212 ISAAC A. HOURWICH 



this condition was met with very early in its mining history, 

 at levels varying from 100 to 200 feet. 



The problem was not fully solved until the eighties, when 

 the process of concentration was introduced. Only rich ores 

 could bear the expense of shipment to distant smelters. As 

 a result, low grade refractory ores which couldrnot be treated 

 by amalgamation were thrown away. The new process of 

 concentration, which reduced the volume of ore to be shipped 

 and treated, was tantamount to a discovery of new gold mines. 



The following description of this process is condensed 

 from the report of the Colorado bureau of mines : 



''The system of ore dressing known as concentration is 

 one of the most important of all processes applied to the 

 treatment of ores carrying low values in gold, silver, lead, 

 and copper. There is probably no other line of ore dressing 

 so universally used. Notwithstanding the new devices in- 

 troduced, all are in line with the early and original designs, 

 differing only in the manner of application of principles in- 

 volved. The theory of concentration is based upon the vari- 

 able specific gravity of the different minerals. Its applica- 

 tion is to separate the various metals, collect those having 

 value, and reject the remainder. A large proportion of the 

 concentrates marketed are derived from the stamp mill tail- 

 ings. When the value in tailings is in form of pyrite, chalco- 

 pyrite, and galena, with gold and silver associated, the ore 

 passes direct from the plates to different patterns of oscil- 

 lating, or bumping, tables, and the separation made. Where 

 the base minerals occur in comparatively large crystals the 

 stamp battery is often preceded by crusher, rolls, sizing 

 screens, and hartz jigs, the jigs yielding a coarse concentrate, 

 and the tailings from jigs being recrushed in battery over the 

 plates and tables." 



The problem of an economical process for the treatment 

 of low grade ores for many years tempted the inventive spirit 

 of mining men. Many processes were devised only to be 

 rejected by experience. The process man became the object 

 of cheap ridicule even in official publications. It was, how- 

 ever, owing to the efforts of one of the multitude of these 

 process men that the cyaniding process was invented, which 



