No. 23S5. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 505 



« (345), 28 (382), ^ (455), g) (6.15.2)^,3 (766), fi (768), i (11.11.10), 

 i (T42), ! (143), 1 (167), m (1.6.12),_o (1.6.14), p (1.10.30), g (L12.10), 

 t (T.20.15), u (211), j (229), j_(263), (295), « (296), p (2.10.5), 

 x t (342), 6- (343), T (351), e, (353), / x (361^ ^ (362), \ (3.10.20), 

 i\ (461), /, (5.6.30), ^ (794), Zj (9.16.2), m y (12.25.1). 



Of the latter, however, a considerable proportion were observed 

 only once as very small faces giving faint signals, and they therefore 

 require additional confirmation as emphasized below. Of the forms 

 previously recorded from the locality the following 19 were not 

 found : 



12 (018), m z (0.1.10), n y (T.1.10), n (132), T (214), & (118), J t (311), 

 23' (313), r (314), x (534), l, (911), Y x (134), A (210), y (601), j (502), 

 fc: (3.0.10), S (136), /' (412), (11.0.2). 



The combinations on the majority of the crystals are more or less 

 similar. In general the number of forms present varies with the size 

 of the crystal, many of the rarer forms being confined to the larger 

 crystals, while small crystals are usually simple in combination. 

 The rarer forms are erratically distributed. It might be possible to 

 measure 100 crystals and find only relatively common forms, while 

 one additional crystal might show 10 or more new or rare forms. 

 Rare forms seldom occur singly, more frequently occurring as small 

 nests of several unusual faces. Although the present examination 

 covered a relatively large number of crystals, it is entirely probable 

 that the examination of additional crystals from this locality will 

 reveal additional rare and new forms. The different forms observed 

 are described in detail below. 



COMMON FORMS. 



a (100) is practically never absent except where it has been 

 destroyed by breaking of the crystal or by confinement during 

 growth. The faces of this form vary from small triangles, sometimes 

 minute, on crystals of types 1 and 2, shown in figures 3, 4, and 5, to 

 large polygons on crystals of types 3 and 4. The largest development 

 of this form is shown by type 3, illustrated in figure 6, where the front 

 pinacoid is one of the most prominent faces. The faces of this form 

 are invariably brilliant and never show any characteristic etching or 

 irregularity. 



b (010), the clinopinacoid, is usually though not invariably pres- 

 ent. It is commonly a linear face, varying from a fairly broad to a 

 very narrow line. It is almost invariably bright and yields good 

 signals even when very narrow. 



c (001), the basal pinacoid, is usually present, although it occa- 

 sionally fails, as in crystals of type 2 shown in figure 5, and also occa- 

 sionally on crystals referable to other types, as shown, for example, 



