220 



plains, have been represented by Mr. Holmes with wonderful accuracy 

 and fidelity, and in among the high peaks are the usual gorges or cafions, 

 and all the proofs of terrific erosion in past times. 



An important fact, however, is illustrated by the profile, which, so far 

 as I know, was first observed by me in 1869 and published in the annual 

 report of that year — that is, the dying out of the mountain-ridges in 

 the plains. The great ranges are usually composite in their character, 

 that is, are made up of a number of smaller ranges. These minor divis- 

 ions may be continuous for long distances or may break off suddenly, 

 and form some other ridge. The general trend of the aggregate mass of 

 the eastern Eocky Mountain group is slightly west of north, and in 

 many cases, and perhaps in the majority of instances, the minor ranges 

 have an axial trend about northwest and southeast. The consequence 

 is, that all along the eastern side of the great range, the smaller ranges 

 tend to die out in the plains. So that, to one traveling along the eastern 

 base, the ends of these smaller ranges or ridges present a sort of "ew 

 eclielon''^ appearance. This feature is much more clearly exposed on the 

 Big Thompson Cre ek about forty miles north of Denver. Section 1, 

 opposite page 20 in the annual report of the survey for 1873, illus- 

 trates the manner in which these ridges die out in the plains more 

 clearly. Cheyenne Mountain, at h, is an example of a short mountain- 

 range ending abruptly. There is still another point of interest which 

 I believe to be capable of demonstration, though I have not accumulated 

 a sufficient number of observations to express it with confidence. It 

 seems to me that these great composite ranges of mountains have grad- 

 ually grown to their present size by additions on either side ; that, for in- 

 stance, the single ridge appeared first, as a nucleus, and that, at different 

 periods of time, minor ridges were elevated on either side of the main 

 ridge. This seems to be quite evident, from the different mineral char- 

 acters which compose the Pike's Peak group. This seems probable, also, 

 from the fact that at one point the red beds, which are at least as old as the 

 Jurassic, appear to have been deposited on the granites after they had 

 been elevated nearly to their present position, while in many other in- 

 stances the ridges have been lifted up after the deposition of the Lignitic 

 strata. On the south side of the Fountain Creek, Mr. Holmes made 

 some very interesting sketches and studies, which tended to show that 

 these red beds were made up of materials taken from the granites in 

 the immediate vicinity ; that the red beds in immediate contact with the 

 granites are made of rather large rounded granitic masses, cemented 

 with sand, that the coarse conglomerate soon became a fine pudding- 

 stone, and fine sandstones, as we gradually move eastward, from the base 

 of the granitic ridge. It is possible tliat the great Eocky Mountain 

 range was outlined in form, far back in the past, perhaps, even during 

 the Carboniferous period, though it received vast additions during the 

 Cretaceous and Tertiary epochs. 



