75 



axes of aegiriiie being parallel to this pinacoid. One extinc- 

 tion direction, which is also the direction of the greatest 

 velocity of light, makes with the vertical axis in the obtuse 

 /3-angle an angle of about 4"^. I have found the same value 

 for this angle in several perfectly homogeneous sections. This 

 angle is, however, a little smaller than the one previously stated 

 for aegirine. 



Pleochroism and varying light absorption are very strongly 

 marked. The absorption of light is strongest in the extinction 

 direction that is nearest to the vertical axis, and the colour of 

 the light vibrating in this direction is a tolerably deep yellowish 

 green. The other direction of vibration transmits light of 

 pale yellow colours. Most sections prove to be optically per- 

 fectly homogeneous, but in a few a zonal structure has been 

 observed. This is specially the case with sections prepared 

 from crystals found at either of the localities No. l and 8. The 

 zonal structure is shown by the fact that the portions of the 

 section which border on the traces of the faces P and a are 

 of a lighter colour and have a weaker pleochroism than the 

 central parts. These faintly-coloured zones are sharply defined 

 and border rectilineally on the normal parts. Often there are 

 several zones parallel to P; of these the outermost shows the 

 faintest colour, the next a somewhat stronger tint a. s. o. The 

 boundary-lines between the successive zones are very distinct. 

 However, they have extinction-directions in common with one 

 another as well as with the central part. 



Sections of crystals from the locality No. 8 often show a 

 typical hour-glass structure. Also here the different parts differ 

 in colour and light-absorption. But here different parts of the 

 section also show different angles of extinction. The angle 

 varies from 0° to 6°. 



The light zones occurring at the ends of some aegirine 

 crystals are probably, in the majority of cases, due to a partial 

 alteration of the aegirine substance. Under the microscope one 



