238 GEOLOGICAL SURVEY OF THE TERRITORIES. 



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 surf ace- 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 off 

 of the wall-rocks, carrying with them portions 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 w T ould 

 to me appear as the most 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 hav T e 

 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, 

 about 5,000 feet above the valley. Lithologically these groups must 

 be 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 

 formation of the present Assures, which subsequently were filled from 

 that source which supplied so much mineral matter to other neighbor- 

 ing rocks in the form of impregnation. It is extremely difficult to de- 

 cide questions of this kind, involving so many different factors, after 

 having made any but the most complete investigation into the subject. 

 I therefore only offer this explanation as a suggestion, without any fur- 

 ther elaboration. 



