AUT. 8. 



BLACK SANDS FROM IDAHO SHANNON. 



11 



bubble-like cavities and also minute microlite-like prismatic crystals 

 of a transparent colorless mineral having a lower index of refraction 

 than the zircon. Many of the translucent smoky-gray crystals appear 

 to owe their color to minute inclusions of iron oxide. In form the 

 zircons are most frequently prismatic, with the length three or four 

 times the diameter, and the most abundant types show the first order 



M 



7^ ^ 9 



Figs. 1-9. — Crystals of zircon. 



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

 development and are terminated by the ditetr agonal pyramid a; (311), 

 either alone (fig. 1) or together with the unit pyramid p{lll). 

 Crystals of these habits are by far the most abundant in the mona- 

 zite-bearing sands and also occur, though in lesser amount, in all 



