MICROSTRUCTURE AND COMPOSITION OF HYPERSTHENE. 345 



give an obtuse bisectrix in converging light. Hence the optical character 

 of the mineral is negative, since the vertical direction is that of least elas- 

 ticity. 



The inclosures so commonly found in massive hypersthene are entirely 

 absent. The mineral has the same microstructure as the hypersthene which 

 occurs in the porphyritic rocks. Well defined crystals of feldspar and grains 

 of augite are nearly always inclosed in the hypersthene crystals. Bauds of 

 augite are sometimes intergrown with the hypersthene. 



An intergrowth between hypersthene and a colorless mineral, which is 

 probably plagioclase, exhibits granophyre structure in a section from the 

 " Dumpling." 



The material for the analyses I and II, given in the table below, was sep- 

 arated by the Klein solution at the density 3.356, after removal of the 

 magnetite. The grains seemed to be fresh and pure when examined under 

 the microscope. 



Analyses of Hypersthene. 



I. — Twins. II. — Twins. Mean. Ratio. III. — Buffalo 



Peaks. 



SiOj 52.06 52.256 52.16 ^886 .886 50.043 



ALO3 2.97 3.03 3.00 .030 ) ^qo 9 qha 



FeA 0.26 0.64 0.45 .003 [ "^^^ ^'^^^ 



FeO 15.16 15.16 15.16 .210'] 17.812 



MnO 0.37 0.35 0.36 .005 ' ^^^ 0.120 



CaO 6.00 5.88 5.94 .106 j '^^^ 6,696 



MgO 21.82 21.96 21.89 .547 J 21.744 



K2O 0.04 0.04 0.04 .0004 ) ..^^ ^ „.. 



Na20 0.16 0.16 0.16 .0026] '^"'^ ^'^'^ 



Ignition 0.08 0.08 0.08 



98.92 99.55 99.24 99.595 



KO : R2O3 : SiO, = 27 : 1 : 28. 



Upon comparing the analyses of the hypersthene from the " Twins " (I 

 and II) with the analysis (III) by W. F. Hillebrand* of the hypersthene 

 occurring in the hypersthene-andesites from Buffalo peaks, Colorado, it will 

 be seen that the resemblance is as striking as that shown by the examination 

 of thin sections. 

 V Associated Minerals. — Biotite occurs in minute crystals here and there 

 throughout the rock. Its outline is usually irregular, but occasionally angles 

 of 120° were measured in basal sections. It occurs usually among the crys- 

 tals of feldspar, either alone or surrounding magnetite. It may also be seen 

 within the hypersthene and augite or along the edges of these minerals. 



The apatite occurs inclosed in the feldspar or in the other minerals, having 

 the form of fine needles. 



*"0n hypersthene-andesite," etc., by Whitman Cross: Bulletin U. S. Geol. Surv., no. 1,1883, p. 29. 



