AUT. 3. 



BLACK SANDS FROM IDAHO SHANNON. 



17 



Idaho City described below. The monazite is, for the most part, 

 in sharp and perfect crystals although many of the larger crystals 

 are broken or abraded. The average diameter in the mineral in the 

 screened sands studied is less than 1 mm., but in one " oversize " 

 sample rough crystals up to 5 mm. in diameter were observed, and 

 larger masses may have been discarded by screening. In form the 

 monazite from all of the several localities represented is very similar. 

 The figures reproduced were all drawn from measurements made 

 upon crystals selected from a sand from Centerville, and subse- 

 quent examination of the numerous other sands did not reveal 

 any additional forms, combinations, or habits. The smaller crystals 

 are often flawless and perfectly transparent, while the larger in- 

 dividuals are more or less opaque from Ihe presence of niunerous 

 cracks and rifts. The forms noted on the crystals are few in num- 

 ber and perhaps 90 per cent of the crystals seen had almost precisely 

 the habit shown in figure 10, and 9 per cent had the form shown in 

 figure 11. Figures 12, 13, and 14 represent quite unusual habits. 

 The very simple habit shown in figure 14 is characteristic of some of 

 the very largest as well as of the colorless and green monazite 

 crystals seen in the samarsldte concentrate. The more prominent 

 faces on the majority of the crystals gave fairly good reflections and 

 the agreement between the angles measured and the theoretical 

 angles completely dissipates any doubt which may remain regarding 

 the identity of the Idaho mineral. The averages of the angles meas- 

 ured on the several crystals examined are compared with the theo- 

 retical angles in the following table: 



Angles of monazite from Idaho. 



The monazite of one sample from Pierce City is in unusually 

 coarse, imperfect crystals which have an internal grating structure 

 which may be due to multiple or polysynthetic twinning. Inclusions 

 are not abundant, and the monazite is rarely attached to any other 

 mineral. In one case a hexagonal tablet of biotite was seen imbedded 



