122 H. 8. Washington & H. E. Mermn — Mineralogy. 



same as that for the calculation of the norm of a rock, 

 and to nse the standard mineral molecules thus obtained 

 for making the necessary readjustments. 9 The recal- 

 culation of norm into mode, or mode into norm, is thus 

 much simpler than if the ratios of the various con- 

 stituents of the mineral are used, as was advocated in the 

 original publication of the quantitative classification. 10 

 The norm of the Haleakala augite is given here, as it will 

 be of use in calculating the modes of many Hawaiian, and 

 probably other, lavas. 



Norm of Haleakala Augite. 



Anorthite 15.57 



Nephelite 3.12 



Diopside 70.68 



Olivine 2.23 



Magnetite 4.87 



Ilmenite 3.65 



Chromite 0.37 



Attention may be called to the presence of the outer 

 film of more highly ferric material, which was noted on a 

 previous page. This points to a state of more highly 

 oxidizing conditions in the magma during the last stages 

 of crystal growth. Possibly a similar relation might be 

 detected in other zonally built augite phenocrysts by 

 observation of the differences in extinction angle between 

 the border and the interior, if a definite relation between 

 the extinction angle and the ferric oxide content could be 

 established. The case of our augite seems to be analo- 

 gous with that of the acmite-aegirite group, in which it 

 has been often noted that mixed crystals of acmite and 

 aegirite generally show a border of acmite with an 

 interior of aegirite. This is readily observable because 

 of the very pale yellow color of acmite and the bright 

 green, pleochroic color of aegirite. This subject of the 

 higher state of oxidation of the iron in the outer parts of 

 pyroxene crystals suggests some interesting lines of 

 thought, but discussion of them must await another 

 occasion. 



Geophysical Laboratory, 

 Washington, D. C. 



9 Cf . H. S. Washington, The Eoman Comagmatic Eegion, Carnegie Publica- 

 tion No. 57, p. 134, 1906. 



10 Cross, Iddings, Pirsson, and Washington, A Quantitative Classification of 

 Igneous Bocks. Chicago, 1903, p. 211. 



