308 GEOLOGY OF THE EUREKA DISTRICT. 



the time the main ridge was uplifted. Other faults indicate lateral thrust, but 

 they are less effective than the former and may be of later origin, due to sub- 

 terranean forces connected with the period of volcanic energy. Accom- 

 panying these are innumerable small fissures, seams, crevices, chambers 

 and channels of varied shapes and sizes. Many of these owe their origin 

 simply to the dynamic effects of upheaval. Others are best explained on 

 the theory of surface waters percolating downward along lines of least 

 resistance, widening fissures and enlarging cavities. If these waters were 

 charged with carbonic acid, chambers and irregular shaped galleries and 

 drainage channels must necessarily have been dissolved out of the limestone. 

 A study of these channels and their intricate connections tends to the belief 

 in the theory of preexisting caves and underground water courses before 

 the introduction of ore. 



Fining of Fissures. The coming in of the volcanic period would be quite 

 likely- to disturb and dislocate any previous system of subterranean drain- 

 age, in some places completely closing and in others opening new channels 

 by the formation of fresh cracks and crevices. Subsequent to the penetra- 

 ting of the main fissure by rhyolite came the filling of minor fissures and 

 other openings in the limestone by the ascending mineral solutions and 

 gaseous currents. Wherever the narrow fissures admitted of it these open- 

 ings and chambers were more or less filled with mineral matter precipitated 

 from solution, the passage ways in many instances being left neai'ly ban-en 

 or only carrying stringers and slight indications of earthy, ocherous 

 material probably deposited before the dying out of the active mineral 

 currents. In some instances the narrow connecting channels between the 

 larger openings are richer in mineral matter than the chambers themselves. 

 It would be useless to speculate on the reasons why certain fissures and 

 chambers earned ores and others were left barren. The freaks of deposi- 

 tion from ascending currents in some places rapid, in others slow and the 

 varying conditions of temperature and pressure brought about by the vary- 

 ing intensity of solfataric action would produce endless differences in the 

 mode of occurrence. Channels which at one time presented conditions 

 most favorable for deposition might at a later period become entirely cut 

 off from ascending currents. Anyone who has observed carefully the 



