BLACK HILLS GEOLOGY. 299 



edge to seven or eight feet. Above is a roof of lime shales. 

 The width of the shoots is roughly from 20 to 150 feet, and in 

 length they are much greater. The central portion of a shoot 

 is usually the thickest, and on the flanks the ore thins out lat- 

 erally, with irregular boundaries. It sometimes passes into 

 shales in alternating layers of different degrees of silicification, 

 and again it thins down to a feather edge. When a shoot has 

 been mined out, narrow vertical fissures filled with ore can be 

 detected, which are roughly parallel to the longer diameter of 

 the ore body, and may be seen both in the shale roof above and 

 the quartzite floor beneath. In the latter they are much con- 

 stricted and frequently thin down to a mere streak. They often 

 fork, and are sometimes cut by cross verticals, but in general 

 they follow roughly the longer direction of the shoot. Through- 

 out the entire mine ramifying dikes and sheets of phonolite oc- 

 cur in great profusion, and in some instances the ore verticals 

 can be seen alongside of them, the solutions evidently having 

 had access through contact zones. The character of the ore is 

 essentially oxidized, and is a very hard siliceous material, which 

 is heavily coated and intermingled with iron oxides. The ore 

 Ts in all cases a replacement of the calcareous material of the 

 shales and sandstones by siliceous solutions, which contained 

 the valuable mineral. 



In many places a gray to blue, dense ro~k is met, which has 

 almost the appearance of a diorite, and which forms the borders 

 of the ore- shoots. It is called " sand-rock " by the miners and 

 was found on examination to be a quite pure crystalline lime- 

 stone, but to contain disseminated sand grains and considerable 

 pyrite. 



The bluish variety, when exposed to oxidizing conditions be- 

 comes red, and in the most highly altered occurrences is a 

 light, reddish, sandy material from which the. greater part of the 

 lime has been dissolved. It is then termed " red gouge " by 

 the miners. This rock, in either the blue or unoxidized, or the 

 red "and oxidized state, is present in all mines of siliceous ore, 

 and seems, from its highly calcareous nature, to have eveiy- 

 where formed the ore -bearing horizon. That the siliceous ore- 



