R^.PORT ON THE MINES AND GEOLOGY OF THE SAN JUAN COUNTRY. 



By F. M. Endlich, S. N. D. 



EegardiDg the geographical location and extent of tbe San Juan 

 mining region, so called, nothing further need be said, as the subject 

 has been sufficiently treated by the topographer in other i)ortions of 

 this bulletin. 



For more than four thousand five hundred square miles the country 

 is covered, in one continuous area, by volcanic rocks, beginning immedi- 

 ately west of the San Luis Valley, and extending in that direction as 

 far as the high masses of mountains and plateaus. Their southern 

 boundary may be characterized as following the line where the high 

 and rugged peaks change for the lower densely-wooded ridges. The 

 Gunnison may be said to indicate api^roximately the northern limit. 

 One exception to the southern line must be taken, however, caused by 

 that group of mountains which we have been accustomed to designate 

 as the Quartzites. It is a densely-packed mass of very rugged peaks, 

 with numerous, almost inaccessible canons cut through it, and contains 

 the headwaters of the Eio Pinos, and a number of swift mountain- 

 streams, that thus far have not been distinguished by special names. 

 Advancing from the east and north toward the quartzites it will be 

 observed that gradually the volcanic strata begin to thin out, revealing 

 the rock they overlie. This will be spoken of in more detail at the 

 proper place. 



The character of the volcanic rocks throughout the district is one 

 of extreme interest, demonstrating an enormous amount of activity 

 during a probably short i)eriod of time, (geologically speaking,) 

 which activity was nevertheless accompanied by a comparatively large 

 number of changes in, the chemical and physical qualities of the ejected 

 material. Local phenomena, for instance — the sudden occurrence of a cer- 

 tain class of rocks intimately associated with others entirely different — 

 are frequent, and their explanation not always to be rendered satisfac- 

 torily. Traveling for many miles over this continuous volcanic area, I 

 gradually became familiar with the different appearances and physical 

 changes in masses that, lithologically, I could not separate. Thanks to 

 the regularity that was observed in the succession of the different 

 flows, thanks furthermore to the barren nature of the highly-elevated 

 portions of the country, the geologist can treat these rocks almost like 

 well-defined sedimentary formations. Basing upon this, I prepared in 

 the field a schedule that roughly divides the various characteristic 

 groups or strata and found it to answer very well. Due changes were 

 made upon future observations and corrections applied when needed, 

 so that now it will give a good idea of the successions of strata, 

 leaving of course a considerable amount of room for the investigating 

 lithologist. 



No. 1. Bight hundred feet of white, yellow, green, orange, red, brown, 

 and gray trachytes, decomposing readily, frequently weathering in pic- 



