268 GEOLOGY OF THE BLACK HILLS. 



stream. Among- the rocky hills these flats are wanting, and the forest 

 extends nearly to the water's edge. 



Rapid Creek flows in a parallel course to Spring Creek, at a distance of 

 four to eight miles to the north, and cutting through the same belt of clay- 

 slates and quartzites ; the description of the rocks of the Spring Creek 

 district will apply, with some slight modifications, to the rocks of this 

 valley. 



The immense belts of quartz and quartzite so marked on Spring 

 Creek are not so prevalent on Rapid Creek, but the soft gray and black 

 clay-slates, with small ferruginous quartz seams, are here extensively devel- 

 oped. The clay-slates have a tendency to split, under the action of frost 

 and the atmosphere, into long, slender prisms, parallel to the bedding of the 

 rock. In some of the cliffs benches of these slates have weathered out, 

 and the fragments falling down lie in regular piles like cord- wood along 

 the face of the rocks. These prisms are about 4 feet long, with an irregu- 

 lar rhombic section 3 or 4 inches across They readily split into smaller 

 prismatic fragments an inch wide and a foot or more in length. The slates 

 contain minute crystals of iron pyrites, and the surfaces of the prisms are 

 stained with oxide of iron resulting from its decomposition. Other strata 

 of slates are lamellar in structure, and split into thin plates like roofing 

 slate. Both forms make excellent bed-rock to catch and retain gold. 



A mile above a small branch entering Rapid Creek from the north, is 

 a low hill, composed of trachyte intruded between, the slates. This was the 

 only occurrence of igneous rock seen in this geological formation south of 

 Custer Peak, with the exception of the granite of the Harney Peak range. 

 On this branch float quartz was found, which on breaking showed small 

 particles of gold, but it could not be traced to the vein from which it had 

 been derived. 



Samples of quartz brotight in by the miners from the hills between Rapid 

 and Box Elder Creeks gave, on crushing, two or three ounces in a mortar, 

 and panning from four to six fine colors of gold. Gold was discovered in 

 paying quantities in a number of places along this portion of the valley; 

 but the miners who staked claims in this district did not open the placers, 

 and prospected the bars only enough to prove the presence of gold. These 



