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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 50. 



bination. The danger of committing errors of orientation is naturally 

 much greater with simple crystals than with highly modified crystals 

 showing a large number of forms. In some rare instances it is possible 

 to completely measure a crystal of this mineral without obtaining 

 measurements which will show conclusively how it should be oriented, 

 the difficulty then having to be settled by optical examination. The 

 various characteristic etchings and irregularities which occur on the 

 faces are of greatest value in orienting the crystals, and for this 

 reason these peculiarities are described in considerable detail below. 

 While every crystal measured by the writer has been carefully con- 

 sidered and the possibility of error in orientation carefully weighed, 

 it is possible though improbable that one or more crystals were 

 measured in a wrong position. 



Table 1. — Datolite. 



Giving a list of forms previously recorded for datolite 'with equivalent indices and 

 symbols in the Dana (Levy) and Goldschmidt (Rammelsberg) orientations. 



