No. 2385. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 497 



in the photographs, plates 104 and 105, especially by the large crystal 

 in the center of the specimen illustrated in the plate 105. 



HABIT. 



Whitlock apparently did not distinguish more than one type in 

 the crystals examined by him. From his description and figures it 

 is obvious that these were essentially like those of type 1 of Kraus 

 and Cook described below. 



Kraus and Cook distinguish four types differentiated by variation 

 in habit, which are described below: 



Type 1. — This type, which was observed on 34 crystals, is pyramidal in habit and 

 shows the forms a, b, c, »i, r, o, I, x, v, u, m x , g, t, m y *, m*, n, /3, Q, v, b, e, X, m, *> 

 v*, M, i, a, r, e', //. a (100) is always present as a brilliant face, while b (010) occurs 

 on about 50 per cent of the crystals of this type. When present it appears as a narrow 

 edge, giving good reflections, c (001) is always present, sometimes as a wholly dull 

 face, which is commonly not large, x (102) is the predominating form, the other 

 orthodomes v (103) and u (104) occurring only rarely and as narrow faces, t (013), 

 g (012), and m x (Oil) are always present, m x always being large. m y (067) and m z 

 (0.1.10), which are new, occur on crystals of this type. The prisms r (230) and o (120) 

 are usually present as somewhat dull faces. I (130) was observed once on 35 crystals. 

 The pyramids n (111), (121), v (Til), e (112), X (113), and m (114) are usually all 

 present, v (111) commonly predominating. Q (122) is frequently present, often as a 

 dull face giving no reflection, b (223), sometimes large, is among the forms commonly 

 observed. M (122), i (123), and a. (124) are also frequently observed, d (148) and 

 M 7 (1.4.10), first described by Whitlock, were seen on several crystals. fx / (1.4.10) 

 gave very good readings but e' (148) was identified by zonal relationships. n z (1-1-10) 

 also occurs on crystals of this type. 



Type 2. — This was observed on seven crystals, is also pyramidal, and similar to 

 type 1, from which it is distinguished mainly by the absence of c (001) and b (010). 

 The forms noted on crystals of this type are a, x, rn x , g, m, o, n, b, e, X, m, "• « (100) 

 occurs as a small triangular face giving good reflections. As in type 1, x (102) is the 

 predominating form. The prisms m (110) and o (102) are always present, though 

 generally small, o (120) beveling the edge between the face3 v (111) and m x (011). Of 

 the pyramids e (Il2) and X (Il3) present large uneven faces, b (223) is usually dull. 

 The other pyramids occur as very small faces. 



Type 8. — This was observed on four crystals. It is characterized by a prismatic 

 habit with a (100) prominent. All forms are well developed, the following being 

 noted: a, b, c, x, $, m x , g, t, m, n, v, e, X, n. All forms except v (Inland £ (102) are 

 brilliant, giving good reflections, v (111) occurs as a dull form and £ (102), in addition 

 to being dull, was so small that it could only be identified by zonal relationships. 



Type ■£.— This was observed on 2 crystals, is tabular in habit, the base c(001) 

 being prominent. The forms noted on crystals of this type are a, b, c, m, m x , g, t, x, 

 v, n, v, and e. Of the clinodomes g (012) predominates, m x (Oil) and g (013) being 

 comparatively narrow faces. The orthodomes x (102) and v (103) are both dull forms. 



Ungemach writes that there are, both on his crystals and on those 

 examined by Gorgey and Goldschmidt, two well-defined types dis- 

 tinguished according to the development of certain typical forms 

 rather than upon habit which is subject to great variation: 



Very characteristic for distinguishing between these are the forms of the orthodome 

 zone. In the first type there are no steeper domes than x (102) and £ (102). There 

 27177— 21— Proc.N.M.vol .59 32 



