REPORT OF TEE CHIEF ASTROXOMER 403 



SESSIONAL PAPER No. 25a 



the acute bisectrix showed that the optical angle is relatively small (2V esti- 

 mated to be between 40° and 50°). 



The first notable difference from true anorthoclase was discovered in sec- 

 tions nearly or quite parallel to (010). Such sections are abundant in the slides 

 and are readily recognized by their rhombic outlines. The obtuse positive 

 bisectrix emerges centrally in these sections. The extinction as determined in 

 sixteen zoned individuals, all cut nearly parallel to (010), varies from 2° to 14°. 

 In these cases equal illumination of core and outer shell occurred at angles 

 varying from 30° to 10° with respect to the trace of the basal cleavage. Using 

 this principle of equal illumination after the method invented by Hichel-Levy, 

 it seems possible to orientate the directions of extinction on (010) ; they are 

 characteristically negative and read, on the average, about -10°. The core is 

 generally inclosed in a single thin shell with extinction on (010) varying from 

 44° 30' to -rl2°, but there is often a yet thinner intermediate shell with an 

 extinction close to 0°. The outer shell is never twinned, is glass-clear, and has 

 the single and double refraction of orthoclase, or, in many cases, soda-orthoclase. 

 The intermediate shell is optically and, doubtless chemically, a feldspar transi- 

 tional between the core feldspar and the outermost orthoclase. 



Though in other respects resembling anorthoclase the feldspar of the cores 

 shows the anomalous average extinction of -10° (maximum at -14°) on (010), 

 thus contrasting with the angles of + 6° 30' to +10° for anorthoclase. This 

 behaviour suggested that the mineral might contain a notable amount of the 

 barium feldspar (celsian) molecule which, in hyalophane, has the property of 

 developing negative angles of extinction; those angles increase in size as the 

 celsian constituent of hyalophane increases in amount. A quantity of cleavage 

 fragments of the rhomb-feldspars were accordingly broken out of the rock, 

 cleansed from adhering material of the ground-mass and submitted to ]\Ir. 

 Connor for analvsis. The analysis resulted as follows :— 



SiO» 54-60 



TiO, -60 



A1 2 3 22-17 



Fe-0 3 2-00 



MgO 1-30 



CaO 4-62 



SrO -80 



BaO 1-09 



Xa.0 4-46 



K 2 5-58 



H 2 2-50 



99-72 



The material was evidently impure, the microscope showing that augite and 

 titaniferous magnetite are the more important primary inclusions in the feld- 

 spar. On account of the presence of the minute but heavy inclusions, separa- 

 tion by heavy solutions is, in this case, a highly objectionable method. It 

 seemed better to use the hand-picked material and then calculate the feldspar's 

 composition after eliminating the oxides introduced into the analysis by the 

 inclusions. For this purpose the augite is regarded as having the composition 



25a — vol. ii — 26| 



