EVIDENCES— GEOLOGY 121 



to us as to our ancestors of Revolutionary times, yet they were 

 once as great a range as the Rocky Mountains. 



The hill on which Denison University stands is one hundred 

 feet above the nearest stream and a thousand above the sea, l)ut 

 at its top there may be found the fossil remains of Brachiopods 

 and Crinoids, animals which once lived in the ocean. The greatest 

 cataclysms of modern times have produced no such movements of 

 material, yet in the terrific forces of a great earthquake we may 

 find some slight conception of the vastness of geological processes 

 in the past. 



The evidence is before us that these things have occurred. We 

 may see in a mountain range the twisted, folded remnants of 

 rocks which could only have been produced by sedimentation. 

 We may walk in deep grooves ground by the glaciers in solid rock 

 and select from undisturbed masses above them the remains of 

 corals which once lived in the ocean. And with these facts we 

 may consider the visible modifications of the earth's surface to- 

 day, infinitesimal but relentless. 



Time Divisions. On the basis of all changes including climate, 

 organic development, and modifications of the earth's crust, the 

 earth's history has been divided by geologists into eras, these into 

 periods, and these finally into epochs. The first are named from 

 the prevailing types of life, the second largely from regions where 

 their characteristic deposits are found, and the last with various 

 descriptive terms. Each division is determined by characteristic 

 sedimentar}^ rocks and the fossils included in them, and by the 

 vertical relationship of these strata the relative chronology of the 

 various periods has been determined with reasonable accuracy. 

 It is obviously impossible to estimate such periods in terms of 

 years with anything approaching exactness. Estimates of the 

 age of the earth run up to 1,000,000,000 and more years, within 

 which a few thousands are a slight margin of error. All of these 

 facts are briefly expressed in the following geological table : 



