Weed and Pirsson — Bearjpaw Mountains of Montama, 193 



We owe the opportunity of examining and analyzing it to the 

 kindness of Professor Dana, and we hope to give a full account 

 of it at another time in connection with a description of the 

 geology and petrography of these mountains ; the analysis 

 introduced here for comparison shows a remarkable similarity 

 in the relations of the alkalies, a regional peculiarity. They 

 are potash tinguaites. 



It might have been expected that in a rock containing such 

 a high potash percentage, leucite would have formed ; it does 

 not, however, occur, and all of this potash is present as the 

 orthoclase molecule, which explains clearly the sanidine-like 

 feldspars. 



The amount of lime and magnesia is somewhat higher than 

 these rocks usually contain ; it is all present as the diopside 

 molecule, as shown below. 



If we take the chief rock-making oxides shown in the anal- 

 ysis reduced to 100 per cent, with the molecular proportions 

 they yield, these ratios will furnish the table of mineral mole- 

 cules given below, and the chemical composition of a rock so 

 composed is shown in the calculated theory. It will be noticed 

 that there are no differences except in the case of the soda, 

 which shows that the calculated composition agrees correctly 



SiO, 



AlA 



Fe,03 



FeO 



MgO 



CaO 



Na.O 



K„0 



Found. 



58-94 

 15-80 

 5-00 

 •89 

 1-41 

 2-66 

 5-62 

 9-68 



Molec. 

 Ratio. 



9823 

 1534 

 0312 

 0123 

 0352 

 0475 

 0905 

 1024 



Calc. 



58-98 

 15-80 

 5-00 

 •89 

 1-41 

 2-66 

 5-01 

 9*68 



Mineral Composition. 



Diopside Ca(Mg Fe)SiPg 20*6 



^girite NaFe Si^O^ 13-6 



Orthoclase KAl SigO^ 44-4 



Albite Na Al 8130, 12-1 



Nephelite Na Al biO, 9-3 



Total 



100-0 



with everything in the analysis except with this element, 

 which is a little too high ; this is because the small amount of 

 soda which goes with the carbon dioxide to form cancrinite 

 and with the chlorine to form sodalite has not been deducted ; 

 it is just sufficient to satisfy them. 



It appears at first peculiar that a rock containing so much 

 soda should have so little of the albite molecule present ; this is 

 a result of the fact that a large amount had been exhausted by 

 the production of aegirite before the feldspars commenced 

 forming, as we have shown above ; later the amount of silica 

 was so low that nephelite was forced to form. Probably a 

 little of the potash is with the nephelite and the amount of 

 albite is somewhat higher, but this will make no essential 

 difference. It will be noticed in the molecular ratios that the 



