16 Analyses of Igneous Rocks [214 



magnesia and similar silica, ferric and ferrous iron. These 

 variants may be due to chance and their distinctness to 

 paucity of example, but such differences as may exist are 

 obliterated by the use of an " average " or even " typical " 

 analysis. 



Lherzolite (Fig. 3) consisting of olivine, bronzite and 

 diallage shows on the whole a wide but uniform distribution 

 in the content of the various constituents in the analyses at 

 hand. The graph shows the abnormality of the Iherzolite 

 from the Protrero, San Francisco with its low magnesia and 

 high lime which raises a question as to the applicability of 

 its name. 



All of the examples thus far considered have been named by 

 workers of different experience and training and the sugges- 

 tion comes that the variations in composition are due in part 

 at least to incomplete comprehension of the content of the 

 terms used or to personal variations in usage. To illustrate 

 range acceptable without these factors two sets of examples of 

 unique types studied by single workers were selected. 



Koswite. Composed essentially of olivine and magnetite 

 with diopside, and some hornblende, and chromite includes a 

 series of magnetite pyroxenites described from the Urals. 

 They are characterized chemically by high ferric and ferrous 

 irons. While the analyses are incomplete through lack of 

 alkalies their summation is in every instance over 100, show- 

 ing that this lack does not vitiate them for the present 

 purpose. 



The departures from the mean of values for the individual 

 constituents is here usually only two or three per cent, or less 

 than 50% of the value of the dominant constituents. 



Ariegites as defined by Lacroix are a group of pyroxenites 

 characterized by the constant association of one or more 

 pyroxenes and a spinel with varieties due to the presence of 

 garnet and hornblende. 



In this series the absolute range is 4% to 8% and the de- 

 partures in the case of the principal constituents is not over 

 10% of the mean values. 



