No. 2385. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 481 



all the unusual forms and variations in habit occurring in upward of 

 a thousand crystals. After much work had so familiarized the 

 writer with the datolite and the peculiarities of outline and etching 

 of the various forms that almost any given crystal could be oriented 

 and its forms identified with a fair degree of accuracy by simple 

 vision, the Westfield datolite specimens in the Brush collection of 

 Yale University and in several private collections were examined 

 without revealing any habits or any prominent forms not represented 

 in the writer's final series of crystals. 



METHODS. 



The study of the datolite was begun and carried through the exami- 

 nation of some 30 crystals by zonal measurement with a Fuess 1- 



FlG. 2.— StEEEOGEAPHIC PEOJECTION SHOWING THE SAME FOEMS AS FIGUEE 1 BUT PLOTTED IN THE 

 GOLDSCHMIDT OEIENTATION. THE POOEEE EXPEESSION OF THE SYMMETEY IS APPAEENT. 



circle goniometer, the symmetry relations of the various forms being 

 worked out in stereographic projection, many of the forms being 

 identified graphically. Despite the pronounced zonal relations exist- 

 ing between the various forms on datolite this method was more or 

 less unsatisfactory, and the attendant trigonometric calculations were 

 so tedious and consumed so much time that the problem was finally 

 laid aside for more than a year, since it was obvious that the work 

 could be done much more easily and simply by using a Goldschmidt 

 2-circle goniometer and attendant methods. Recently such an instru- 

 ment has become available and the work which had been so difficult 

 by the older system became fairly easy. The measurements made 

 by the 1-circle goniometer were all rejected and the crystals remeas- 

 27177— 21— Proc.N.M.vol.59 31 



