666 CROSS, IDDINGS, PIRSSON, WASHINGTON 



approximately the same composition as other amphiboles and 

 micas which have been analyzed. 



Further consideration of the problem will convince one that 

 where augites, amphiboles, and micas occur with femic minerals, 

 such as diopside, hypersthene, and olivine, the problem cannot 

 be solved by simply determining the kinds of minerals present 

 in the rock. The algebraic equations involve too many unknown 

 quantities. In other words, there may be variable amounts of 

 the same minerals developed from chemically similar magmas. 

 It becomes necessary then to determine the relative amounts of 

 several of these minerals, according to the number of them, in 

 order to reduce the number of unknown quantities in the alge- 

 braic equations. Then it is possible, with part of the problem 

 solved by microscopical study, to complete it by estimating the 

 remaining factors from the chemical analysis of the rock. That 

 is to say, in most cases the microscopical and chemical methods 

 must supplement one another. 



Thus it is possible to calculate the probable composition of 

 a hornblende in a given rock when all the other minerals have 

 comparatively simple, or fixed, molecules, and when the quantity 

 of the hornblende has been determined optically. In another 

 case, if hornblende has been separated from the rock and 

 analyzed, it is possible to calculate the probable composition of 

 a biotite present, when the proportions of these two minerals are 

 known, and the other minerals in the rock have fixed molecules. 



The same process may be used to determine the composition 

 of the groundmass, when the character and percentages of the 

 phenocrysts have been determined. 



The method of calculation, which is illustrated by the case of 

 the Butte granite given on a subsequent page, may be stated as 

 follows : 



Starting with the chemical analysis of the rock, reduce it to 

 molecular proportions by dividing the percentage of each chemi- 

 cal component by its molecular weight. 



Deduct from these molecules the molecules belonging: to such 

 minerals as have been chemically and quantitatively determined. 



