220 REPORT UNITED STATES GEOLOGICAL SURVEY. 



facilitated the disruption, which seems rather difficult of explanation. 

 Seismic action, in preference to any other, may be regarded as the cause. 

 It will be noticed that mineralized springs not unfrequently occur in 

 this number of the trachyte. Its comparatively ready decomposition 

 and its usual orographic position explain this feature. The most promi- 

 nent among them are the hot springs of Wagon-Wheel Gap.* 



As occupying a position between trachytes No. 2 and No. 3, we may 

 regard the trachytic breccia or conglomerates. It is evident that the 

 horizontal distribution of this conglomerate must necessarily be, to a 

 certain extent, commensurate with the distribution of the volcanics. In 

 1874 I found but few localities where the conglomerate was exposed. 

 Altogether our explorations were carried on too much within the mount- 

 ains to show us the former shores of the volcanic land. Dr. Pealet 

 found the conglomerates covering an extensive area north of the Gunni- 

 son Kiver. This is the northerly continuation from the main San Juan 

 group of volcanics. He found the conglomerates overlying Cretaceous 

 beds, and covered, in turn, by rhyolite. Inasmuch as the inner limits 

 of the conglomerates must be regarded as the shore-lines of the ancient 

 volcanic land, its distribution becomes a matter of importance. Cer- 

 tainly large quantities of tiie readily-disintegrated material have been 

 removed from their original places of deposition, and, in consequence, 

 the exact limits cannot be accurately drawn. Essentially, as is seen 

 from the occurrence of the conglomerates, the volcanic land had at one 

 time the same general shape that the entire area shows at present. 

 Eastward of the conglomeritic beds found by Dr. Peale, they seem to 

 disappear. No connection was observed between the group north of 

 the Gunnison and the one appearing a short distance north of Del 

 Norte.f Along the eastern edge of the San Juan Mountains the con- 

 glomerates appear in the deep cartons of the Rio Grande drainage. On 

 the Conejos particularly the walls have been carved into fantastical shapes 

 out of the soft material. It will be remembered that the volcanic mount- 

 ain area becomes narrow as we reach a locality as far south as Eio San 

 Antonio. West of the headwaters of this stream the connection of the 

 conglomerates of the two sides is almost accomplished. Along the en- 

 tire western border of the mountains we find the conglomerate again. 

 It occupies an absolutely higher position than farther east, owing to the 

 easterly dip of the volcanic strata. Near the Eio Grande Pyramid it is 

 lost. Again it appears on the drainage of the San Miguel and of the 

 Uncompahgre.§ 



In thickness this conglomerate shows considerable variation when 

 distant points are compared. Near the Eio Grande Pyramid the thick- 

 ness is about 200 feet. As we proceed southward along the western 

 base of the mountains, it increases considerably. At Pagosa Peak we 

 find it to be 1,200 feet, and on the headwaters of Eio Navajo about 1,500. 

 Crossing over to the eastern slope of the range, and examining as we 

 proceed northward, the same conditions are apparent. On the Conejos 

 the thickness of the conglomerates is about 1,200 feet, on the Alamosa 

 700, and north of Del Norte about 300. North of the Gunnison Eiver 

 Dr. Peale has found the thickness of the breccia to be 400 feet. At no 

 other locality in Colorado, except in connection with the main San 

 Juan volcanic group, was this breccia observed. Its existence is owing 



* Eep. U. S. Geol. Snrv., Ib75, p. 154 ; Rep. Expl. and Surv. west lOOth Mer., vol. iii, 

 1875, p. 623. 

 tRep. U. S. Geol. Surv., 1874, p. 170. 

 I Eep. U. S. Geol. Surv.. 1875, p. 1853. 

 $ Eep. U. S. Geol. Surv., 1875, p. 94. 



