BLACK SANDS FROM IDAHO SHANNOiST. 



13 



translucent crystals of zircon of two types, both of which are some- 

 what different from the normal transparent crystals occurring in 

 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(lOO), as shown in figure 5. The average angles obtained upon 

 measurement of a crystal of this type are given in the following 

 table : 



Average angles of tabular crystal of zircon from. Idaho City. 



Other crystals from the same lot have a short pyramidal habit with 

 p{lll) prominent and w(331), m(llO), and a(lOO) about equally de- 

 veloped, as shown in figure 6. These are irregular and appear as 

 though made up of numerous very small individuals in parallel posi- 

 tion. The reflection from the faces are consequently not very good. 

 The angles measured on a crystal of this habit are given below. 



Angles of zircon crystal from Idaho City (fig. 6). 



Reflection. 



Very good. 



Good 



Medium 



Fair 



Although the majority of the tetragonal crystals in the Snake 

 Eiver sands are similar to those shown in figures 1 to 4, several other 

 types were seen, A sand from Rosa, Bingham County, contains scat- 

 tered translucent crystals of a brownish color whidi are short pris- 

 matic, with the length about twice the thickness. These show the 

 prism m(llO) and the pyramids p{lll) and 1^(331) developed as 

 shown in figure 9. A crystal of this type yielded the following 

 measurements : 



3136— 22— Proc.N.M.Vol.60 6 



