ELEMENTS OF THE GEOLOGICAL TIME-SCALE. 287 



ing of the underlying red sandstone and shale, and the resulting 

 unconformity with the succeeding formations. The evidences 

 of the revolution are not widely extended nor is the time rela- 

 tion of the termination of the revolution sharply defined, but it 

 is sufficiently so to form a natural boundary line separating the 

 Jura -Trias from the Cretaceous. After this point of time there 

 occurred nothing in the eastern half of the continent which 

 deserves the name or rank of a geological revolution. The 

 western part of the continent is conspicuous for its grand 

 geological construction after the Triassic at least ; along the 

 coast the Sierra Nevada revolution marked the same general 

 interval of time recorded by the Palisade revolution of the east. 

 These events on the opposite borders of the continent are alike 

 at least in preceding the Cretaceous and in terminating the 

 formations which are of Jura -Triassic age. 



The Rocky Mountain revolution, which resulted in the 

 elevation and disturbance of the rocks in the region of the 

 Rocky Mountains, and extended from them to the border 

 ranges, is distributed along the time from the close of the 

 Cretaceous to the Miocene, or possibly later. It -is altogether 

 probable that the actual length of time, taken in the eleva- 

 tion, tilting and disturbance of strata after the last marine 

 deposits of the pre -Laramie formations, which resulted in the 

 permanent adding to the continent of its western third, was 

 not longer than that consumed in the various events termin- 

 ating the Palaeozoic, and making into permanent land the great 

 mass of the eastern half of the continent. This Rocky Moun- 

 tain revolution resembles the Appalachian revolution, in extend- 

 ing over and affecting a large area of the continent, in its general 

 upward -lifting of that area, which process extended over a long 

 period of time, and in the great accumulation of coal or lignite 

 which was associated with the gradual emergence of the conti- 

 nental mass above the sea -level. 



Another feature in which the two revolutions resemble each 

 other is found in the wide extent of the disturbances recorded. 

 The elevation of the mountain ranges from the Pyrenees east- 



