No. 2385. CRTSTALLOGRAPHIC STUDY OF D AT '0 LITE— -SHANNON. 511 



as a small triangular face, which is somewhat dull on crystal B9, 

 which is illustrated in figure 9. The angles for this form are as follows : 



(308) Calculated ^ = 90° 00' p = 36° 54' 



Measured, maximum <p = 91° 28' p = 37° 48' 



Measured, minimum <p = S9° 28' p = 36°49' 



Measured, average <? = 90° 26' p = 37° 14' 



A = 0° 26' A = 0° 20' 



£ (T02) was seen several times on crystals of types 1 and 4. On 

 type 1 crystals it occurs usually as a very narrow line, beveling the 

 edge between e (112) and e' (TT2). On crystals of type 4 it may be 

 present as a relatively broad face, yielding measurements agreeing 

 closely with the calculated angles for the form. It is shown in 

 typical development on crystals 36 (fig. 7) and 39 (fig. 15). 



2 (302) occurs occasionally as a relatively small face, which, 

 though slightly etched and having a satiny sheen, yields sharp signals. 

 It is shown in typical development on crystal 39 (fig. 15), crystal 27 

 (fig. 7), and crystal B10 (fig. 23). 



II (T01) occurs occasionally with S (302) as a narrow face, which is 

 sometimes dull and etched. 



A (1 12) is a rare form on datolite from Westfield. Only one crystal 

 (B4) showed measurable faces of this form, which gave the angles 



(112) Calculated ^ = 57° 40' p = 49°48' 



Measured ^ = 57° 40' p = 49° 50' 



A= 0° 00' A= 0° 02' 



The form was seen on two or three other crystals as very small and 

 dull faces. 



Mi (231), a form previously described by Ungemach on datolite 

 from Sainte Marie, was doubtfully identified on crystal BS as a nar- 

 row line face beveling the angle o (120) An (111), as shown in figure 12. 

 The signal from the narrow face was so faint that it could be measured 

 only approximately, owing to lack of visibility of the cross hairs. 

 The angles are: 



(231) Calculated <p = 46° 26' 



Measured «p = 44°48' 



A= 1° 38' 



q (312) should perhaps be included with the common forms, as it 

 occurred on approximately one-third of all the crystals examined. 

 On type 1 crystals it frequently forms a narrow line, beveling the 

 edge x (102) A (Til). On type 4 crystals it is almost invariably 

 present as small triangular faces at the upper corners of the front 

 pinacoid. Although normally clear and brilliant, yielding excellent 

 signals, the faces of this form are occasionally somewhat etched. 



