PHYSIOGRAPHY. CLASS II. 



composition is frequently found in St Gothard. 

 52. Axis of revolution parallel to the principal axis, 

 face of composition parallel either to the right. Fig. 

 80., or to the left faces, Fig. 81., of Pr + oo. 

 Both are found near Elbogen in Bohemia. 3. Axis 

 of revolution perpendicular, face of composition 



parallel to ^ (HAUY. Traite. 2de Ed. T. 



III. p. 91. PI. 82. Fig. 256). 4. Axis of revolu- 

 tion perpendicular, face of composition parallel to 



a face of . This and the preceding variety are 



found at la C layette in the department of Saone and 

 Loire in France ; a beautiful crystal of the latter is 

 preserved in Mr ALLANS cabinet. Sometimes there 

 occurs composition according to several of these 

 laws at once. Massive : composition granular, of 

 various sizes of individuals, sometimes lamellar. 



OBSERVATIONS. 



I. The forms, as contained in the preceding description, 

 have been calculated from the observations of ~ being 

 = 126 12', and (Pr + cs) 3 = 118 52'. The inclination 

 of the axis has been supposed = 0. It will depend upon 

 future observations, whether this is in fact the case, or 

 whether there exists also in the present species an inclina- 

 tion, as in most other hemi-prismatic forms. The angle of 

 (Pr + co) 3 has been found in several varieties = 119 10'. 



Besides the present species of prismatic Feld-spar, there 

 are several whose forms possess a hemi-prismatic or tetarto- 

 prismatic character, which differ so much in this respect, as 

 well as in their angles, and the specific gravity, that ac- 

 cording to the principles of Natural History, they must be 

 considered as particular species. Their occurrence is most 



