JBascom and Goldschmidt — Anhydrite Twin. 489 



They are bounded only by the three pinacoids and show twin- 

 ning striations parallel to the twinning plane. Later Preis- 

 work* described some more highly developed crystals twinned 

 according to the same law. 



The crystal in question is still richer in forms : The parallel 

 orientation of the faces b(0 oo) and r (10), which is characteristic 

 and genetic, is the decisive factor for the twinning law of 

 anhydrite. They are the chief planes of coincidence, and 

 moreover all planes of the zone br coincide. We therefore 

 call this a zone of absolute coincidence. This zone is next in 

 importance to the dome-zones ac and ah, which are the most 

 richly developed zones in anhydrite. It includes the forms 

 b.jp.f.n.o.r. The gnomonic projection (figure 2) makes quite 

 apparent these relations. 



■ It should be observed that the b faces of both individuals 

 fall into a single plane, as is also the case with several 

 of the twins figured by Preiswork,+ and further that the 

 f face is a plane of contact (grenzflaehe) for both individuals. 

 The transformation of the elements and symbols of the 

 Winkeltabelle (WT) to those of the projection on b — oo. (B) 

 is made according to the formulae. 



py(WT) = I p - (B) 



M,(WT)= ^ • f- . (B) 



JO 20 



The elements and table of angles on p. 490 belong to the pro- 

 jection on b and may sometimes prove convenient for use. 

 The following forms, which are distinguished thus (?) in 

 the table, are uncertain : 



p 



02 

 Orientation (B) = fO fo fo ^0 £0 



Orientation = Of 04 Of 02 03 = Hessenberg 



The forms a and p are given by W. H. Miller;}: but desig- 

 nated by him as uncertain. The forms t, /jl, and a are figured 

 by A. Schrauf§ without further description. They also cannot 

 be considered assured forms. All these forms should be omitted 

 from the list until they have received additional confirmation. 

 Heidelberg, July, 1907. 



% Jahrbuch Min., 1905, vol. i, p. 39, pi. 3, fig. 5-8. 

 f Jahrbuch Min., 1905, vol. i, pi. 3, figs. 5, 7, 8. 

 % Phil. Mag. 1874, vol. xlvii, p. 124. 

 t Atlas 1871, pi. 15, fig. 4, 5. 



