No. 2385. CRTSTALLOGRAPHW 8TUDY OF DATOLITE— SHANNON. 517 



£ (205) was observed on five crystals as well-defined faces yielding 

 good measurements. It is among the best established of the forms 

 added here. The form is shown typically in crystals 10 (fig. 16), 

 B8 (fig. 11), and 28 (fig. 8). The angles are as follows: 



(205) Measured, maximum <p = 90° 13 ' p = 39° 20' 



Measured, minimum ^ = 89° 28' p = 37° 54' 



Average (of five) <p = 89° 55' p = 38° 25' 



Calculated <p = 90°00' p = 38° 41' 



A = 0° 05' A= 0° 16' 



Vicinal (16.0.39). Although vicinal forms occur frequently on the 

 datolite crystals, those of the orthodome zone deserve especial men- 

 tion. On crystals of type 4 this zone contains certain vicinal forms 

 as prominent faces. The present form was measured on three crys- 

 tals, on which it was present as a prominent face, and was seen as a 

 narrower line on several other crystals of type 4. The angles are: 



Vicinal (16.0.39) Measured ^ = 89° 49' p = 39° 22' Crystal B4. 



«P = 90° 00' p-39° 05' Crystal B5. 



<p = 90° 00' p = 39°50' Crystal 10. 



<P = 89°56' p = 39°26' Average. 



Calculated <p = 90° 00' p = 39° 22' 



A = 0° 04' A= 0° 04' 



9)2 (403) was observed once as a very narrow and etched line face 

 on crystal 36. The angles are: 



(403) Measured <p = 90° 00' p = 68°55' 



Calculated <p = 90° 00' p = 69° 25' 



A= 0° 00' A= 0°30' 



Vicinal (12.0.25) is another characteristic vicinal form, which 

 occurs very frequently in the orthodome zone of crystals of type 4. 

 This face occurs with x (102), the two forms, which are only about 

 1° apart, being represented by faces of equal size and brilliancy. 

 The angles are: 



Vicinal (12.0.25) Measured ^ = 90° 26' p = 44° 09' 



Calculated <p = 90° 00' p = 43° 50' 



A= 0° 26' A= 0° 19' 



O (134) is a new form which occurred on crystal 51, as shown in 

 figure 17, as a narrow face between U (123) and m* (011). The face 

 is somewhat dull from etching and is correspondingly difficult to 

 accurately measure. The angles are : 



(134) Measured ^ = 28° 51' p = 46°48' 



Calculated <p = 27° 50' p - 47° 02' 



A= 1°01' A= 0° 14' 



