508 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.59. 



face, often a mere line, but may be more prominent, as shown, for 

 example, by crystals 10 (fig. 16) and B7 (fig. 10). On type 4 crystals 

 it is usually narrow and brilliant, yielding excellent measurements, 

 while on other types of crystals the form is usually larger and more 

 or less dull and etched. 



b (223) is a characteristic form on Westfield datolite occurring on 

 one-third of the crystals measured. Unlike v (Til) it is practically 

 confined to crystals classed as type 1 by Ungemaeh, and occurs almost 

 invariably on crystals of type 1, as defined here, as a moderately 

 narrow to relatively broad face between v (Til) and e (Tl2). It is 

 often more or less etched. Typical occurrences are shown in figures 

 10, 11, and 17. 



e (Tl2) is a universally present form, and ordinarily it is the most 

 prominent pyramid. It determines the form of crystals of type 2, 

 shown in figure 5, and of the similar type shown in figure 8. The 

 faces are horizontally striated and are often more or less irregular 

 yielding multiple signals. 



X (Tl3) is invariably present where not destroyed by interference. 

 It is commonly a narrow face which is bright and gives a good signal. 



n (Tl4), like X (113), is commonly present as narrow bright faces, 

 giving good signals. 



k (T15) is of relatively infrequent occurrence, being found on about 

 1 in every 10 crystals. It is narrower than ju(Tl4), but yields good 

 signals. 



i (T23) is a common form, occurring on half of the crystals studied. 

 It forms a narrow but brilliant face, beveling the angle between 

 e (T12) and m x (0TT). Its characteristic form and position are well 

 shown by figures 4 and 6. 



a (124) occurs on about 1 in every 4 crystals studied, being 

 similar to i (T23) in form and occurrence. It forms a narrow but 

 bright face beveling the edge between #(012) and e (Tl2), as shown 

 by figures 4 and 10 (crystal B7). Occasionally it is present as a 

 broader face, as shown in figure 16 (crystal 10). 



t (231), elsewhere a rare form for datolite, is a characteristic form 

 at Westfield, occurring on every second crystal. It occurs com- 

 monly as a narrow face, beveling the angle between o(120) and v (Til), 

 as shown by figures 17 (crystal 51) and 16 (crystal 10). Where v (Til) 

 is absent this form occurs as a larger triangular face, as shown, for 

 example, in figures 23 (crystal B10) and 14 (crystal Bll). The most 

 conspicuous development of this form is shown in figure 24 (crystal 

 33), where it forms the center of a small group of rare and new forms. 

 Commonly the faces of -k (231) are more or less delicately etched, so 

 as to give them a silky sheen, although the form usually yields clear 

 and brilliant signals. 



