NOTES ON THE TOPAZ FBOM MINO. 75 



The prevailing faces in this type are o and i, accompanied by r 

 in rare cases. The brachydome / is always present, y occurs fre- 

 quently and the brachypinacoid h is rare. The prisms, M, I, and m 

 are pretty often found, while many other prisms are difficult to 

 distinguish. 



Type III. Base-pyramidal type. (fig. ll-fig. IS). 



Crystals of this type show the most complex combinations, and 

 are rather common in smaller individuals. The pyramids w, o, i, r, v, 

 and x, belonging to the vertical zones of the prisms M and Z, are all 

 here represented. The brachydomes y, /, and X are developed nearly 

 of equal size. All the prismatic faces occur in this type. 

 Type. IV. Tabular type. (fig. 19). 



The peculiarly flattened form is due to suppression of two parallel 

 faces in one of the prisms M and L Such an alteration in the 

 development of faces has not yet been noticed in crystals from the 

 lirais, Schneckenstein, and South Africa. These crystals are, how- 

 ever, in general aspect, very like the different types of our specimens. 



Etched Figures. — Natural etched figures are comparatively fre- 

 quent in our specimens, but it is difficult to find out their crystal- 

 lographic significance. 



On c. (E. 1.) There is observed a group of small rectangular pyra- 

 mids, with their bases extending in the direction of the macro-axis. The 

 figures seem to have been produced just in the same way, as in the 

 Omi topaz, where we find on the same face long rectangular impres- 

 sions (fig. 20. 6.) parallel to the macro-axis. In a further stage of 

 corrosion they appear in the shape of grooves leaving between them 

 high rectangular pyramids, as represented in fig. 20. a. The figures 

 observed by Laspeyres 1 and Pelikan 2 have not been found. 



1. Groth, Zeitschr. f. Kryst. u. Min. 1877. 



2. Tscherniak, Min. u. Petro. Mitth. 1890. 



