652 CX OSS, IDDINGS, PIRSSON, WASHINGTON 



words, that the calculation of the former of each pair precludes 

 that of the latter, and conversely. 



With quartz there will be no nephelite, leucite or olivine. 



With hypersthene there will be no nephelite or leucite. 



With corundum there will be no diopside or acmite. 



With anorthite there will be no acmite. 



With wollastonite there will be no hypersthene or olivine. 



With leucite there will be no albite. 



Percentage weights of minerals. — Having estimated the rel- 

 ative number of molecules of the various mineral components, 

 their relative masses may be obtained by multiplying each by 

 the molecular weight. This is readily accomplished by means 

 of tables, both for finding the molecular proportions corre- 

 sponding to percentages of the chemical components given in 

 analyses, such as those lately published by J. F. Kemp 1 and 

 others, for finding the percentage weights of the minerals with 

 constant composition when their molecular proportions have 

 been calculated. The weights of minerals like olivine and 

 pyroxene in which the component (Mg, Fe) O is variable must 

 be calculated from the proportions of MgO and FeO present 

 in the rock after deduction of FeO allotted to magnetite and 

 ilmenite, the same ratio between these oxides being used for 

 each kind of -molecule containing both of them. The weights 

 of diopside, hypersthene and olivine, in which MgO and FeO 

 occur in varying amounts, may be computed from the sums of 

 the simple molecules CaSi0 3 , MgSi0 3 , FeSi0 3 , and Mg 2 Si0 4 , 

 Fe 2 Si0 4 . 



Tables for finding the molecular proportions of the constitu- 

 ent oxides, and those for the percentage weights of the standard 

 minerals will be found in the reprint of this paper, already 

 alluded to. 



EXAMPLES OF CALCULATIONS. 



It will be useful to give some examples illustrative of 



the method of calculation and of the various possibilities, 



selected from several thousand calculated by us. To simplify 



1 Kemp, J. F., " The Recalculation of the Chemical Analyses of Rocks," Schoot 

 of Mines Quarterly, Vol. XXII, pp. 75-88. 



