No. 2385. CRYSTALLOGRAPEW STUDY OF DATOLITE— SHANNON. 509 



LESS COMMON AND RARE FORMS. 



Z (130) may be considered one of the rarer forms, as it occurs on 

 only about one-tenth of the crystals studied. It is usually present 

 as a very small and more or less dull face. Its typical occurrences 

 are illustrated by figures 12 (crystal B8) and 9 (crystal B9). 



9 (140) a prism, first described by Ungemach as a new form occur- 

 ring on datolite from this locality, was seen once as a narrow and 

 dull face on crystal BlO between M (T22) and m x (Oil), as shown in 

 figure 23. The signal from the face was very faint, due to etching, 

 although the face was moderately conspicuous. The angle measured 

 does not compare very well with the calculated values as shown 

 below: 



(140) Measured <^ = 20° 55' p = 90°00' 



Calculated ^ = 21° 30' p = 90° 00' 



A = 0° 35' A = 0° 00' 



$ (530), another prism described as new from Westfield by Unge- 

 mach, was observed on one crystal, No. 43, as bright faces, as shown 

 in figure 21. The angles measured compare as follows: 



(530) Measured ^ = 69° 

 Calculated <p = 69° 

 A = C 

 The form is thus confirmed. 



ra y (067) was doubtfully identified once, on crystal Bo, as a small 

 face giving a poor signal. Some doubt attaches to the identification, 

 however, as the face may have been m* (011) of a crystal not quite 

 in parallel position. The corresponding face on the opposite side of 

 the crystal was m* (011). The angles measured are as follows: 



</> (101) was recorded as occurring on Westfield datolite by Gorgey 

 and Goldschmidt. Ungemach takes exception to their identification 

 as follows: "4> (201), which is here given as new was apparently 

 observed by Gorgey and Goldschmidt, but owing to the curvature 

 of its faces it was only tentatively identified by them and then as 

 (j> (101). The authors say that the signals from the curved face 

 yield a train of light in the center of which the position of (101) lies. 

 In my crystals there is no trace of 4> (101), while the train of light 

 begins quite sharply at the position of \p (201) and extends through 

 the short arc to the position of a (100)." In the present examination 

 the phenomenon described by Gorgey and Goldschmidt — that is, a 



