B.N-DUCH.] CRETACEOUS. 221 



by contraction would most likely have produced disturbances, either on 

 one side or the other of the river. No such disturbances were observed, 

 however. The beds show no folding or dislocation from north to south. 

 Throughout the region the gentle dip southward is retained. A num- 

 ber of facts spealj for a separation by directly-acting forces. On both 

 sides of the river the forojations are identical, and could the valley be 

 reduced to a line, the two rows of bluffs on the opposite sides would be 

 found to correspond very closely. On both sides the faces of the bluffs 

 are nearly vertical, and there seems to be sufficient evidence that they 

 have been so for a very long time. The nearly straight course pursued 

 by the river since its entrance into the sedimentary country points to 

 the fact that the formation of its first bed was probably due to some 

 agent exercising a very considerable amount of force. Only through 

 the older sedimentary strata has this force made itself perceptible, as 

 the river, after leaving them, is obliged to cut its own way through the 

 younger Middle Cretaceous beds. 



About four miles north of the conflux of Junction Creek with the Animas, 

 the Lower Cretaceous strata overlie the red sandstone unconformably, 

 which latterextends westward totheLa Plata group. There it iscovered 

 by the trachytic flows,which have their origin near the higher portions of 

 the group, and some of which extend over into the Cretaceous area. While 

 the volcanic rocks are stratified nearly horizontally, the sandstone still 

 retains its southerly dip, thus producing an uuconformability similar to 

 the one east of station 31. The western limits of this formation were 

 not reached during our work in 1874, but will probably not be found to 

 extend much farther than the slope of the higher ridges. 



A few isolated patches of the red sandstone occur in the northwest- 

 ern portion of the district, showing its considerable horizontal distribu- 

 tion. At the upper end of the creek, a tributary of the San Miguel, and 

 nortb of which station 32 was located, it crops out covered by trachyte, 

 which forms the higher portions of the ridge running from station 30 

 towards Mount Snefiels. I did not succeed in establishing to my own 

 satisfaction the relation it bore to the Cretaceous beds of that neighbor- 

 hood, but from a'll examinations that our limited allowance of time 

 enabled us to make, it would appear that it had been deposited and 

 placed into its present position before the Cretaceous waters reached 

 the locality. Another outcrop of the sandstone occurs in the Uncom- 

 pahgrecanon, about five miles westof station 10. Thecauon is considered 

 inaccessible, owing to the precipitous character of its walls, the lower 

 portion of which belongs to the formation above named. Trachyte 

 covers it here as well as near station 32. With this the occurrence of 

 Carboniferous beds in our district is exhausted. They are very uniform 

 in cljaracter, and can readily be recognized, and although covering quite 

 a considerable area, show but slight variation in vertical development. 



CRETACEOUS. 



. This formation covers a considerable area in the district. Joining on 

 to the southern boundary of the preceding one, it extends southward 

 beyond tire limits of our district, forming the characteristic low bluffs 

 of that region. Some of the highest points, where the Cretaceous was 

 found, show an elevation of 10,500 feet, while it reaches down below 

 6,000. As everywhere in Colorado, the single groups are well defined 

 and characteristic, both regarding their lithological and orogrtiphic 

 features. The southern portion, in the vicinity of the Animas, resembles 

 more closely in the latter respect the parallel groups of other localities, 



