228 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



for nearly all vein systems. If we have, so near the place of lode-oc- 

 currences, the agents which can and undoubtedly have produced vio- 

 lent seismic action, we will rarely go amiss in assigning to them the 

 cause of producing such fissures. Not far distant from the lode-bearing 

 locality we have evidence that younger, more recent eruptions must 

 have taken place. I allude to such instances as the Mount Wilson 

 group, the isolated volcanic groups near the headwaters of the Dolores, 

 and to the enormous quantities of basaltic lava that have been ejected 

 northeast and east of the mining districts. Accompanying the erup- 

 tions, the usual phenomena are fully adequate to have produced the 

 effects we now observe. 



In previous pages the impregnation of the red stratum by pyrite has 

 been mentioned. This mineral was probably segregated during the 

 I)eriod of the cooling of the rock. Its presence denotes nothing save 

 the existence and ejection of a large amount of iron and sulphur at the 

 time of eruption. Such conditions, also, must have prevailed, as were 

 favorable to the combination of this metal and metalloid. In case, 

 therefore, if this mineral, or rather its component parts were drawn 

 from the same source that furnished the trachytes, it may justly be pre- 

 sumed, that the resources were not thereby exhausted. Inasmuch as 

 the red stratum is one of the okler members of the mountain group, it 

 may be inferred that its source lay nearer the surface of the earth than 

 that of the succeeding Hows. Assuming that the fissures, either as 

 such, or in ramifications and other forms reach downward to this source, 

 or approximately to it, we may explain the filling of the fissures with 

 metalliferous and other matter. Infiltration, in its widest sense, has no 

 doubt produced the ores as we now find them in veins. Whether such 

 infiltration occurred by the means of mineral substances in solution or 

 in a state of \olatilization cannot always be determined. In this in- 

 stance we have a case where, perhaps, both methods were employed. 

 From the vertical distribution of minerals in the vein much can be 

 learned, and, no doubt, future developments of the San Juan mines will 

 furnish many data of the highest interest and value. 



Special features of trachytic roclcs. — Some very peculiar effects of ero- 

 sion were observed in the trachytic rocks. The fantastic forms generally 

 exhibited by members of the White Earth group and the conglomerates 

 have been mentioned above. A very striking instance of the former 

 was found on a small creek flowing into Henssen's Creek.* On the 

 steep slope of a hill an accumulation of tuffs had taken place. Basaltic 

 bowlders have rolled down upon the hillside from the edge of a plateau 

 and have lodged there. Temporary streams and other aqueous erosion 

 carved out columnar " monuments," each one surmounted by an erratic 

 bowlder of basalt. Their height is 20 to 30 feet. 



On South Eiver, which joins the Rio Grande a short distance below 

 Antelope Park, a unique group of similar monuments was discovered.! 

 They have been carved directly out of the conglomerate which reaches 

 a thickness of about 600 feet at that locality. Some of the large bowld- 

 ers of the conglomerate act as protecting caps and have preserved the 

 slender columns supporting them from destruction. Thousands of these 

 monuments are crowded into a small space, that for its unique beauty 

 surpasses any other spot in Colorado. While we are accustomed to see 

 these monuments — at best — 30 feet high, they here rise to the towering 

 dimensions of 400 feet. Archesf ana caves are found at the same 

 locality, remarkable for their regularity and symmetry of form. 



* Eep. U. S. Geol. Surv., 1874, p. 195. t Rep. U. S. Geol. Surv., 1875, p. 156. 



X Ibid., p. 158. 



