522 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 50. 



g (T. 12.10) occurred on crystal 10 as a small though bright face, as 

 shown in figure 19. The angles arc: 



(1.12.10) Measured <p = 7° 25' p = 56° 15' 



Calculated <? = 7 ° 23 ' p = 56 ° 53 ' 



A = 0° 02' A= 0° 38' 



The order of agreement is good, and this is regarded as a well- 

 established form. 



t (T.20.15) occurs on crystal B7, as shown in figure 10, as a small 

 triangular face. The angles are : 



(T.20.15) Measured <p=4 6 51' p = 58° 50' 



Calculated <? = 4° 25' p = 59°27' 



A = 0° 26' A = 0° 37' 



u (211) occurs on several crystals as a narrow face between v (Til) 

 and a (100). In most instances, however, the face is entirely dull 

 and can not be measured. The best face of this form which occurred 

 on crystal 44, shown in part in figure 20, gave the following angles : 



Although the agreement between the calculated and measured 

 angles is not as close as might be expected, this is due to the faintness 

 of the signal from the etched face. The form may be regarded as 

 well grounded. 



r (229) was noted first upon crystal 41, shown in figure 18, as a 

 narrow line face between p. (Tl4) and k (T15). The angles are as 

 follows : 



(229) Measured ^=58° 04' p = 27°33' 



Calculated ^ = 57° 28' p = 27° 36' 



A= 0° 36' A= 0° 03' 



The same form was also noted on crystal 55 as an etched face which 

 gave very poor signals, not accurately measurable because of their 

 faintness. These gave the angles: 



(229) Measured *> = 55° 40' p = 25° 08' 



A= 1°4S' A= 2° 28' 



Since the form does not agree with the normal series as shown in 

 the discussion of zone 4 below, it must be regarded as a disturbed 

 form. 



