28 



E. J. CHAPMAN ON THE MEXICAN TYPE IN THE TOPAZ. 



The following figures exhibit iu their more distinctive aspect the four types described 

 above : 



APPENDIX. 



The system of crystallographic notation employed in this paper has been long followed 

 by the author. It possesses the advantage of being readily translated into words corres- 

 ponding more or less closely with its symbols, and of thus bringing before the mind's 

 eye, at once and without effort, the position of any given form. It is based on the self- 

 evident fact that the forms present in any crystal must be either Basal, Yertical, or Polar, 

 forms ; no other kind of form (as regards position) can possibly occur. These — apart 

 from crystals of the Regular or Isopolar System, in which there is no distinction of axes, 

 and where consequently the initial of the form's familiar name is of more convenient 

 usage — are denoted respectively by the symbols B, V, and P. A basal form consists, of 

 course, of a single plane at each extremity of the vertical axis, and therefore lying 

 parallel with the middle or basal axes, to which it essentially conforms. It is evident, 

 therefore, that in all cases the sign B is sufficient to define this form. Vertical forms 

 consist of planes which stand parallel with the vertical axis. The symbol V indicates 

 this. But these forms are of three general kinds : Front Verticals {i.e. Macro-Verticals or 

 Ortho- Verticals, according to the crystal system) ; Side- Verticals {i.e. Brachy-Verticals or 

 Clino- Verticals) ; and Vertical prisms, the planes of the latter being parallel only to the 

 vertical axis. All front forms whether Verticals or Polars are distinguished by the hori- 

 zontal sign (-) placed above their symbol ; and all side forms by a short or inclined sign 

 {- or /), representing the sign of the basal axis to which they are parallel. Polar or 

 Pyramidal forms incline upon or towards the vertical axis or poles of the crystals. They 

 lie thus above and below the vertical planes when present, connecting these with the 

 basal planes, or, in the absence of the latter, with the crystal poles. They are also of 

 three kinds: Front Polars {i.e. maero-polars or ortho-polars=macrodomes or orthodomes) ; 

 Side Polars {i.e. brachy-polars or clino-polars = brachydomes or clinodomes) ; and Pyramids 

 or Polars proper. The symbols of these vertical and polar forms thus become : V, V or V, 

 Vto ; jmP, r/iP or mV, and mP : orally. Front V ; Side V ; V; Front P ; Side P ; P. A numerical 

 value {m or J) placed after a symbol refers to the frontal axis, the right-and-left axis being 

 unity. A value of this kind placed before a symbol refers to the vertical axis. There 

 is thus no necessity to place the sign of the axis, referred to, over the numeral ; although 

 to avoid any risk of misconception this has been done with regard to the notation of the 

 Topaz crystals given in the present article. In Clino-Rhombic and Anorthic or Triclinic 

 crystals, in which the polar forms at the top and front differ from those at the bottom or 

 back, the distinction is indicated by the use of brackets, or by some other simple sign. 



