26 ARCTIC BASALT PLATEAU 
are specimens representing partly a dense rock of darker or lighter 
steel gray coloured groundmass with sparingly dispersed phenocrysts 
of yellow olivine not exceeding 1 mm. in diameter; the air exposed 
surface of this rock is covered with a thin and smooth, rusty brown 
oxidation crust suggestive of desert varnish. Partly the specimens 
are slaggy and vesicular, representing lava flows, with rusty brown 
or glassy black surface and empty, no mineralized and smoothvalled 
vesicles, the rock being without visible phenocrysts. All specimens 
are of great freshness. 
Under the microscope the darker dense rock reveals two genera- 
tions of all chief mineral constituents: olivine, augite, the lenads 
and perofskite, the feldspars quite wanting. 
The phenocrysts of olivine reach the greatest dimensions, and 
the greatest among them show a zoned structure with stronger 
birefracting envelop and rounded crystal forms of hyalosideritic 
character. The axial angle is always negative. On a section | 7 
of a well zoned phenocryst a measurement of refractive indices 
gave the following results: 
in the core a ==1.670-+0.001, 8 =1.687 + 0.001 
in the envelop =1.706+-0.001, = 1.31-+ 0.001; 
the double refraction measured with the compensator of Babinet 
upon a microphenocryst without zonar structure, thus corresponding 
to the envelop of the greater crystals, gave the following figure 
7—0 = 0.0423, thickness = 0.022 mm. 
consequently y = 1.748 + 0.0015. 
From the determination diagram of the olivine group, based upon 
the change of refractive indices in connection with the varying che- 
mical composition, * the core of the olivine is found to contain about 
19 %/o Fe,SiO,, or 18 2/0 FeO, and the envelop about 36—37 %o of 
the Fe-silicate, or 32°/o FeO, thus the latter nearly according with 
hyalosiderite (34 %/o Fe,SiO,, or 30°/o FeO). Since the micropheno- 
1H. Backlund, Uber die Olivingruppe. Trav. Mus. Geol. Acad. Imp. Sciences. 
St. Pétersbourg. 3. 1909. p. 77. 
