

 to tell it from sodalite in the section. The minute amount of 

 phosphoric anhydride shows that the fluorine is to be referred 

 to the fluorite present ; after deducting enough lime for this 

 small amount of fluorite, the remainder exactly balances the 

 lime and magnesia and is present as the diopside molecule. 

 Hence the feldspars are entirely alkaline. After deducting 

 enough soda to turn the ferric iron into segirite and to satisfy 

 the acid radicals, other than silica, which are present, a consid- 

 eration of the remaining molecular proportions shows clearly 

 that the orthoclase molecule is by far the dominating feldspar 

 present, the soda being mostly taken up by nephelite and 

 sodalite. 



Taking these facts into consideration and regarding the 

 nosean as a pure soda compound, the following calculation has 

 been made of the proportion of the various mineral molecules 

 present : 



Diopside 



Ca(Mg,Fe)Si,0, 



5*4 



^girite 



NaFeSi.O, 



9-2 



Sodalite 



Na3Al3Si30,,NaCl 



9-8 



Nosean 



Na AlSiO^NaSO, 



4-2 



Nephelite 



NaAlSiO, 



16-2 



Orthoclase 



KAlSi303 



54-5 



Fluorite 



CaF, 



•7 



100-0 



This, of course, is approximate, as a small portion of the pot- 

 ash is probably present with the soda in the nephelite, while 

 conversely the feldspar, as already stated, contains some soda. 

 Relatively the amounts given in the table may be taken as 

 indicating closely the average composition of the rock. 



For comparison we have introduced the analysis of a similar 

 type of rock from Arkansas, and it will be seen that on the 



