506 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.59. 



by crystal 28, figure 8, and crystal B9, figure 9. The faces of this 

 form vary from minute to large. They are usually not brilliant, 

 being characteristically marked by triangular pits or ruled depressed 

 lines, as further discussed under another head. Occasionally the face 

 is wholly dull, reflecting no light at all, but usually it gives a distinct 

 although somewhat furred signal. On crystals of type 1 the base 

 is usually more or less triangular in outline, as shown in figures 3 and 

 16. It ma} r also be represented by a narrow line, as shown in figure 4. 

 On crystals of type 4 (type 2 of Ungemach) the base is the most 

 conspicuous form, many of the crystals of this type being distinctly 

 tabular parallel to this face. 



m (110), the unit prism is not absent from any crystal examined, 

 although it varies from the thinnest line on some crystals of types 1 

 and 2, figures 3 and 5, to a large and predominant face on crystals 

 of types 3 and 4, as shown in figure 6. It is usually a plane and 

 brilliant face, yielding excellent signals. The only characteristic 

 irregularity present is a grooving by reentrant angles, produced by 

 oscillatory alternation between m (110) and n (111), as described 

 above. 



o (120) occurs on somewhat more than half of the crystals examined. 

 It varies from a very small to a fairly large face, being exceedingly 

 variable in outline. On crystals of types 1 and 2 it is oftenest shown 

 as a relatively narrow faces truncating the angle v (Til) A)»i (Oil), 

 as shown in figures 5 and 16. Other characteristic outlines are shown 

 also in figures 4,7, and 23. The most prominent type of development 

 of this form is shown by crystal 51, illustrated in figure 17. The luster 

 of the face is almost never perfect, the etching varying from a faint 

 satiny sheen to complete lack of luster. Usually the faces of this 

 prism, like others in the same vertical zone, yield clear and sharp 

 though faint signals, which are sometimes blood red in color and 

 appear as though viewed through a haze. 



r (230) occurs on about one-fourth of the crystals examined, 

 usually as a small face, as well illustrated by crystal 51, shown in 

 figure 17, and crystal B10, shown in figure 23. The face lies between 

 to (110) and o (120) and usually yields a fair signal. It is frequently 

 etched lightly but is not so characteristically so as is o (120). 



to x (011) is invariably present, usually as one of the most promi- 

 nent and brilliant faces. While sometimes slightly wavy, it is always 

 brilliant and } T ields excellent signals. 



g (012) is practically always present as a prominent and brilliant 

 face, yielding good measurements. 



t (013) is present on about half of the crystals studied, occurring 

 as a narrow face, giving good signals. It is much less prominent 

 than the two steeper clinodomes to x (011) and g (012). 



