678 IDAHO MINING DISTRICTS. 



grated rocks. The heavy minerals found in the granite are also 

 found in the sand, and may be easily separated by washing in the 

 miner's pan; they are always deposited in the sluice boxes with the 

 gold. 



In all parts of the basin a yellow or brownish-yellow mineral forms 

 a considerable quantity of the heavy substances remaining with the 

 gold. It is usually referred to as "yellow sand," and is also given the 

 picturesque name of "Bummer Hill sand," from a locality near Cen- 

 terville, where it was particularly abundant, but I am not aware that 

 its true character has ever been investigated. 



The mineral has been shown to be monazite, this being the first 

 time its occurrence has been noted from the Western States. As is 

 well known, it occurs abundantly in the granite and gneissoid rocks 

 and gold-placer mines of the Southern Appalachians, and in several 

 of the North Atlantic States, also in Brazil, the Ural Mountains, and 

 other places. There is no doubt that it forms an original constituent 

 of the granite of the Idaho Basin. 



One of the samples was obtained in washing a few pans of the sandy 

 lake beds occurring as "false bed rock" in a gravel bench at the 

 junction of Moore Creek and Granite Creek, 3 miles east of Idaho 

 City (see fig. 59). The heavy residue consisted largely of small yellow 

 grains and amounted to about 2 grams per pan of 8 kilograms, which 

 would correspond to 0.025 per cent. The microscope revealed the fol- 

 lowing minerals: Ilmenite in sharp hexagonal crystals, but no magne- 

 tite; zircon, also in extremely sharp crystals of a slightly brownish 

 color, and abundant yellow or greenish-yellow grains rarely showing 

 crystallographic faces. The refraction and double refraction of the 

 latter mineral were very high; the hardness not much over 5. The 

 ilmenite was eliminated by the electro-magnet, and the remaining pow- 

 der, containing about 70 per cent of the yellow mineral, was analyzed 

 by Dr. W. F. Hillebrand. The result showed it to be a phosphate of 

 the cerium metals, the approximate amount of the oxides of the latter 

 being 48 per cent; in these approximately 1.20 per cent of thoria was 

 found. This result identifies the mineral with monazite, the only 

 other similar mineral being xenotime, which is mainly a phosphate of 

 yttrium with but little cerium. The samples also contained a con- 

 siderable amount of titanium, which would indicate that some titanite 

 is present. Practically all of the ilmenite was extracted by the 

 magnet. 



Another sample, furnished me by Mr. T. Myer, of Placerville, 

 came from the alluvial gold washings in Wolf Creek, near that town. 

 Cleaned from quartz, etc., it appeared as a heavy dark sand consist- 

 ing of a black iron ore (ilmenite), rounded crystals of red garnet, 

 sharp crystals of zircon, and irregular grains of a dark yellowish- 

 brown mineral with waxy luster, sometimes showing crystallo- 

 graphic faces. It was found impossible to extract more than a small 



