14 Analyses 'of Igneous Rocks [212 



approximately 16% CaO, the most abundant with approxi- 

 mately 10% and a third with about 3%%. While the usual 

 " average " analysis of anorthosite would represent in a gen- 

 eral way the more abundant variety the breadth of range, the 

 discontinuity of the series and the existence of grouping is 

 obscured. 



Dunite (Fig. 1) is another rock consisting largely of a single 

 mineral. Here the analyses shows little variation except in 

 the ferrous iron which shows an absolute range of 10% and a 

 relative range of 156% of its mean amount. Magnesia shows 

 an absolute range of 7% or about 15% of the average content. 

 The isomorphism of the olivine group would suggest more 

 uniform departures of the iron and magnesia from their aver- 

 age values. 



Horriblendite (Fig. 2) is a third representative of rocks 

 composed essentially of a single mineral or mineral group. 

 Here the possibility of alumina in the molecule suggests 

 wider departures from the average but the analyses show an 

 absolute range in alumina of scarcely 9%, although nearly 

 80% of the average content. Ferric iron shows about the 

 same absolute range but a much greater relative range because 

 of its lower average content. Ferrous iron, on the other hand, 

 with an average content similar to that of alumina, shows a 

 relative range of 175%, while magnesia and lime show total 

 ranges of less than 100% of the average content. 



From the foregoing it seems reasonable to infer that con- 

 clusions based upon " expansions " of averages in accordance 

 with the known isomorphism of constituent minerals are un- 

 certain even in monomineralic rocks. 



Turning to rocks consisting essentially of olivine with one 

 or more pyroxenes or hornblende two rocks were selected. 



Uarzburgite or Saxonite (Fig. 4) composed of olivine and 

 an orthorhombic pyroxene shows several variant types which 

 are entirely obscured in an average analysis. The rocks which 

 have been named harzburgite compared with those called 

 Saxonite generally show lower alumina and lime with higher 



