2l6 



WILLIAM H. EMMONS 



To ascertain the relative amounts of hypersthene and augite 

 the following ideal compositions for the two pyroxenes were 

 assumed: 



Hypersthene (Mg, Fe)0 • SiOz 



/ (a) CaO • (Mg, re)0 • 2SiO. 

 Augite (b) Na.O • AUO3 • 4SiO. 



( (c) (Mg, Fe)0 • (Al Fe)303 • SiO. 



In the adjustment the TiOa was regarded as isomorphous with 

 SiOj and the MnO isomorphous with FeO. A quantity of silica 

 amounting to 3 . 24 per cent, cannot be accounted for by any probable 

 adjustment, and it may have resulted from decomposition of adhering 

 feldspar after treatment with acid, the insoluble silica remaining 

 behind. In the hypersthene, iron : magnesia as i : 2. In the augite, 

 diopside (a) : soda pyroxene (b) : aluminous pyroxene (c) as 204 : 5 : 32. 

 With this adjustment of molecules the proportion of the two pyroxenes 

 as they occur in the specimen gives : 



Hypersthene 38 . 84 



Augite 56.12 



Quartz 3-24 



Total 98 . 20 



Hypersthene is 40.91 per cent, of the p3rroxene, and augite 59.09 

 per cent, of the pyroxene. 



Biotite is present as irregular plates, and magnetite occurs as 

 bodies up to 0.8™"^ in diameter. Apatite occurs as prisms in all 

 the other minerals. The order of crystallization was the same as 

 in D. The chemical analysis of E, taken one quarter-mile west 

 of Mud Lake (see Fig. 3) is as follows : 



ANALYSIS OF VARIETY E. GEO. STEIGER, ANALYST 



SiOa 54.09 



AI2O3 16 . 00 



FeaOa 2.92 



FeO 5-54 



MgO 5-19 



CaO 7.37 



Na,0 3.38 



K,0 2.67 



H.O- o 



H,0+ o 



TiO^ o 



P^Os o 



MnO o 



BaO o 



SrO ._o 



Total 99.78 



,20 

 ■77 

 99 

 ■35 

 15 

 ,10 

 ,06 



