226 REPORT UNITED STATES GEOLOGICAL SURVEY. 



by most European geologists. The term " trachyte " was there by no 

 means so limited as might have been desired, but comprised a number 

 of distinct rock-species, or even genera. Bielz proposed for this rock 

 the name of timazite. Cotta * states that where he saw it it was decom- 

 posed, "frequently bleached almost white, commonly containing pyrite 

 disseminated through it." At Nagyag and Ofi'enbanya the ore-bearing 

 rock is the timazite. This is synonymous with Eichthofen's propylite. 

 Were it not for the fact that we have a direct contact between the older 

 trachytic rocks and the ore-bearing group of the San Juan region, the 

 miueralogical and petrographic similarities of the two would probably 

 lead to its identification as propylite. One of the most favorable local- 

 ities to study the relations of the Mountain group to those underlying, is 

 found near the headwaters of the Rio Grande. Distinct stratification 

 of the lower members of No. 4 is noticeable. There they rest directly 

 and conformably upon the upper strata of No. 3. This feature may 

 distinctly be traced all along the line of junction. At the time the tra- 

 chytes of No. 4 were ejected the region where now the miues are located 

 was already considerably corrugated. A ridge extended northward 

 from the Quartzite Mountains, which barred the progress of No. 3. The 

 succeeding flows, however, utilized the equalization oi niveau, and, pass- 

 ing over this ridge, penetrated beyond it. Evidence of this ancient 

 ridge may be found in Arrastra and Cunningham Gulches and on the ele- 

 vated plateau east of them. This fact accounts for the absence of the 

 three lower trachytic groups along that entire border of the volcanic 

 area containing the headwaters of the Animas and San Miguel Elvers. 

 Although, consequently, the ore-bearing rocks of the San Juan region 

 appear as almost an isolated group, they are in reality in direct and har- 

 monious connection with the older groups of the trachytic division. In 

 the direction of Mount Sneffels and Uucompahgre Peak this connection 

 can more readily be found, and will be clearly understood. 



At the time of my visit the region had not even been thoroughly 

 prospected. Subsequent discoveries made since those in and near 

 Baker's Park, mostly occur within the limits assigned to the Mountain 

 group. As significant I regard the presence of the " red stratum." I 

 do not mean to insinuate that such localities that do not exhibit it will be 

 found barren of ore, but I am of the opinion that the bountiful mineral 

 impregnation with which this group is supplied, indicates the former ex- 

 istence of such causes as led to the formation of minerals that are 

 classed as " ores." 



Wherever any definite structure can be observed in the rocks of No. 

 4, it will be found to correspond with that of underlying volcanic rocks. 

 As it is the last one of the series showing such character, I have in- 

 cluded it under trachyte, and have closed the trachytic division with it. 



No members of the Mountain group were found in the San Juan 

 Eange. Their southern border is immediately south of Sultan Mountain, 

 where they unconformably overlie Carboniferous sedimentary beds.t 

 From there it swings around to the headwaters of the San Miguel and 

 follows the edge of the mountains northward beyond Mount Sneffels. 

 Between Mount Sneffels and Uncompahgre Peak the limits of the Mount- 

 ain group turn eastward. They include within their area the Station 

 12 group, and, alter having crossed the Eio .Grande, disappear a 

 short distance south and east of Silverton. At this latter locality they 

 become thin, overlying metamorphic, Devonian, and, in part. Carbonif- 

 erous rocks. In Cunningham Gulch the metamorphics, chloritic schists, 



* Ore-deposits, p. 5i77 tEep. U. S. Geol. Surv., 1874, p. 217, section II. 



