70 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



are some of them which can be termed fissure or contact veins, but in most 

 cases they are very irregular in form, and are better described by the Ger- 

 man word "Stock" than by any mining term used in English. The word 

 "pipe" can be applied to many of them, but in its ordinary use a "pipe" 

 implies a "rake," while the ramified structure of the Cumberland deposits 

 is rarely well marked at Eureka. They are often lenticular. This term, 

 however, cannot always be used to describe their form, as they have off- 

 shoots in all directions. Any classification, however, which is dependent 

 on mere differences of form must be more or less imperfect. 



sha? e of the deposits. — The deposits sometimes spread out into immense cham- 

 bers that measure more than fifty feet in each direction, and which are com- 

 pletely filled with ore, with the exception of an occasional cave or limestone 

 pillar. From the sides of these chambers, which scarcely ever present 

 smooth walls, there are branches, and auxiliary pipes lead up or down, or 

 in a horizontal direction, to other bodies. The ore bodies do not seem to 

 follow any particular direction, either as regards dip or strike, and at first 

 sight they appear to be distributed throughout the ore-bearing formation 

 without any regularity. This is not wholly the case, and although no well- 

 defined law can be found governing their occurrence, this is connected with 

 that of certain phenomena in the country rock, such as fissures, caves, and 

 broken limestone. 



Relation of the deposits to the limestone. — The distribution of the ore has been deter- 

 mined almost entirely by the physical character of the limestone in which 

 it is found, and not by any chemical or mineralogical differences in the rock. 

 In other words, whether the limestone was dolomitic or not, and whether it 

 was nearly pure or somewhat argillaceous, it was always a rock which would 

 fulfill the chemical conditions necessary to the deposition of ore. Even sup- 

 posing that dolomitic limestone is generally better adapted to induce deposi- 

 tion — and this is something which has never been proved — the greater facili- 

 ties offered by a crushed and broken limestone, no matter what its charac- 

 ter, to the percolation of the metal-bearing solutions, would more than 

 compensate for any chemical advantages which a particular kind of lime- 

 stone offered. Although the typical fissure vein is found in limestone 

 in many parts of the world, its occurrence in that rock in the Great Basin 



