-UiT. 3. BLACK SANDS FROM IDAHO SHANNON. 9 



rarely does a magnetic grain show the typical octahedral form of 

 magnetite, and it may be that the black mineral is practically all 

 ilmenite. The ilmenite of the Snake River sands thus is in consid- 

 erable part magnetic as contrasted with the nonmagnetic character 

 of the same mineral in the sands of the granite regions. 



The source of this mineral in these fine sands of the Snake River 

 is not certain. It is noteworthy, however, that the thin sections of 

 the diabasic rock, which occurred abundantly in sand from Minidoka, 

 contained numerous scattered tabular crystals of ilmenite, as noted 

 under " augite." 



GARNET. 



Garnet is almost if not quite invariably present in the sands ex- 

 amined, occurring either as grains, irregular fragments, or rough to 

 highly perfect crystals. As seen under the binocular this mineral 

 is of two varieties, which are distinguished by a very striking dif- 

 ference of color. The most abundant and widely distributed variety 

 is clear bright red to slightly brownish red in color and is probably 

 almandite. It occurs most abundantly in many of the sands, being 

 especiall}^ prominent in those which carry abundant monazite, as in 

 samples from Centerville, Idaho City, and other localities in Boise 

 County. Here it is in part in small, irregular worn, or broken grains, 

 but for the most part the grains are small, highly perfect limpid 

 and transparent crystals which are either trapezohedrons, dodecahe- 

 drons, or combinations of the two. In the sand from Bear Creek 

 almandite garnet occurs in rare small model-perfect crystals up to 

 a millimeter in diameter, associated with titanite and allanite. In 

 the fine sands of the Snake River similar brown-red almandite is 

 present in subordinate amount as small irregular grains. The abund- 

 ant garnet of these sands is rather pale rose pink in color when in 

 small grains. This pink garnet is present in greater or less amount 

 in every Snake River sand examined, in some cases being the only 

 garnet present, while in other sands both varieties occur. Aside from 

 the Snake River samples pink garnet was noted only in a few sands 

 from the vicinity of Pierce City and Orofino, in Clearwater County. 

 In some of the coarser sands from these latter localities the garnet 

 occurs in masses up to 1 centimeter in diameter, and in the larger 

 grains its color is rose purple, being comparable to that of the rhodo- 

 lite garnet from North Carolina. The pink or purple variety in the 

 heavy sands is more or less coincident in occurrence with augite, and 

 this fact, together with its color and general appearance, supports 

 the conclusion that it is probably a magnesian variety, high in the 

 pyrope molecule. As contrasted with the abundant and highly per- 

 fect crystals of the brown-red garnet, the pink variety rarely occurs 

 in recognizable crystals, being usually in water-worn grains or angu- 



