365 



mineral be nosean such crystals will not be formed at all or only 

 to a very small extent. 



Augite. Several varieties of augite occur in this group. The 

 augite of the more basic members does not call for any special 

 description. It is worthy of note, however, that idiomorphic augite in 

 the form of large porphyritic crystals, and also in the form of small 

 microlites, is far more common in the nepheline- and leucite- than 

 in the felspar-bearing rocks. In the normal elseolite-syenites and in 

 the phonolites and leucitophyres green augites are very common. 

 These are in many cases segirine. The brown alkali- bearing augite 

 (akmite) also occurs. The optical properties of segirine and akmite 

 are markedly different from those of ordinary augite. The mineral 

 is usually elongated in the direction of the vertical axis. The forms 

 in the prismatic zone are (110) and (100). The negative bisectrix 

 makes an angle of only 4 or 5 with the vertical axis. The 

 extinctions, therefore, in the prismatic zone are always very low even 

 lower than in the hornblendes. Notwithstanding the fact of these 

 low extinctions it is always perfectly easy to distinguish between 

 segirine and hornblende, for in the former the major axis of depolari- 

 zation is the one which most nearly coincides with the vertical axis 

 whereas in the latter it is the minor axis of depolarization which is 

 most nearly coincident with this axis. The green microlites in the 

 phonolite of the Wolf rock may thus be proved to be segirine and 

 not hornblende. Both akmite and segirine are pleochroic. There are 

 probably intermediate forms between typical akmite or segirine and typical 

 augite. The analyses made by MANN (I) are particularly interesting from 

 this point of view. This observer separated the pyroxene from several 

 rocks belonging to the present group and showed that the maximum 

 extinction angle in the prismatic zone decreased as the percentage of 

 iron and alkali increased. 



Total Iron Total Extinction 



as FezOs Alkalies. Angle. 



Hauynophyre ) 



19-fi7 1-QQ 89 



(Melfi) ... } 



Leucitophyre 1 Ig . g2 3 . 3g QQO 



(Bieden) ... j 



Ph / 'f e t 26-85 ... 13-88 ... W 



(Honentwiel) ) 



Hornblende. This mineral appears to be less common than was 

 formerly supposed. It does, however, occur. It is often green in 

 colour. The alkali-hornblende, arfvedsonite, is present in the elseolite- 

 syenites. In thin section it is extraordinarily pleochroic, varying from 

 clear yellow to dark brownish black. Sometimes greenish tints may 

 be observed. The extinction in the prismatic zone is not much less 

 than 15. 



(1) N.J. (1884). Band II., p. 205. 



