No. 2385. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 501 



Crystal 56 is distinctly tabular parallel to the base and otherwise 

 resembles crystals referred to Ungemach's type 2, shows v (Til) and 

 b (223) with flat positive orthodomes 8 (205), £ (308), v (103), and 

 u (104), all type 1 forms, with the rare form w (324), characteristic 

 of type 2. 



Crystal B8, shown in figure 11, shows v (Til), with 8 (205), v (103), 

 and u (104), characteristic of type 1, with a well-developed face oi 

 a (312). 



Crystal 36 shows q (312) as brilliant faces and also a steep dome 

 Tl (403), both characteristic of type 2, together with brilliant faces 

 of v (Til) and of the flat domes £ (308), v (103), and u (104); char- 

 acteristic forms of type 1. 



Numerous other examples might be cited to show that the forms 

 regarded as characteristic of the two separate types by Ungemach 

 frequently intermingle, or at least that the orthodomes are subject 

 to no laws regarding their distribution and that the forms q (312) 

 and v (Til) are common to both types. In fact q (312) very fre- 

 quently occurs on cr} T stals of the type here designated as types 1 

 and 2 (following Kraus) as a narrow line beveling the edge x (102) 

 Ae (112). 



Ungemach 's rigid statements as to the degree of etching on the 

 faces of the two types are not borne out by the crystals studied. 

 The crystals showing the series of relatively equally developed ortho- 

 domes show q (312), together with other features characteristic of 

 t}^pe 2 of Ungemach. These are all characterized chiefly by a rela- 

 tively large development of the basal pinacoid. It is believed that 

 type 4 of Kraus and Cook was essentially of this type, the crystals 

 which they examined happening to lack the several unusual forms 

 which are regarded as characteristic of the type by Ungemach. 

 Many of the crystals of this type are more or less prismatic on the 

 a axis, although others are nearly equidiniensional. In the positive 

 orthodome zone, instead of x (102) developed as the only prominent 

 form, there often occurs a series of several narrow faces, which may 

 include not only five or six well-defined forms but also several vicinal 

 planes 1° or more from the established forms. This series of front 

 domes, which during the study of the crystals was designated as 

 "scintillating," is highly characteristic of crystals of this type, as 

 shown in figures 7 and 15. It is not always present, however. Thus 

 in crystal Bll, shown in figure 14, the only forms in this zone are 

 x (102) and v (103). The crystal B10, shown in orthographic pro- 

 jection on the front pinacoid in figure 23, contains a larger number 

 of the forms listed by Ungemach as characteristic of his type 2 than 

 any other crystal examined by the present writer, namely 2 (302). 

 z (201), P (332),$ (546), t) (811), and 9 (140). This crystal was taken 

 from the small specimen shown in plate 106, all of the crystals of 



