ENDLicH.] ACIDIC VOLCANIC EEUPTIVES TRACHYTE. 227 



crop out, and in Arrastra Gulch they have been reached by the Little 

 Giant Mine. Within the Station 12 group we find a very excellent de- 

 velopment of the red stratum. Again it is observed at Mount Canby, 

 near the junction of Pole Creek with the Eio Grande. On Bear Creek, 

 west of Baker's Park and along Mineral Creek we find it well represented. 

 Near Mount Sneffels the evident stratification of the volcanic rocks can 

 be most satisfactorily observed, and a connection can from there be es- 

 tablished of trachyte No. 3 eastward to the Rio Grande. At Handle's 

 Peak and other localities we noticed a large number of very regular quartz 

 veins. Since our visit many of them have been "located," and some are 

 worked. 



In Bulletin No. 3, second series, and in United States Geological Re- 

 port for 1874, a discussion of the San Juan mines may be found. It is 

 needless, therefore, to repeat any information there contained. Nearly 

 all of the lodes located near Baker's Park were found in trachyte No. 4. 

 Subsequent investigation has shown that they penetrate beyond the 

 lower limits of this group, and, without any appreciable change of 

 course or character, enter the metamorphics, which are covered by the 

 trachyte. All the veins that I had occasion to visit, located within the 

 trachyte, were argentiferous. The only one which was at that time worked 

 in the underlying schists, the Little Giant, is auriferous. I am not 

 aware whether this feature has since been established as the rule. 

 Mining is not unfrequently carried on in a very desultory manner in our 

 western country, and it is very difficult to obtain any reliable data, ex- 

 cept by personal inspection. A surprising regularity of the metalliferous 

 veins may be observed. I know of no place where the veins appear so 

 undisguised on the surface. A photograph taken at Howards ville has 

 been reproduced.* For a vertical distance of about 1,400 feet we can 

 trace the veins of this hill-side. 



As to the cause which produced the formation of the fissures which 

 we now find filled with metalliferous matter, several suggestions may 

 be ottered. 



First. They may have been produced by the contraction of the vol- 

 canic masses upon cooling. 



Second. Subsidences of certain portions may have given rise to the 

 formation of fissure-systems. 



Third. They may owe their existence to plutonic or volcanic seismic 

 action. 



We notice that nearly all the veins have a strike lying between north . 

 450 west, and north 45° east. This indicates a common cause for their 

 formation. Generally the fissures will be found nearly at right angles 

 with the beds or flows of trachyte. 



The fact that the veins enter the underlying metamorphic rocks, and 

 do so without changing their character, excludes the first proposition. 

 No doubt the irresistible force of contraction would probably have suc- 

 ceeded in carrying the fissure downward into the harder, metamorphic 

 rocks. In case this had occurred, however, the fissures would have 

 changed their character. Slides would either have occurred, or the veins 

 must grow narrower with increasing depth. Neither of these instances 

 was observed. 



If subsidences had occurred after the established rigidity of the vol- 

 canic rocks, it would be shown in their stratification. Declinations of 

 some extent, or faulting, must have taken place. 



In preference, I accept the third explanation, not only for these but 



* Eep. U. S. Geol. Surv., 1874, p. 232. 



