163 



ing tlie persistency of the veins in a vertical direction, a suflicieut num- 

 ber of observations have been made to lead to the conclusion that their 

 general character in that respect is satisfactory. The ores contained in 

 the veins are of such composition that they will oifer no serious obstacles 

 in any smelting establishment that may be founded upon principles that 

 are not totally at variance with chemical and physical laws. 



Geologically, the veius of our district are very young, probably having 

 been formed at the close of the Cretaceous or the beginning of the Ter- 

 tiary period. The enormous eruptions of the trachytic lava, covering a 

 ■ continuous area of more than live thousand square miles, must have 

 taken place at the geological period above indicated. In the beginning 

 of this paper particular stress was laid upon the impregnation with 

 mineral matter of certain volcanic strata — a phenomenon that occurs 

 over a large tract of country. This shows that at the time of the 

 eruptions such conditions existed as were favorable to the formation of 

 that class of minerals generally termed ores. It is furthermore to be 

 observed that these impregnations occur mainly in the younger strata. 

 Although the inference cannot be drawn that the fissures were formed 

 at the same time, or shortly after the deposition of the trachytic lava, it 

 is allowable to assume that at such a period the material for filling these 

 fissures was existing near the locality where but lately so thorough an 

 impregnation had taken place. The fact that the fissures extend, at a 

 number of points, downward, through the older metamorphic rocks, 

 makes it improbable that they should have been formed by contraction 

 of the cooling masses. Singular as it may seem, these lodes are devoid 

 of that ore which is generally classed as surface-ore. Immediately from 

 the surface the perfectly fresh minerals are taken out. The gangue is 

 hard and solid. An exception is made, of course, although only to a 

 slight extent, by pyrite, which decomposes very readily when exposed 

 to the action of atmospheric influences. This characteristic may be 

 explained in various ways— by the rapid decomposition and breaking oft 

 of the wall-rocks, carrying with them jjortions of the gangue and ore ; by 

 the less intense effects of atmospheric agencies ; by the character of the 

 minerals composing the ore, and by the comparatively short time that 

 these fissures have been filled. The latter view is the one that would to 

 me appear as the ijaost acceptable. 



A difficult question arises, when a decision is to be made, as to the 

 causes that have produced the formation of the fissures that were after- 

 ward filled. Accepting, as I have always done, the theory that vol- 

 canic or plutonic earthquakes have probably produced the larger num- 

 ber of all lode systems — and such we have in this case — it will be neces- 

 sary to find whence came the requisite force. Along the highest por- 

 tion of the quartzite mountains we have an anticlinal axis which can 

 be traced westward for nearly forty miles, an upheaval that must have 

 a very perceptible effect on regions adjoining. The idea at first pre- 

 sented itself that this might have given rise to the formation of the 

 fissures, but evidence subsequently discovered demonstrates that long 

 before the eruption of the trachyte this disturbance had occurred. 



About twenty miles west from the center of the mining region is a 

 series of isolated groups of volcanic peaks. The highest one of these, 

 Mount Wilson, reaches an elevation of 14,285 feet above sea-level, 

 be about 5,000 feet above the vallej^. Lithologically these groups must 

 considered younger than the lode-bearing rock of the Animas, and must 

 therefore have become eruptive later. It seems quite possible, that the 

 disturbance produced by these eruptions may have resulted in the 



