GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 



543 



3 

 '5 



Colour of Olivine between 

 crossed nicols. 



Maximum phase 

 difference feX) 

 of labradorite. 



Thickness of 

 slice in neigh- 

 bourhood of 

 olivine crystal. 



.". Minimum 

 Popt-ible D.R. 



ft 



Interior. 



Outer Zone. 



Intei ior. 



Outer 

 Zone. 



Y 



B 



Green IV 



Ord. 



Red IV 



Ord. 



Red IV 450 w 



Ord. 

 Green V ! 505 ,, 



Ord. 



*050 mm. 

 •058 „ 



•038 

 •037 



•042 

 •041 



The chief element of uncertainty was found to be the 

 D.R. of labradorite, which is here taken as *009. Levy 

 and Lacroix give '008 for both albite and labradorite, and 

 '013 for anorthite, but their determination for labradorite 

 is admittedly accurate only to within '002. If *008 is 

 correct for albite, and this seems to be in accordance with 

 general experience, 1 the probability would seem to be that 

 the D.R. of labradorite is *009 at least, assuming that the 

 progressive change in properties with composition holds 

 good for D.R. as with all the other optical properties. The 

 results in the two slides agree very closely when allowance 

 is made for the section in B not being quite parallel to the 

 optic axes. If the D.R. of labradorite be assumed to be 

 *0085 the D.R. of the olivine must be reduced in each case 

 by *002. 



The figures obtained certainly agree with the hypothesis 

 that the olivine is more than usually ferriferous. There 

 can be no question as regards the difference of D.R. by 

 *004 between the interior of crystals and the outer zone, 

 so that, even if the interior of the crystals be olivine of 

 normal composition, nearly one-third of the volume of the 

 phenocrysts, and the whole of the not very numerous small 

 crystals of the groundmass, must be unusually rich in iron. 



In the two specimens in which the composition of the 

 augite was known, it was found that the silica was deficient 

 for producing albite with all the soda. At first thought the 

 presence of nepheline would be inferred from this, but 

 there is no microscopic evidence whatever of the original 



1 Cf. the determination of the D.R. of the andesine of Baskerville as 

 •0081 by Off ret (Bull. Soc. Min., France, 1890, xin, p. 64.8.) 



