W. Lindgren — Monazite from Idaho. 63 



Art. IX. — Monazite from Idaho; by W. Lindgren. 



The intermontane valley of " Idaho Basin " is situated thirty 

 miles north-northeast of Boise City, Idaho, in the great granite 

 area of the southern part of that state. Its placer mines have 

 been of extraordinary richness and still contribute a consider- 

 able proportion of the gold production of Idaho. The gold- 

 bearing gravels are of Pleistocene and Neocene age and, near 

 Idaho City, there are also some Neocene lake beds containing 

 only a slight amount of gold. 



The sand of the gravels and lake beds of the Idaho basin is 

 entirely derived from the granite and associated dike rocks. 

 It consists of relatively angular and sharp-edged grains indicat- 

 ing its manner of formation by extremely rapid accumulation 

 from the deeply disintegrated rocks. 



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

 forms a considerable quantity of the heavy substances remain- 

 ing with the gold. The mineral has been shown to be mona- 

 zite, this being the first time its occurrence has been noted 

 from the western states. As well known, it occurs abundantly 

 in the granite and gneissoid rocks and gold placer mines of the 

 Southern Appalachians and in several of the northern Atlantic 

 states, also in Brazil, the Ural Mountains and other places. 

 There is no doubt it forms an original constituent of the gran- 

 ite of the Idaho Basin. 



A sample washed from the lake beds near Idaho City con- 

 sisted of the following minerals : ilmenite in sharp hexagonal 

 crystals but no magnetite ; 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 this mineral were very 

 high, the hardness not much over 5. The ilmenite was elimi- 

 nated by the electro-magnet and the remaining powder, contain- 

 ing 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. 



Another sample furnished me by Mr. T. Smith of Placer- 

 ville came from the alluvial gold washing in Wolf Creek near 

 that town. Cleaned from quartz, etc., it appeared as a heavy 

 dark sand consisting of a black iron ore (ilmenite), rounded 

 crystals of red garnet, sharp crystals of zircon, and irregular 



