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University of California Publications. 



[Geology 



Habit. — The most common habits are distinctly trigonal and 

 pyramidal. Although prism faces are almost universally pres- 

 ent they are always comparatively small, and no approach to 

 a prismatic habit has yet been observed. The largest crystal 

 yet seen is about two and a half inches across, but is of unusual 

 size. The majority are less than one inch across. The negative 

 pyramid generally shows the largest faces, p commonly not 

 reaching to c, and thus giving a triangular outline to the basal 

 plane. The prisms appear as narrow bands. The basal plane 

 may more rarely be absent, and p is then quite small. This gives 

 the two habits shown in figures 1 and 2, plate 37. The positive 

 pyramids reach the basal plane in about 20 per cent, of the 

 crystals and then a hexagonal outline is produced, the edges gen- 

 erally being distinctly in two alternate groups (pi. 37, fig. 3). 

 Only one crystal was seen wherein was produced a pseudo- 

 hexagonal symmetry. A peculiar habit that has been found in 

 a few crystals is produced by a predominant basal plane, the 

 crystal being very thin, and, if growing from one side, has the 

 appearance of an orthorhombic table. 



Only one crystal was found not showing prism faces. The 

 positive prism m may be narrower than the negative prism as 

 in plate 38, figure 1, and in about 3 per cent, of the crystals 

 examined it was absent, as in plate 38, figure 4. On the other 

 hand, it may be considerably broader than the latter, even when 

 its corresponding pyramid p is smaller than the negative 

 pyramid n, as in figure 2. 



The form r (1012), has been found on considerably less than 

 half of the crystals in which p does not reach the basal plane, and 

 then always as a narrow truncation of the tt edges. This is shown 

 in figure 4, plate 37, together with x (10 1910) which is here 

 represented as complete for the crystal, although actually it is 

 not found in all the sectants, and occurs only on a few crystals. 

 In a certain number of crystals the place of r is then taken by a 

 strip of horizontal striations or narrow planes due to oscillatory 

 growth. The striations are shown in plate 38, figure 3, and in 

 figure 4 is represented a set of coarser oscillations showing the 

 origin of the striations as alternations of c and p. 



The prism of the second order a (1121) was observed in 



