Feb. 15, 1877] 



NATURE 



339 



show that the elevation of these ridges did not take place 

 until after the deposition of the older Tertiary rocks, and 



that this elevation was not merely due to the protrusion of 

 hypogene masses, but was part of a general and prolonged 



Fig. 3. — Head of Labyrinth Creek, looking South-easterly. 



Fig. 4.— Head of Cafion, Colorado. Erosion of Triassic Scries.! 



movement of plication by which the present axis of the 

 North American continent was determined. The whole of 



the sedimentary formations ot the plain are found bent 

 up against the granitic nucleus, which, like a wed^e, has 



