G. W. Tyrrell — Bekinkinite of Barshaw, Renfrewshire. 365 



Comparing Tables II and III, it is evident that the mineral 

 composition of the bekinkinite, as obtained by calculation from the 

 chemical analysis, does not agree well with the micrometric analysis. 

 The greatest difference is in the relative amounts of the light and 

 dark minerals. The chemical analysis gives a much more leucocratic 

 type than does the micrometric analysis, and, moreover, provides no 

 nepheline or analcite. These discrepancies may arise from three 

 causes : the assumptions made as to the chemical composition of the 

 constituent minerals may not be correct ; the chemical analysis may 

 be incorrect ; or the rock analysed by the Survey may not be identical 

 with the one investigated in this paper. In connexion with the 

 first-mentioned, a calculation of the mineral composition utilizing 

 the analysis of a Brazilian augite gave better results in so far as it 

 was possible to obtain a little nepheline (No. II, Table III), but the 

 relative amounts of the light and dark constituents remain much 

 the same as before. The second hypothesis may be dismissed as 

 improbable in view of the high quality of the Survey analyses. It 

 is more probable that, assuming the accuracy of the analyses, the 

 discrepancy between, the results obtained by the two methods arises 

 from the fact that the rock analysed by the Survey evidently 

 contained little or no nepheline, and was rich in the anorthite 

 molecule, thus differing from the type described in this paper. The 

 amount of lime and alumina in the analysis is much in excess of that 

 required for the bisilicate minerals, and could only go into anorthite. 



For the lugarite, however, there is a satisfactory agreement 

 between the results of the two methods of analysis. In the calcula- 

 tion of mineral composition from the chemical analysis it was 

 assumed that segirine and olivine were present in the amounts fixed 

 by those of the norm, and that analcite totalled 12 per cent. The 

 result of the calculation shows that these assumptions were justified. 

 The calculated mineral composition agrees best with varieties 1 and 2 

 (Nos. I, II, and III of Table II). In the one case there is an excess 

 of pyroxene, in the other an excess of barkevikite in the micrometric 

 analysis as compared with the quantities of these minerals in the 

 calculated mineral composition. It is probable, however, that the 

 rock analysed contained both types of lugarite, and that the chemical 

 analysis therefore represents a mixture of these two types. The 

 quantitative relations of titanaugite and barkevikite vary considerably, 

 even within the limits of a single thin section. 



There are one or two minor discrepancies. For example, no 

 anorthite can be calculated out of the chemical analysis on the 

 assumptions made, although the anorthite molecule must be present 

 in small amount. Moreover, the percentage of Ti0 3 in the analysis 

 is too small to allow any ilmenite to be calculated after titanaugite 

 and barkevikite have been satisfied in this constituent. As ilmenite 

 is undoubtedly present, the above minerals must either be poorer in 

 titania than assumed, or the analysis must be defective in this 

 particular. In spite of these discrepancies the agreement between 

 the calculated and the measured modes may be taken as good, making 

 all allowance for disturbance caused by the alteration of some of the 

 constituents of the rock. 



