36 MINERALOGY AND LITHOLOGY. 



than any other crystals that I have ever tried ; and, when examined with 

 the microscope, its surface is seen to be covered with depressions, one of 

 which, with its relationship to the octahedral face, is shown in PL 3, Fig. 

 J a. These etch figures on fluor spar are considered by H. Baumhauer* 

 as being made by the faces of a tetragonal trisoctahedron, because, if the 

 cubic faces of fluor spar are etched, four-sided pyramidal depressions, 

 first observed by Wyrouboff, are found on them, the sides of which are 

 parallel to the combination edge of the cube and octahedron, and which 

 can consequently also be explained by referring them to the same figure. 

 These same figures were obtained by A. von Lasaulx,t in his studies on 

 Silesian fluorite, and referred to the same crystalline form. Both these 

 gentlemen also obtained more complicated figures, which were referred 

 by them to the combination of the same figure, with a trigonal trisocta- 

 hedron. 



These simple etch figures on the octahedral faces can with equal pro- 

 priety be referred to the faces of a cube, since they bear the proper 

 relationship to the octahedron ; and to this form I refer the figures ob- 

 tained on our octahedrons, for the following reasons. Although they are 

 not capable of measurement, the faces look as though they stood at right 

 angles to one another. Some of the larger and more isolated depressions 

 possess more facets. One of these depressions is represented in Fig. jb. 

 This figure, somewhat different from any obtained by Baumhauer, corre- 

 sponds to the combination of a cube and dodecahedron. Now, octahe- 

 drons of fluor spar are found in a great many places that need no etching 

 to bring out this structure. Octahedrons are found that are entirely 

 made up of little cubes, and these cubes possess at times the dodecahe- 

 dral modification. Hence I think it may be inferred that there are struct- 

 ural directions in these perfect, smooth octahedrons at the Notch which 

 are parallel to the faces of a cube and dodecahedron, just as there are in 

 those common cases, which are made so evident by the more predom- 

 inating influence of the last named figures over the octahedron. 



These crystals, when heated, phosphoresce with a very beautiful violet 

 light ; they also, under the influence of heat, decrepitate violently at a 

 comparatively low temperature. Possibly this may be due to the fact 



* Neues yahrbuch f\lr Min, 1876, p, 605. 



t Zeitschri/t fWr Krysiallosraphie, vol. 1, p. 360. 



