154 SPAEKS FROM A GEOLOGIST'S HAMMER. 



save in extraordinary cases, demands a century to render its 

 results perceptible. Even if we only claim that portion of 

 the gorge below the whirlpool as the record of post-glacial 

 work, and then reflect upon the almost stationary position 

 of the falls since first observed by civilized man, we re- 

 ceive a profound impression of the length of the passing 

 geological period. Much of the peninsula of Florida, within 

 times geologically modern, has been undergrown by a coral 

 reef and added to the domain of the land. The delta of 

 the Mississippi has taken the place of a broad estuary which 

 penetrated deep into the heart of the land. There are 

 those who would have us believe that even the monuments 

 of human activity date back a thousand centuries, while 

 the decline of the continental glacier, the extinction of the 

 last fauna, the wastage of the pre-glacial surface of North 

 America, these are events which stretch aeons upon aeons 

 into the remoter past. 



Now let us gaze the ages steadily in the face. Let us 

 see if it be impossible to take in the compass of a geological 

 period. Let us seek for a unit of measure with which we 

 may gauge the cycles of terrestrial evolutions. Let us 

 grope for a parallactic base-line of known dimensions, from 

 which we may take the bearings of events gleaming down 

 upon us from primeval time. 



Not all great geologic events date back to a high antiq- 

 uity. Here has been the first error in our premises. Man 

 did not come upon a world in which history had closed. 

 He came in the midst of the progress of events. Man him- 

 self was one in the series of events. Great vicissitudes pre- 

 ceded his coming; great vicissitudes have even followed 

 his coming. We have thought that when man appeared, 

 the work of geologic agencies had been completed, and 



