658 IDAHO MINING DISTRICTS. 



granite is easily washed down by atmospheric agencies. The width 

 of the valley between the summits is not more than 2 miles. Above 

 the mouth of Grimes Creek the valley contracts and the steep slopes 

 project, the creek forming a more V shaped valley, which continues 

 to 44 miles above the junction. A few small alluvial flats lie in the 

 bottom of this canyon. 



The basalt flow. The valley bottom is filled with a basalt flow rarely 

 over one-fourth of a mile in width ; its top rises to about 100 feet above 

 the creek level. Through this flow the stream has cut down to a depth 

 somewhat exceeding that which it had attained before the basaltic 

 eruption. The joints in the basalt have produced perpendicular 

 cliffs, the whole being an exact illustration of a canyon within a valley. 

 "While much of the basalt flow has been eroded, enough remains to 

 form a nearly continuous fringe of cliffs on either or both sides of the 

 streams, reaching from the mouth of the creek up to 1^ miles above 

 the junction with Grimes Creek. The level top of the flow has been 

 covered by a sloping, sandy wash from the adjoining hills. The 

 character of the valley, the basalt flow and its covering, is well illus- 

 rated in PI. XCII. Above the point mentioned the basalt suddenly 

 ceases, and no more of it, either in outcrops or in pebbles, is seen 

 above. It also extends a few miles up on Grimes Creek, and it is 

 probable that the vent was located at some point in the Grimes Creek 

 Canyon, and that the flow backed up for some distance on Moore 

 Creek. The basalt is about contemporaneous with the flows of the 

 Snake and Boise rivers geologically speaking recent being referred 

 to the late Neocene (Pliocene) period. 



The present stream gravels. The bed occupied by the stream is 

 generally narrow, seldom exceeding 100 feet in width. The bed rock 

 is only rarely exposed. The creek is filled with coarse gravel, largely 

 made up of tailings brought down from the Idaho Basin by the winter 

 freshets. The maximum depth of these tailings is 20 feet. The 

 gravels in the present stream have been and are still extensively 

 washed, mostly by Chinese using the ordinary appliances for river 

 mining dams, Chinese pumps, and derricks. The old channel cov- 

 ered by the recent debris has been exposed by mining at many places, 

 being naturally richer than the tailings. The many basalt bowlders 

 found in it are a considerable obstacle to cheap mining. On Moore 

 Creek, about 4 miles above the mouth of Grimes Creek, a steam 

 dredge was operated in 1896, with the purpose of striking the rich 

 gravel 10 to 20 feet below the surface. The gravel is reported to have 

 been reached, though the large bowlders interfered somewhat with 

 the work. On the whole, the present stream gravels of lower Moore 

 Creek can not be considered to have been extremely rich. 



The gravels below the basalt. The basalt along Moore Creek is 

 found to rest on stream gravels accumulated in the bed before the 

 molten flow poured down the valley. The old channel is sometimes 



