Yol. 65.] WITH PORPHTEITIC LABEADOEITE-CEYSTALS. 



85 



No. 1) approximately of 61 per cent, of anorthite, 33 of albite, 

 and 6 of orthoclase (certain peripheral zones are a little richer still 

 in albite). The ground-mass-labradorite consists (as shown by 

 Analysis No. 2) almost exactly of 52 per cent, of anorthite, 42 of 

 albite, and 6 of orthoclase. 



To these correspond the calculated compositions Nos. lb &2b- — 





No. 1 b. 



No. 2 b. 



55-30 



28-29 



10-45 



495 



101 



SiO 



5299 

 29-84 

 12-26 

 '3-90 

 101 



Alo0 3 



CaO 



Na 2 



K*0 





Totals 



100-00 



10000 



According to the precise determinations of Messrs. Day & Allen 

 (cited below) the specific gravities are as follows : — 



Pure anorthite (CaAl 2 Si.O s ) 2765 



Pure albite (NaAlSi 3 8 ) 2"605 



Orthoclase (KAlSi 3 O s ) may be put at 2'57. 



By a percentage calculation the following specific gravities are- 

 obtained : — 



Specific gravities. 



Calculated. 



Found. 



61An, 39Ab, 60r 



2-701 

 2-686 



2-708 

 2-685 



52An, 42Ab, 60r 



The small difference discernible between 'calculated' and 'found' 

 may be taken as confirming the approximate accuracy of the 

 compositions calculated for the two plagioclases on the basis of 

 Analyses Nos. 1 & 2. 



In most statements hitherto published regarding the optical 

 constants and specific gravities of the plagioclases, only the influence 

 of anorthite and albite has been considered. Yet, in fact, most 

 plagioclases of eruptive rocks are ternary mixed crystals with a 

 small proportion of orthoclase, the influence of which on the 

 physical constants must also be taken into account. 



The pyroxene-minerals. — In the ground-mass diallage and 

 hypersthene occur in approximately equal quantities, both showing 

 the customary interpositions and occasionally having a common 

 crystallographic orientation. 



