BY A. B. WALKOM. 



165 



seems to be shown by the fact, that an examination of three dif- 

 ferent specimens of this type, showed the relative orientation of 

 the two individuals to be different in each case. 



The crystals all show distinct curving of some of the faces ; the 

 prism-faces are generally plane, and give straight edges, but the 

 pyramid- and dome-faces are decidedly curved. In measuring the 

 curved faces, the method used by Anderson and Jevons^* in 

 measuring opal-pseudomorphs from White Cliffs, N.S.W., was fol- 

 lowed, namely, "making the goniometer-arms tangent to the part 

 of the faces close to the edges." 



Glauberite has been suggested as the probable original mineral 

 for these pseudomorphs, and all the measurements of these crystals, 

 from the Lower Marine Series, tend to confirm that suggestion. 



The habit is monoclinic, and measurement shows that there are 

 three forms present, the angles between homologous faces of which 

 are, 94°, 63-3°, and 67°. These three forms correspond fairly well 

 with with m (110) , s ( 111 ) , and / ( 023 ) of glauberite. Two of these 

 forms were described on the crystals from Huskisson, f but on these 

 crystals, the clino-dome present was #(021), while on the crystals 

 now being discussed, the clino-dome is /(023). The following table 

 shows the measurement of interfacial angles, compared with those 

 of glauberite: — 



* Rec. Austr. Museum, vi., 1905, p.33. 

 t Op. cit., p 175. 



