542 H. S. JEVONS, H. I. JENSEN, T. (i. TAYL.OR AND 0. A. SUSSMILCH. 



for the biotite, but is conditioned by the remainders of PeO 

 and MgO after allotting for the augite and magnetite in 

 specimen I, which contains an insignificant quantity of 

 biotite as will be seen on referring to the table of mole- 

 cular proportions for that specimen. Confirmation of the 

 high iron content of the olivine is to be found in the deep 

 colour of the serpentine, whether brown or green, resulting 

 from its alteration. Further verification of its ferriferous 

 nature seemed desirable, however, and the only readily 

 available means appeared to be the determination of the 

 strength of double refraction, for according to the figures 

 of Levy and Lacroix 1 that of olivine is "036, that of natural 

 fayalite '049 and of artificial fayalite *043. Sections of 

 specimens B and Y in which the olivine is abundant and 

 little altered were searched, and the section giving the 

 highest colour examined in convergent light, with the 

 result that a section was found in Y very nearly parallel to 

 the optic axes, in B one not so nearly parallel, but sufficiently 

 so to be worth examining further. The highest colour 

 shown by any of the labradorite crystals in the vicinity of 

 the olivine crystal chosen was then found. In both cases 

 it lay in the first order yellow to red, and was accurately 

 determined by addition and subtraction with a quarter- 

 wave plate. 



The olivine crystals were found to be heterogeneous in 

 composition. Most of the crystals have an outer zone 

 distinctly stronger in double refraction than the interior of 

 the crystal section, the strength of D.R. increasing gradu- 

 ally but rapidly on nearing the edge of the section. In one 

 or two cases, however, the zone of stronger D.R. was 

 wrapped round an irregular centre of weaker D.R., the 

 boundary between the two being perfectly sharp. The 

 readings, and the results obtained by us of Michael Levy's 

 chart are shown in the following table. 



1 See Mineraux des Roches, pp. 248-9. 



