568 GEOLOGY AND MINIIfG INDUSTRY OF LEADVILLE. 



over, that deposits of mineral matter made in pre-existing cavities are in 

 more or less regular layers, parallel with the walls of the cavity, and that 

 where this approaches a spherical shape, even in a slight degree, these layers 

 are concentric. The most perfect type of this arrangement is seen in the 

 agates which fill geodes. 



Were the ore bodies of Leadville the filling of pre-existing cavities, 

 not only would it be expected that a certain jjarallelism within the walls or an 

 arrangement in layers of their various mineral constituents should have ex- 

 isted, but these walls would have been defined by a distinct cla3'ey selvage, 

 all of which could hardly have been entirely obliterated by the secondary 

 alteration which has taken place. Further, an examination of the outlines 

 of these ore bodies afi'orded by the maps and sections shows the physical 

 impossibility of their having once been open cavities. What would have 

 supported the roofs of such broad continuous openings as they would rep- 

 resent, or, in cases where they occupy sensibly the entire space between 

 two sheets of porphyry, wh)^ did these sheets not close together? Again, 

 how could such cavities have been formed at the depth at which tliese 

 deposits were originally formed, which it has been shown must have been 

 about 10,000 feet below the rock surface? 



The caves which now exist in the limestones of this region are of 

 extremely recent origin, and, as has been shown, cut through limestone and 

 ore bodies indiscriminately. The action of the surface waters which formed 

 them is therefoi'e not only recent, biit more recent than that which pro- 

 duced the greater part of the secondary alteration of the ore bodies. 

 Those who maintain that the deposits in limestone have necessarily been 

 deposited in pre-existing cavities do not in all cases,' it is true, distinctly 

 state that these cavities must have been formed by surface waters; but it 

 yet remains to be proved that any of the caves which are so commonly 

 found in limestones have been formed at any great distance from the sur- 

 face. The majority certainly have not, and, since it is generally admitted 

 that the power of easily dissolving limgstones is acquired by the waters 



' Prof. J. S. Newberry (School of Mines Quarterly, March 1880) distinctly states that the west- 

 ern deposits in limestone have been deposited in caves which, like the Mammoth Cave, were formed by 

 surface watei-s, :ind that probably these deposits will prove of limited extent in depth, since the exca 

 vation of limestones " must be confined to the zone traversed by surface drainage." 



