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ELEMENTS OF THE GEOLOGICAL TIME-SCALE.
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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 relation 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 elevation, 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 terminating the Palæozoic, and making into permanent land the great mass of the eastern half of the continent. This Rocky Mountain revolution resembles the Appalachian 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 associated with the gradual emergence of the continental 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-

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