856 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



fortified on every side by vertical walls. Below all this was the bright 

 green valley with its meandering river, which reflected the rosy hue of 

 an evening sky." 



This "ruined city" is built upon a rapidly rising slope, in terraces, 

 resembling somewhat in its general plan Oriental arrangement. Dark 

 shadows are cast into the narrow streets, and curious detail-erosion has 

 peopled the city with fantastic beings. Altogether it produces the im- 

 pression of a weird spot, resembling the former abode of living creatures, 

 but now desolate, haunted scarcely even by a shy, cringing wolf. Upon 

 closer examination, however, much of its mythical character is dispelled. 

 Too plainly are recognized the forces that have been at work to accom- 

 plish the result we observe. What has been said about the composi- 

 tion and formation of mural forms will here apply. On a grander scale 

 the agents employed have been able to perforin their duties, and have 

 built for themselves, in this ruined city, a monument most instructive 

 and imposing. 



Forms resembling castles, towers, and spires can readily be found 

 within this sandstone area, due to the same causes operating with the 

 same effects. 



LA PIEDRA PARADA. 



Near the junction of Rio Piedra and Eio Nutria, at about west lon- 

 gitude 107° 18' and north latitude 37° 17', is located a famous landmark, 

 La Piedra Parada. On the summit of a narrow ridge stands an isolated 

 mass of rock. It is only with difficulty that the top of it can be reached. 

 Eising nearly vertically on all sides, this remnant of formerly exten- 

 sive strata attains a height of about 400 feet from its base. It is over 

 600 feet long, and about 120 feet wide.* Alternating beds of shale and 

 sandstone compose it, and heavy strata of yellow sandstone form the 

 top. During the progress of maximum erosion in that region, enormous 

 masses of material were swept away, but this huge block remained. 

 Subsequent weathering and disintegration have ornamented it with 

 small towers and turrets, so that to-day it resembles some ancient, dis- 

 mantled castle. Constantly fragments, loosened by frost, are falling 

 down. Joint-fissures, very pronounced, facilitate the wedging action of 

 frost and growing vegetation, so that, in the course of time, this prom- 

 inent feature will no longer remain a i)ortion of the scenery. 



B. — Second Group. 



GUNNISON EIYEK. 



North of the Gunnison, in the regions es:amined by Dr. Peale during 

 1874, are large outcrops of trachytic " breccia". This material has been 

 eroded into innumerable forms representing spires, columns, turrets, and 

 castle-shaped masses. Its composition, here as well as elsewhere, fits it 



* Compare Report Exploring Expedition, J. N. Macomb, 1859, 1876, p. 78. 



