81 



strongly curved plane, which, as such, would hardly have been 

 determinable. However, a few small crystals were terminated 

 by a face that was quite plane and brilliant and could be ac- 

 curately determined by measurement as the third pinacoid с 

 (Fig. 1, Plate 5). Some of these small crystals show a slight round- 

 ing towards the edges, obviously indicating the transition lo 

 the common crystals with curved terminal faces. This curved 

 termination is, consequently, to be regarded as the third pina- 

 coid with a most imperfectly developed prism of the third 

 order, s. 



With regard to their interior structure these arfvedsonite 

 crystals are remarkable for enclosing a kernel of aegirine. This 

 seems lo be the case with all of them. At least in the four 

 sections 1 have had made such a kernel is observable. The 

 minerals show parallel orientation with coinciding Ъ- and c-axes. 

 The aegirine kernel is sometimes small, sometimes so large that 

 the arfvedsonite substance only forms a thin shell round it. 



In a section orientated parallel to the first pinacoid the ab- 

 sorption of light is nearly complete parallel both to the Ъ- and 

 the c-axis. The section consequently remains dark and opaque 

 on a revolution of 360°, although it is rather thin. The axial 

 colour is, in both directions, deep blue to black. In a section 

 cut parallel to the second pinacoid one extinction-direction makes 

 in the obtuse /9-angle an angle of about 5° with the vertical axis. 

 In this direction the absorption of light is as complete as in 

 the section first described, and the axial colour is a deep bluish 

 black. At right angles to this direction the section is tolerably 

 translucent, vsith greyish brown colour inclining to green. 



In this variety of arfvedsonite, consequently, the light-absorp- 

 tion, as well as the pleochroism, is considerably more intense 

 than in that of the first type; at the same time the angle of 

 extinction is perceptibly smaller. 



Arfvedsonite of type II has been chietly found at the locality 

 No. 17, where it occurred associated with crystals of feldspar, 



XXIV. 6 



