318 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



the zircons are most frequently prismatic with the length 3 to 4 times 

 the diameter and the most abundant types show the first order 

 prism m(110) and the second order prism a(100) in almost equal 

 development and are terminated by the ditetragonal pyramid :r(311) 

 either alone (fig. 83) or together with the unit pyramid p(lll). 

 Crystals of these habits are most numerous, although other habits 

 occur occasionally as noted below. While, except as before men- 

 tioned, these crystals are colorless, a sand concentrate consisting 

 largely of monazite from Idaho City contains numerous zircons of 

 this habit which are very like the associated monazite in color. 



The crystals vary considerably in development from the typical 

 forms illustrated in Figures 83 and 84, the first order prism in some 

 cases being the larger form with its angles truncated by the second 

 order prism (fig. 85) while in other cases the relative development of 

 the two prisms is reversed (fig. 86). The terminations vary also, in 

 some cases the ditetragonal pyramid being alone or merely truncated 

 at its extreme summit by the pyramid p(lll), while in other crystals 

 the unit pyramid 2>(1H) is largely developed reducing the ditetragonal 

 pyramid to small faces (fig. 85) . Not infrequently these two extremes 

 are observed at opposite ends of the same crystal, producing a pecu- 

 liarly hemimorphic aspect. Some variants of the common develop- 

 ment are difficult to orient as, for instance, when the faces of the form 

 x(311) are very unequally developed and reduce the prism faces to 

 irregular polygons of small size. The pyramid u(S31) is occasionally 

 present, usually as small and dull faces which give poor measure- 

 ments. A coarse screened portion of a sand from Idaho City con- 

 sisting largely of ilmenite contains translucent zircon crystals of two 

 types, both of which are somewhat different from the normal small 

 crystals occurring in the finer screenings of the same lot of sand. 

 The most abundant of these two types is peculiarly tabular to a 

 face of the second order prism a (100) as shown in Figure 87. The 

 other crystals of this lot have a short pyramidal habit with £>(111) 

 prominent and it (331), m(110), and a(100) about equally developed 

 as shown in Figure 88. These are irregular and appear as though 

 made up of numerous, very small, individuals, in parallel position. 

 The reflections from the faces are consequently poor. 



A sample of concentrate rich in columbite and samarskite from 

 Idaho City contains a few small brownish crystals of the simple 

 pyramidal habit shown in Figure 90. 



CAMAS COUNTY 



A sample of placer concentrate from Cow Creek, Camas County, 

 contains rare crystals of zircon like those of Boise County associated 

 with gold, garnet titanite, etc. 



