91 



no indication of pleochroism. The preparations consist partly 

 of feebly doubly-refracting, partly of isotropic, portions. The 

 doubly-refracting portions occur along cracks, which traverse the 

 mineral reticularly. Between this mesh-work of doubly-refracting 

 substance which follows the cracks, occur isolated isotropic 

 portions. There is no sharp line between the isotropic and 

 the doubly-refracting portions. Whether the double refrac- 

 tion is positive or negative, could not be decided. As the 

 above-mentioned cracks are evidently secondary, the double 

 refraction of this eudialyte must also be secondary. How- 

 ever, this seems to be at variance with the fact that all 

 the doubly-refracting portions of the same section have uni- 

 form extinction. The trigonal eudialyte must originally have 

 been doubly-refracting. Through some change in the molecular 

 arrangement the whole mass has then become isotropic. In a 

 third stage the mineral has become traversed by fine cracks, 

 and on both sides of each crack it has again become doubly- 

 refracting. Such a process seems to me to be more likely than 

 that original double refraction should have remained along the 

 cracks, Avhile the portions not in contact with them should -have 

 become isotropic. 



At all events the molecular rearrangements, from which 

 the optical phenomena now described have resulted, are of no 

 great consequence. This appears from the specific gravity 

 of the mineral as well as from the loss on ignition, both of 

 which agree with those of normal eudialyte. 



A complete alteration resulting in the formation of new 

 minerals has taken place in the outer parts of the eudialyte 

 masses and along the larger cracks in them. In such cracks 

 aegirine and zeolite substance have been deposited in crusts up 

 to 5™™ in thickness. These alteration products together with 

 several others also occur arranged in zones along the sides of 

 the cracks and in those portions of the eudialyte, changed in 

 colour and lustre, that border on the other minerals. The 



