
ROCKY MOUNTAINS—GENERAL CONCLUSIONS. ig 
The recurrence of magnesian rocks at so many different horizons 
is also noteworthy. | 
176. The sudden nature of the break between the nearly horizontal 
strata of the plains, and the crumpled rocks of the mountains, is a 
remarkable feature on the forty-ninth parallel. A similar peculiarity of 
structure has, however, been observed in other and distant parts of the 
range. Near the head waters of the Powder River, the Cretaceous and 
Tertiary beds, beyond ten to fifteen miles from the mountains, are not 
flexed.* ‘ 

* Geological Report, Yellowstone and Missouri Expedition, p. 66. Compare also Mr. Marvin’s Sec- 
tions. U.S. Geol. Surv. Territ., 1873. 


