THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 43 



revolution sharply defined, but it is sufficiently so to form a 

 natural boundary-line separating the Triassic- Jurassic 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, except the glacial revolu- 

 tion which is defined further on. The western part of the 

 continent is conspicuous for the late occurrence of its geological 

 construction, which was chiefly after the Triassic ; along the 

 western 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. 



Rocky Mountain Revolution. The Rocky Mountain revolu- 

 tion, which resulted in the elevation and disturbance of all 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 elevating, tilting, and disturbing the 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 terminating the Paleozoic and making into 

 permanent land the great mass of the eastern half of the 

 continent.* 



This Rocky Mountain revolution resembles the Appa- 

 lachian revolution in extending 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 asso* 

 ciated with the gradual emergence of the continental mass 

 above the sea-level. Another feature in which the two revo- 

 lutions resemble each other is the wide extent of the disturb- 

 ances recorded. The elevation of the mountain ranges, from 

 the Pyrenees eastward to the Himalayas and to the islands 



* See further regarding this revolution Dana, "Manual of Geology," 4th 

 ed. t 1895, p. 875, etc., paragraph on " Post-Mesozoic Revolution: Mountain- 

 making and its results," also pp. 932-939. 



