712 IDAHO MINING DISTRICTS. 



These placers also probably have their origin in the gold veins of 

 Crown Point Hill. 



RHYOLITE. 



The sharp point of Prospect Peak and the hill 1 mile to the west of 

 it consist of rhyolite. They are necks from which large flows of the 

 same rock poured down the southern slope of the ridge during the 

 period of the Payette lake beds. In South Willow Creek the sand- 

 stone is seen to overlie the rhyolite. A smaller flow of rhyolite 

 reached down as far as one-fourth mile south of Pearl. The rock is 

 usually reddish or reddish-gray, and is of the ordinary compact lith- 

 oidal variety. In the last-mentioned flow occur, associated with it, 

 rhyolite glass and loose tuffs. 



BASALT. 



The eruption of rhyolite was followed, during the same early Neo-> 

 cene period, by extensive eruptions of andesitic basalt. Smaller 

 masses and necks of this black massive rock occur on the ridge one- 

 fourth mile west of Prospect Peak, near the Leviathan, and at several 

 other places to the west. 



The Payette formation and accompanying eruptives are later than 

 the mineral deposits, and contain no veins. 



THE ORE DEPOSITS. 



GENERAL CHARACTER. 



The gold deposits in the Willow Creek and Rock Creek districts are 

 fissure veins of somewhat varying character. Most of them occur in 

 a belt parallel to that of the porphyry dikes, extending in a north- 

 easterly direction, and being in no place much over 1 mile in width. 

 The veins at Willow Creek are most frequently entirely in the dioritic 

 granite. Sometimes a vein follows a porphyry dike for some distance 

 in foot or hanging, but rarely for a long distance. Again, a vein 

 may cut through a dike, in which case it often splinters up. The 

 porphyry dikes are evidently all older than the veins. The fis- 

 sures which carry gold strike east- west or northeast-southwest. In 

 the Willow Creek district the dip is always to the north from 45 to 

 80 and the direction east- west, but toward Rock Creek the direction 

 gradually changes to northeast-southwest. Toward Horseshoe Bend 

 the direction changes again to east-northeast to west-southwest, and 

 the dip is frequently steep to the south. The individual veins can 

 rarely be traced for a long distance, and though it is probable that 

 some of the veins are a mile long, this can rarely be satisfactorily 

 proved. Narrow veins predominate in the Willow Creek district, 

 while wider deposits occur on Rock Creek. The best exposures are, 

 however, found in the former district, owing to more extensive devel- 

 opment. In Rock Creek the developments are relatively slight, and 



