EOTLicH.] GEOLOGY FOEMATION OF LODES. 283 



siderable number of mines. As an average width for the lode be 

 tween walls, about -4 to 6 feet might be given, although some of them 

 widen out to nearly 30 feet. 



Taking all these observations and facts into account, we may be able 

 to arrive at some conclusions regarding the formation of these lodes. 

 Yon Cotta says,* " It is still most probable, as von Beust in his criti- 

 cism of Werner's theory has clearly shown, that the majority of lode- 

 fissures have been torn asunder by concussions caused by volcanic or 

 IJiutonic activity, or, in other words, by volcanic or i)lutonic earth- 

 quakes." In applying this sentence to the case now under consider- 

 ation, it becomes necessary to inquire into the geological nature of 

 the formations involved. The crystalline gneiss is probably the oldest 

 rock in that section of Colorado, while the granite and porphyry are 

 younger. A simple metamorphosis, a remelting of some one or the 

 other portion of this gneiss at a comparatively shallow depth, would 

 undoubtedly produce the effect of a '^ plutouic earthquake," whereby not 

 only the fissures could have been formed, but the granite have become 

 eruptive. Subsequent disturbances from the same direction may have 

 produced the parallel fissures in the granite, while the eruption of the 

 porphyry seems to have been accompanied by very slight or no such 

 demonstrations. Besides the great uniformity among the lodes jper se, 

 the similarity shown in the character of the ores within certain limits 

 of locality speaks for a common origin of almost ail the vein-matter and 

 ore contained within those certain limits. Although the lodes on the 

 outer edge of the district vary in the character of their ores from the 

 others, this may still not justify assigning to them an age very widely 

 separated either before or after from the latter. 



As ore, mainly pyrite, chalcopyrite, galenite, and sphalerite are 

 found in this district, and in a few isolated points argentite. Experience 

 has shown that the chalcopyrite and the finely-disseminated pyrite 

 yield the best production of gold, while galenite and sphalerite contain 

 silver ; the massive pyrite,. however, only small quantities of gold. Fre- 

 quently bands and veins of this pyrite occur more than a foot in thick- 

 ness, but, as a rule, from two to six inches. 



Toward the surface, the ores decompose through the action of atmo- 

 spheric influences partly, partly through chemical agents. The result 

 of this decomposition is usually termed " surface-ore," and contains, in 

 contradistinction to the ores of lower depths, its gold as free gold. Pyrite 

 loses its sulphur, as also does the chalcopyrite, and more slowly sphale- 

 rite and galenite, and either oxides, carbonates, or sulphates are formed. 

 From the fact that so little native gold is observed, even with diligent 

 search in the deeper portions of lodes that on the surface show or 

 showed it in considerable quantities, it might seem possible that the 

 gold was contained in the original ore as a compound, and became free 

 through the action of decomposing agencies. Thus far, no experiments 

 that may have been made afford any proof that such a compound of gold 

 should exist, although circumstantial evidence points very strongly in 

 that direction. It is a question that would require for its answer, by 

 means of chemical investigation, the most subtile manipulation, unweary- 

 ing attention, and a large, judiciously selected material. 



In speaking of the various lodes and mines in the following pages, 

 only those shall be spoken of at any length with which I have become 

 personally acquainted by one or more visits. No further attention was 

 paid to the names of individuals or companies owning the mines, as they 



* Ore Deposits, translated hj F. Prime, jr., 1870, p. Go. 



