74 GEORGE I. FIN LA Y 



FeO, for which 87 molecular proportion units of SiOj would be 



needed. If we should take the MgO and FeO with silica for olivine 



(ol), 2(MgFe)0.Si02, in the proportion 2:1, then sihca would be 



left over in amount equal to 18 units. The formulated method for 



calculating the norm does not admit of our making olivine at this 



point with (MgFe) O and silica, and then calling the remaining silica 



quartz. This accords with the fact that quartz and olivine are very 



rarely found together in igneous rocks. What we do is to divide the 



MgO, FeO, and available silica between hypersthene and olivine, 



making use of two simple algebraic equations. 



Let ;v=the number of hypersthene molecules 



and y=\ht number of oUvine molecules; 



■ then .-v+;y=the number of units of (MgFe)O 



y 

 and .%-+'- = the number of units of SiO,, 



2 "' 



orx+)' = 87 



y 



and x-\ — =62 

 2 



y 

 2 ^ 



3/ =50= molecules of oHvine 

 and .T=37=molecules of hypersthene. 



MgO and FeO are to be introduced in hypersthene and in olivine in 

 the same ratio in which they were used in diopside. The ratio in 

 this case is 20:67 or, nearly, 1:3^. 



It is to be noted in connection with the use of the tables that 

 olivine is the sum of two parts, 2MgO.Si02 and 2FeO.Si02. We 

 look up the first of these on p. 255, and use in looking it up one-half 



39 

 the amount of MgO units, i. e., — , not 39.; and in the same way we 



look up one-half the amount of FeO units, or 5I, not ir, on p. 256, 



and add our findings together for olivine. 



Analysis K illustrates the same points as J, but in it pyrite is 



introduced, FeSa having been present in the rock. 



x=\he number of hypersthene molecules 

 y=the number of olivine molecules 

 x-l-)'= 139= (MgFe) O 



:x:-f^=88=SiO, 

 2 



y 



-=51, 3'= 102, and x=2)T- 



