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PROCEEDINGS OF THE KATIONAL MUSEUM 

 Measurements of datolite, Figure 2 



VOL. 66 



The letters, symbols, and indices of the above table are for the 

 Dana orientation, while the calculated angles are for the equivalent 

 indices taken from Goldschmidt's Winkeltabellen. 



The larger and more highly modified crystals usually have some 

 small neg'ative pyramid faces developed without the corresponding 

 positive forms or the opposite and, where the forms are the same, 

 the faces of one end are slightly larger than at the other end of the 

 crystal. Where the larger faces are of forms occurring more fre- 

 quently as negative forms they are made negative although the 

 orientation is wholly arbitr'ary. One such crystal is shown in 

 figTire 3 and the measurements are given in the following table. 

 This crystal was measured in the Dana orientation and the angles 

 are so given. 



Measurements of datolite, Figure 3 



Another habit occurring as a variant among crystals of the pre- 

 ceding kinds is shown in figure 4. The same choice exists as in 



