50 



always as one single individual only, so that the same direction 

 of cleavage may even be observed through several cavities that 

 have only a narrow connection with each other. In all places 

 where the calcite is found, the ilvaite occurs in another type 

 than the common one (type 1); otherwise, however, these modi- 

 fications are very different from each other, of which feature it 

 is impossible to find any outer reason. It is, however, an ab- 

 solute rule that all the crystals of ilvaite connected with one 

 individual of calcite, belong to one type. Also with regard to 

 some of the particular types that have no longer any connection 

 with the calcite, there is every possible reason to suppose that 

 such a connection has existed at an earlier time, but has been 

 resolved by the percolating water ; this is the case where the 

 crystals of ilvaite have the dull appearance which is found every- 

 where, when the lime has been partly decomposed, and which 

 is produced by a beginning disintegration of the surface. Some 

 of the particular types, however, have quite fresh and bright 

 surfaces like the principal type, and where it cannot be sup- 

 posed that any process of decomposition has taken place. If 

 the calcite is decomposed in acids, all the crystals will also be 

 seen to have quite bright surfaces. 



In the same way as the calcite, sphalerite and galenite may 

 also be found filling the cavities in a few specimens; but they 

 are far more rarely found and only in small amounts, and have 

 no traceable influence on the form of the ilvaite. 



Type I (the principal type). 



The most common form of the ilvaite from the veins of 

 sodalite-syenite-pegmatite, which is now to be described, has 

 been chosen as a starting point for the crystallographic treat- 

 ment, as it is found very abundantly, and most of it is provided 

 with well developed, bright faces, although, as in all forms of 

 ilvaite, it is provided with numerous vicinal faces that make 

 exact measurings very difflcult or quite impossible. 



