544 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



presence of any lenad — not the slightest suggestion of it. 

 A deficiency of silica is not uncommonly found in calculating 

 modes from chemical analysis, and it is most probable that 

 in this, as in other cases, the deficiency is due to the de- 

 composed state of the rock. A slight under-estimation of 

 ferric iron would have the same effect by producing an 

 underassignment of magnetite ; and it is just possible, 

 considering the difficulties of ferrous iron determinations, 

 that a small part of the deficiency may be due to this cause 

 in specinen B, in which the ferric iron is particularly low 

 and the deficiency of silica rather high. 



The removal of silica during decomposition is undoubtedly 

 the main cause of the deficiency of silica, as shown by the 

 presence of much secondary analcite ; and in calculating 

 the modes, silica has therefore been added sufficient to 

 form polysilicates of the whole alkalies excluded from the 

 alferric minerals. 



Mode of Specimen I. 



The quantity of biotite present was assumed to be a 

 little less than 2 per cent., in conformity with the micro- 

 scopic evidence, and the number of molecules of oxides in 

 the proper proportion necessary to make this quantity was 

 calculated. Allotment then proceeded as in the following 

 table, equations involving lime-alumina ratios being used 

 to determine the augite and anorthite : — 





°4i 



O 00 



4= ° 



o b° 



a o 



4= 



e3 

 Pi 



'a 



a 



43 



o 



i 



o 

 si 



43 

 U 





'Sc 



4-2 



a 



a 



a 



bo 



03 





GO 



a 



33 



o 



S3 



0> 



u 





8$ 



o H 



<j 



M 



m 



O 



< 



< 



<3 

 110 



§ 



O 



CO 



q 



Si0 o 



41-05 



684 







10 



24 



222 



307 





32 



705 



-21 



Al 2 6 3 



12-27 



121 







3 



4 



37 



21 



55 







120 



+ 1 



Fe 2 3 



6-39 



40 







1 







7 





32 



31 



40 





FeO -> 

 MnOi 



11-26 



157 





54 



3 







37 





32 



33 



157 





MgO 



6-38 



159 







5 







121 









159 





CaO 



10-96 



196 



3 











138 



55 







196 





Na 2 



2-43 



40 











37 



3 









40 





K 2 



0-53 



6 







2 



4 













6 





Ti0 2 



4-39 



55 





54 









1 









55 





P 2 5 



0-19 



1 



1 



















] 





H 2 









... 



2 







, 















95-83 





