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



mineral at an oblique junction being counted as if belonging 

 wholly to the former. For the purpose of calculating the 

 mode we have therefore assumed the quantity of biotite 

 present to be 11*5 per cent., and have then proceeded as 

 in the case of the previous specimen (I). It is unnecessary 

 to exhibit the reduction of the mineral composition from 

 molecular proportions to weight percentage as the method 

 followed was exactly the same as in the example just given 

 (Spec. I.). 





a 



& 



O 02 



o go 



a o 



o y 



u * 



CD 



9 



0> 



'3 



CD 



a 



<D 

 O 



ffl 



CD 

 CO 



aS 

 'o 

 O 



U 



O 



0) 



< 



cd 



'a 



ft 



o 



< 



CD 



'So 



CD 



a 



in 



3 



CO 



a 



o 



a 



cd 



q 



Si0 2 



46-26 



771 







7L 



12 



300 



84 



249 





82 



798 



-27 



ALO, 



13-36 



131 







20 



2 



50 



42 



17 







131 





Fe.,0. 



2-34 



15 







7 









6 



2 





15 





FeO 



MnO 



10-53} 



13) 



148 





21 



17 









30 



2 



78 



148 





MgO 



8-87 



222 







37 









98 





87 



222 





CaO 



9-18 



164 



10 











42 



112 







164 





Na„0 



3-27 



53 











50 





3 







53 





K o 



1-23 



13 







11 



2 













13 





H„0 











14 















14 





Ti0 2 



1-78 



22 





21 











1 







22 





P 2 5 



0-42 



3 



3 













JZL. 







3 





Percentage of com- 

















position of the 



10 



3-2 



11-5 



1-1 



26-4 



11-7 



30-4 



0-5 



14-2 



100- 







rock by weight 

























Excess 0*25. 



Sum of mineral weights 99*32. 



Sum of analysis + H 2 + def. Si0 2 99'07. 



Composition of plagioclase : Ab 10 oAn 42 = Ab 7 An 3 



Mode of Coarse Aplitic Rock (Essexo-aplite). 



The problem of calculating the mode of the coarse aplitic 

 rock from its analysis differs from that of the preceding 

 two specimens, for microscopic examination shows that it 

 is devoid of both biotite and olivine, and that the pyroxene 

 present must have a composition different from that of the 

 violet-brown augite of the main rock, as it is colourless to 

 pale-green in colour, and passes in the outermost zone to- 



