346 



University of California Publications. 



[Geology 



These etch figures are illustrated by figure 5, plate 37, wherein 

 the basal plane remaining fixed, the pyramidal planes of the 

 upper half of the crystal and the prisms are supposed" turned 

 on their upper horizontal edges until brought into the plane 

 of the drawing. This gives the actually observed forms of the 

 figures on each face, and exhibits clearly the symmetry. The 

 broken lines are the traces of the three lateral planes of sym- 

 metry. 



The basal plane remains brilliant and gives very distinct 

 figures. These most commonly show the outline of equilateral 

 triangles in position reversed with respect to the similar tri- 

 angular outline of the basal plane, and with its sides parallel 

 to the edges c A tt. On one crystal treated with very dilute 

 acid hexagonal figures were obtained, but the face was also 

 dotted by numerous small reversed equilateral triangles rep- 

 resenting depressions of regular trigonal pyramidal form. 



The negative unit pyramid tt is the most readily attacked 

 form on the crystal, and very soon becomes dull without show- 

 ing any distinct figures. Under very favorable circumstances, 

 peculiar figures are obtained, the upper boundaries of which 

 are very indefinite, but which are distinctly symmetrical with 

 respect to the projection of the c axis. 



The negative unit prism (/x) shows generally rod-like or 

 ellipsoidal depressions with sides or elongation parallel to the 

 c axis. Where exceptionally developed figures of hexagonal 

 outline are produced, two opposite sides are parallel to the e 

 axis and the whole figure is symmetrical with respect to a 

 horizontal and to a vertical line. 



The positive unit pyramid p remains quite bright and gives 

 good figures, usually quadrilateral and symmetrical to a diag- 

 onal parallel to the projection of the c axis on p. In some cases 

 this quadilateral is truncated by a line parallel to the edge 

 p A m at its upper angle, sometimes at its lower angle also. 

 In a specimen treated with hot acid, depressions were obtained 

 corresponding to a truncated hexagonal pyramid (not regular) 

 as shown in fig. 5, plate 37. 



The positive unit prism (m) is with difficulty attacked 

 and generally shows only small pits with sides parallel to the 



