Red 



Yellow 



Green 



1,5416 



1 ,5440 



1,5465 



1,5417 



1,5441 



1,5466 



1,5438 



1,5464 



1,5491 



a 0,0022 



0,0024 



0,0026 



70 



so slight, that the errors of observation are probably no less. 

 The values obtained are 



a 



ß 



r 



For an accurate calculation of the angles between the 

 optic axes it is here necessary that the indices of refraction 

 should be fully reliable also with regard to the last decimal 

 place; for only a unit more or less in this place is of consider- 

 able influence on the size of the angle in question. Such 

 accuracy seems not to be attainable here ; and therefore an 

 error of a degree or two in the calculated axial angles may 

 exist. The values calculated are 



Red Yellow Green 



2V 23° 34' 22° 38' 21° 42' 



14. Aegirine. 



No other mineral occurs on Narsarsuk in such considerable 

 quantities or in such large crystal individuals as aegirine. 

 Together with feldspar this mineral is the chief constituent of 

 the numerous pegmatite masses that are met with on the 

 plateau ; as such it is no doubt of primary origin. However, 

 the mineral also occurs there as a secondary formation, and 

 probably dating from different periods. None of the other 

 minerals probably offers so great resistance to the weathering 

 agents as aegirine. While the feldspar and most of the 

 other minerals are gradually disintegrated through the influence 

 of the atmospheric agents, and the gravel thus formed is carried 

 away by water and winds, the aegirine individuals remain more 

 or less intact, it is these aegirine individuals lying on the 



