No. 2385. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 507 



a (014) occurs on about one-third of the crystals as a narrow face, 

 which is often reduced to a mere line, sometimes so narrow as to 

 yield a very faint signal. This is the flattest clinodome observed 

 during the present work. 



x (102) is the only orthodome which is invariably present, and on 

 many crystals it is the most prominent form. It is nearly always 

 etched to a greater or less extent and in various manner, as described 

 elsewhere. The form commonly yields a good signal. On crystals 

 of types 1 and 2 it may be very large, as shown in figures 3, 5, and 

 16, while on crystals of type 4 it may not be more conspicuous than 

 the other forms in this zone, as shown in figures 7, 8, and 15. 



v (103) occurs on about one in each three crystals, usually as a 

 narrow and relatively inconspicuous face. It varies from bright 

 through various degrees of etching to wholly dull, reflecting no light 

 at all. Quite frequently the form can be measured only by the 

 expedient of moistening the face with alcohol, and some of the less 

 completely etched forms give red signals. 



u (104) occurs much as does v (103), the two forms often occurring 

 together and being similar in size, outline, and degree of etching. 



n (111), the only common positive pyramid in this vertical zone, 

 is always present. It varies from a small triangular face, as in the 

 crystals of type 2, figure 5, to a prominent form. The faces of this 

 form are always brilliant and yield a sharp signal. The only char- 

 acteristic irregularity of the surface of the faces is the presence of 

 the reentrant angles formed by oscillation between n (111) and 

 m (110). 



j8 (121) occurs on about one-fourth of the crystals measured as a 

 small face, which is often more or less etched. Characteristic occur- 

 rences of this form are shown by crystal 10 (fig. 16), 36 (fig. 7), 

 51 (fig. 17), B9 (fig. 9), B10 (fig. 23), and Bll (fig. 14). 



Q (122) occurs frequently with /3 (121) and, like it, is usually 

 etched. It falls between n (111) and m x (011). It varies from a 

 relatively prominent face, as shown in figure 17, crystal 51, to a 

 narrow line, as shown by figure 16 of crystal 10. 



U (123) is about as frequent in occurrence as /3 (121) and Q(122), 

 and like them is often etched lightly. It falls between n(lll) and 

 #(012) and can easily be identified by its position. Typical outlines 

 and positions of this form are shown by figures 4, 6, and 23. 



v (111) is a common form, being present on over half of the crystals 

 examined. This form is considered characteristic of his type 1 crys- 

 tals by Ungemach, but was frequently observed during the present 

 examination in combination with g(312) and other forms character- 

 istic of Ungemach's type 2. This is shown by the figured crystal BS 

 shown in figure 11, etc. This form is usually represented by a narrow 



