370 T. W. E. DAVID, F. B. GUTHRIE, AND W. G. WOOLNOUGH. 



The last three minerals forming an segirine-augite, and the first 

 two a soda-potash felspar. 



Mean percentage composifAon of the rock. 



Si0 2 



... 51-51 



A1 2 3 ... 



... 19-66 



Fe 2 3 ... 



4-21 



FeO 



1-32 



MgO 



... 0-36 



CaO 



1-28 



Na 2 



... 11-81 



K 2 



... 4-39 



MnO 



... 0-37 



Distrib 



uted as 



follows 



: — 





94-91 









Mean 

 Percentage. 



Soda-potash Felspar. 

 Orthoclase. Albite 



Nepheline. 



^Egirine. 



RSi0 3 



MnOSi0 2 



Total. 



Si0 2 



51-51 



9-88 



17-84 



13-85 



6-32 



3-30 



0-32 



51-51 



A1 2 3 



19-66 



2-80 



5-06 



11-77 









19-63 



Fe 2 3 



4-21 









4-21 







4-21 



FeO 



1-32 











1-32 





1-32 



MgO 



0-36 











0-36 





0-36 



CaO 



1-28 











1-54 





154 



Na 2 



11-81 





307 



5-96 



1-63 







10-66 



K 2 



4-39 



2-58 





1-81 









4-39 



MnO 



0-37 





25-97 









0-37 



0-37 





94-91 



15-26 



33-39 



12-16 



6-52 



0-69 



93-99 



The difference (0*92) between the totals as calculated in the 

 last column and the amounts present in the rock being due to 

 the excess of Na 2 (1*15) and Al 2 3 (0 # 03) and to the deficiency 

 of CaO (0-26). 



Reducing these figures to percentages we get the following as 

 the approximate composition of the rock: — 

 Orthoclase and sanidine 

 Albite 



Nepheline... 

 .ZEgirine . . . 

 RSi0 3 ... 

 MnO Si0 2 



1 A» O \ 



97-fi > 43*8 soda-potash felspar 



35-6 

 13-0 ) 



6 *9 > 20-6 segirine-augite 



0-7 I 



100-0 



