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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 60 



other sands examined, including the monazite-free samples from 

 numerous localities along Snake River and the titanite and allanite 

 bearing sand from Bear Creek. Wliile, except as before mentioned, 

 these crystals are usually colorless, a sand concentrate consisting 

 largely of monazite, from Idaho City, contains abundant zircons of 

 this form, which are yellow, being very like the monazite in color. 

 A typical crystal, having this habit, from a monazite concentrate 

 from Grimes Creek near Centerville was measured and gave the 

 following angles: 



Angles of zircon crystal from Centerville {fig. 2). 



The crystals vary considerably in development from the typical 

 forms illustrated in figures 1 and 2, the first order prism being in 

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

 order prism (fig. 3), while in other crystals the relative development 

 of the two prisms is reversed (fig. 4). The terminations vary also in 

 some cases the ditetragonal pyramid a? (311) being merely truncated 

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

 the unit pyramid p(lll) is largely developed reducing the ditetrag- 

 onal pyramid to mere line faces (fig. 3). Not infrequently these two 

 extremes are observed to opposite ends of the same crystal yielding 

 a peculiarly hemimorphic aspect. Some variants in the common 

 development are difficult to orient, as, for instance, when the ad- 

 jacent faces of the form a? (311) are very unequally developed and re- 

 duce the prism faces to irregular polygons of small size. The pyra- 

 mid w(331) is occasionally present as small faces which are dull and 

 irregular and give poor measurements. A coarse-screened portion of 

 a sand from Idaho City consisting largely of ilmenite contained 



