THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 45 



was incident to the vast earth disturbance which raised, to the 

 amount of at least five thousand feet, a large part of the west- 

 ern half of the continent. The long line of volcanoes along 

 the western coast of the two Americas had their origin in the 

 same general period of time. 



Glacial Revolution. — There was, still later, a revolution 

 which has left little record in the way of disturbance or dis- 

 cordance of strata, but was of particular importance in life- 

 history, as it introduced the recent period or the age of man. 

 It constituted 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. 



Erosion of River Caiions as Gauges of Time Duration. — Some 

 of the more definite estimates of the length of geological 

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

 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 of such esti- 

 mates of the length of geological time are those made re- 

 garding the cutting of the Niagara River gorge, the retreat of 

 the falls of St. Anthony from Fort Snelling to their present 

 position, and the cutting of the canons of the Yellowstone 

 and Colorado rivers. But the unsatisfactory nature of these 

 estimates is shown by the fact that different authors reach 

 such divergent results from the same data. The time taken 

 in the cutting of Niagara gorge is estimated by Upham to 

 be 6000 to 10,000 years, by Spencer (1894) to be at least 

 32,000 years. 



Continental Value of Revolutions as Time-Breaks in the History 

 of North America. — The above revolutions are selected, not as 

 the only revolutions interrupting the regular course of sedi- 

 mentary formation of stratified rocks, but as chief examples of 

 such interruptions in the North American column of deposits. 

 All along the course of geological time there are evidences to 

 show that there were constant oscillations of the relations be- 

 tween land- and ocean-level, and at some localities these oscilla- 

 tions were passing across the datum-plane of the ocean surface. 



