224 REPORT UNITED STATES GEOLOGICAL SURVEY. 



but very few places. Had large masses of lava-, such as we now find 

 in the range, issued from the numerous prominent peaks, we would 

 scarcely observe the same uniformity that we now find. At the same 

 time, the peaks in question do not at all exhibit the petrographic char- 

 acteristics that we should expect them to show did they belong to a 

 younger group of volcanics ; i. e., were they remnants of flows subse- 

 quent to those of No. 3. 



At some of the highest localities of the range we meet with the hard 

 variegated beds belonging to, and sometimes forming the top of, the 

 Bluff group. On Banded Peak,* and farther south, they were noticed. 

 Here their thickness reaches about 800 feet. I am inclined to think that 

 a number of the upper beds of No. 3 occur in very much lighter colors, 

 and have therefore been added in the variegated beds. Porphyritic 

 pitch-stones and obsidians occur here also. A large bed of pitch-stone, 

 varying from 4 to 15 feet in thickness, was found near the Kio Grande 

 Pyramid in these strata and traced for the distance of about six miles. 



North and west of the Quartzite Mountains No. 3 is gradually super- 

 seded by No. 4. It again appears, however, on the western and north- 

 western border of the volcanic area. Underlying the younger beds, it is 

 mostly found at the base, hiding from sight the two older groups. In 

 several instances cafions are cut down sulficiently deep to expose them, 

 however. All of these older groups appear to thin out in ihat region, 

 while the younger ones reach a very considerable vertical development. 



Displacements of strata are comparatively frequent in this group. 

 Two of the most prominent are those at Mount Sneffels and at Bristol 

 Head. The former we have named, from its form, the Great Amphithe- 

 atre.t For the distance of 2,000 feet the strata have dropped down 

 vertically, causing an oval depression about four miles in length and 

 nearly a mile wide. On nearly all sides the walls are perfectly perpen- 

 dicular. It seems probable that this "drop" and one near the Eio 

 Grande Pyramid were caused by enormous caves. To all appearance 

 they have occurred but recently, comparatively speaking. At the Great 

 Amphitheater the strata, removed a distance of nearly half a mile from 

 their original position, have retained a certain continuity to the present 

 day. Although cracked and broken into' innumerable fragments, the 

 pieces, often weighing hundreds of tons, lie side by side, separated, not 

 rarely, by only a narrow fissure. At Bristol Headf the cause of the 

 displacement is more apparent. A small ridge, north of Antelope Park, 

 runs parallel with the edge of the Bristol Head Plateau. Upon exami- 

 nation, it was found that the Eio Grande had probably undermined this 

 portion by washing away a deposit of trachyte No. 1. Thus the weight 

 could no longer be sustained, and the unsupported portion dropped 

 down a vertical distance of more than 1,000 feet. The "drop" is not 

 purely vertical ; it was accompanied by a movement toward the south, 

 away from the plateau with which the detached portion was originally 

 in connection. A very pretty little valley, containing Lake Santa Maria, 

 has thus been formed between the two steep, enclosing walls. Local 

 "drops" of small dimensions occur quite frequently in the strata of the 

 Bluff group. In its physical character this group belongs, as well as 

 the preceding, to the massive volcanic eruptions. 



Irachyte JSfo. 4.§ — Only during the summer of 1874 did I have occa- 



* Rep. U. S. Geol. Surv., 1875, page 167. 

 • tRep. U. S. Geol. Surv., 1874, page 206. 



t Rep. U. S. Geol. Surv., 1874, page 199. 



§ Rep. U. B. Geol. Surv., 1874, p. 196 ; Bulletin U. S. Geol. Surv. No. 3, second series, 

 May 15, 1875, p. 152. 



