ABT. 3. BLACK SANDS FROM IDAHO SHANNON. 7 



the Snake River ; the second from the gold placer region of the Boise 

 Basin, and the third from the placer-mining region centering about 

 Pierce City, in Clearwater County. 



The source of the minerals occurring in the sands of the bars of 

 Snake Elver is somewhat problematical. Very fine gold is known 

 to occur in these sands throughout the length of the river and many 

 attempts have been made to work the deposits, but, owing to the ex- 

 tremely fine state of subdivision of the metal, these efforts have not 

 met with much success. The occasional olivine and abundant augite 

 in the sands may be derived from the basaltic lava which covers 

 hundreds of square miles of the upper Snake River Valley or may 

 have been concentrated from earlier sedimentary deposits derived 

 from older formations in the foothills. It is noteworthy that the 

 black sand from Minidoka, which contains a higher proportion of 

 augite and more olivine than any other examined during the present 

 investigation, was shown by Day ^ to contain appreciable amounts of 

 platinum. BelP has discussed the occurrence of platinum in the 

 fine sands of the Snake River and has shown that it is not present 

 in commercial amount. 



The minerals found in the heavy sands of the Idaho Basin are 

 probably all derived from the granitic rock which underlies the 

 region or from the dikes of several types which intrude the granite. 

 The placer districts of Clearwater County are largely derived from 

 the rock of the northern extension of the same granitic batholith. 

 The magnesium minerals, pyrope garnet, and augite, which occur in 

 some sands from the latter section, are in all probability derived 

 from local intru?3ions of basic magnesian rocks. 



The minerals identified in the sands studied are described in some 

 detail below, since the exact form and character of the constituent 

 minerals of such sands have not heretofore been adequately set 

 forth. The numerous figures accompanying these descriptions have 

 been drawn from actual measurements made on cr3'stals selected 

 from the sands. The isometric forms of garnet, magnetite, and 

 pyrite are not figured, as the habits of these minerals in the sands 

 are precisely those illustrated by the figures given of these minerals 

 in any standard textbook of mineralogy. It is hoped that the figures 

 reproduced here may be of some assistance to future workers in 

 identifying the minerals of such heavy sands under the microscope. 



ILMENITB. 



Ilmenite is abundant in the heavy sands, probably being present 

 in excess of magnetite in all of those examined except in one from 



' Day, David T., and Richards, R. H.. Mineral Resources U. S. for 1905, U. S. Geol. 

 Survey, 1906, p. 1195. 



* Bell, Robert N., Mining Industry of Idaho for 1906, p. 116, Aim. Rept. State Inspector 

 of Mines, Boise. 1907. 



