ELEMENTS OF THE GEOLOGICAL TIME- SCALE. 289 



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 leugth of geological 

 time are based upon the rate of erosion or gorge - cutting by riv- 

 .ers, 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 posi- 

 tion, as beautifully elaborated in Winchell's Report on the Geol- 

 ogy of Minnesota, vol. 2. 



The above revolutions are selected, not as the only revolu- 

 tions 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 local- 

 ities 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 Moun- 

 tain revolutions constitute the two grander revolutions. The 

 first closed the Palaeozoic 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 dis- 

 tributed 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- 



