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ELEMENTS OF THE GEOLOGICAL TIME-SCALE.
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particular importance in life-history, as it introduced the recent period, or the age of man. This is the combination of events marking the glacial epoch. In general, it consisted geologically of oscillations of the northern lands for the northern hemisphere, and was associated with the accumulation of ice upon the surface and its continuance as a great ice-sheet for a long period of time. Some of the more accurate estimates of the length of geological time are based upon the rate of erosion or gorge-cutting by rivers, and the period so measured dates back to the last uncovering of the river channels coincident with the northward withdrawal of the ice-sheet. Standard examples are the estimates of the time required to cut the Niagara River gorge, and the retreat of the falls of St. Anthony from Fort Snelling to their present position, as beautifully elaborated in Winchell's Report on the Geology of Minnesota, vol. 2.

The above revolutions are selected, not as the only revolutions interrupting the regular course of sedimentary formation of stratified rocks, but as chief examples of such interruptions in the North American scale. All along the course of geological time there are evidences to show that there were constant oscillations of the relations between land and ocean-level, and at some localities these oscillations were passing across the datum plane of the ocean surface. Wherever this happened, on one side rocks were forming, and on the other erosion and degradation obliterating them as time-records. The Appalachian and the Rocky Mountain revolutions constitute the two grander revolutions. The first closed the Palæozoic life Period, the fossils being chiefly marine until the Devonian, and being associated with marine forms up to the close of the Carboniferous. The deposits are distributed across the continent, with local interruptions. After the Appalachian revolution the eastern half of the continent, except its Atlantic and Gulf borders, became permanently above the sea-level.

The period between the Appalachian and Rocky Mountain revolutions is the period of the Mesozoic life. In the faunas and floras of this period, land and fresh-water species take a promi-

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