484 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 59. 



polished faces that the brilliant signals in the field render it almost 

 impossible for the eye to distinguish the fainter signals. In general 

 no face was considered which did not yield an unmistakable signal, 

 and, conversely, no signal was measured unless its face was readily 

 visible under a lens. In many cases a single crystal will yield a 

 measurable signal from a face representing a form which is repre- 

 sented on all other crystals only as dull etched faces yielding no reflec- 

 tion of light. Were more attention paid to working out the prob- 

 able identity of dull faces by zonal relations and to measuring signals 

 of doubtful authenticity which may represent minute faces or internal 

 fractures, etc., the number of new forms listed might be more than 

 doubled. 



In the working out of the crystals by the Goldschmidt method the 

 writer has been greatly assisted by the articles on this method which 



Fig. 7.— Clinographic projection of a datolite crystal of TYPE 4 HABIT (CRYSTAL 36) SHOWING the 



CHARACTERISTIC POSITIVE AND NEGATIVE ORTHODOME ZONE. 



have appeared from time to time in recent numbers in the American 

 Mineralogist. He has also to here express his obligation to Dr. Edgar 

 T. Wherry and William F. Foshag for valuable advice and assistance. 



ORIENTATION. 

 There are at present in use two prominent orientations for dato- 

 lite — the Levy orientation, which was adopted by Dana and which 

 will be referred to subsequently as the Dana orientation, and the 

 orientation of Rammelsberg, which is followed by Goldschmidt and 

 is best known as the Goldschmidt orientation. In the Goldschmidt 

 orientation the form which in the Dana orientation is the dome 

 #(012) becomes the unit prism m (110). The axes are thus inter- 

 changed, axis d, Dana orientation, becoming axis 6, Goldschmidt 

 orientation, and £ axis 6, Dana orientation, becoming axis a, Gold- 

 schmidt orientation. The axial ratios for the two orientations are 

 then as follows: 



a : I : c = 0.63446 : 1 : 1.26574 Dana orientation. 



a: h : c = 0,63287 : 1 : .63446 Goldschmidt orientation. 



