TTOLicH.] ACIDIC VOLCANIC ERUPTIVES TRACHYTE. 223 



serve the hills to bo terraced. Near Saguache this is very well illus- 

 trated.* 



For the appearance of the members belonging to trachyte No. 3 we 

 must look mainly along the northern and eastern border of the mount- 

 ains, in the interior and in their southern extension. Kegarding the 

 San Juan Mountains from the stand-point of a geographer, we must 

 separate them essentially into two divisions ; the Uncompahgre group 

 cud the San Juan Mountains proper. These latter I have regarded, in 

 the Annual Report for 1875, as the southern extension of the Sawatch 

 Kange. Strictly speaking, they may be considered so. The term " San 

 Juan Mountains" is one, however, so well known, and so indiscrimi- 

 nately applied, that it may be well to retain the name, but restrict its 

 application. While the Uncompahgre group is certainly a very typical 

 arrangement of the most rugged mountains, without order or system, 

 the southern continuation of the entire mountain-mass is definable as 

 a range. It is to this that I apply the name of San Juan Range. Tak- 

 ing a comprehensive view of it, we find it to be a narrow, elevated 

 plateau. Streams and other agents have deeply furrowed it, but the 

 main summits retain a uniform elevation. In harmonious relation with 

 the rocks and beds composing this range do its orographic features 

 show themselves to stand. 



While the most rugged group of mountains is composed mainly of 

 trachyte No. 4, the San Juan Range owes its existence to the Blui]f group. 

 A short distance west and northwest we first find the members of No. 

 3 participating in the structures of small plateaus, hills, and bluifs. 

 Reaching to elevation of 13,000 feet above sea-level, we find deposited 

 the highest members of the group. Conformable with the underlying 

 beds, in perfect accordance with the total arrangement of the volcanic 

 flows, they frequently form the summits of some of the high peaks of 

 that region. As we proceed westward, we find the absolute position of 

 the beds of No. 3 to be higher. Partly an amplification of the strata 

 accounts for this, partly the general easterly dip of all the volcanics of 

 that section. The highest jioint is reached on the summit of Uncom- 

 pahgre Peak, 14,235 feet. None of the flows belonging to this group 

 appear to extend northward beyond the Gunnison River. There the 

 conglomerates are covered by rhyolites.t 



By far the greatest development of trachyte No. 3 is found in the San 

 Juan Range. From the Rio Grande Pyramid we can trace the members 

 of this group along the range to the southern border of Colorado. 

 Forming frequently extensive plateaus that are but little lower than the 

 highest peaks of the range, the flows present one unbroken series 

 throughout. The gentle southeasterly dip is always noticeable. It is 

 owing to this that no prominent i)eaks are found on the eastern slope of 

 the range, while the western is amply supplied therewith. Strictly 

 speaking, the western slope is but the abrupt termination of the plateau, 

 while the eastern is essentially its sloping summit. Erosion and abra- 

 sion have necessarily had a degrading influence, so that now the slope 

 is more than commensurate with the dip angle of the volcanic beds. 

 Whether the isolated peak on the summit of the range should be re- 

 garded as evidence of local eruptions or as the remaining portions of 

 once continuous beds, seems difficult to determine. I accept the former 

 view. Although I find scarcely any variation in the lithological char- 

 acter of the rocks, the flows of No. 3 are so uniform and continuous in 

 their character, that I cannot regard them as having been ejected from 



* Rep. U. S. Geol. Surv., 1873, p. 345. tRep. U. S. Geol. Surv., 1874, p. 170. 



