Handbook of Paleontology 61 



also a rock scale, and the International Geological 

 Congress has adopted the following divisions of the 

 two scales : 



Time Scale Rock Scale 



Era Group 



Period System 



Epoch Series 



Age Stage 



Substage 

 Zone 



In the rock scale the divisions are carried farther be- 

 cause of the generally local character of the minor di- 

 visions. To illustrate these two scales we have the 

 Paleozoic Era of time in which the Paleozoic Group 

 of rocks was laid down ; the Devonian System of rocks 

 was deposited in the Devonian Period; the Erian Se- 

 ries ( Hamilton- Marcellus beds) in the Erian Epoch; and 

 the Hamilton Stage comprises the rocks of Hamilton Age. 

 The geologic age of rocks is much harder to determine 

 than their relative age. Fossils must be identified, and 

 when no fossils are present the ages of beds can be 

 determined only by correlation with other beds of 

 which the geologic age has been established. There 

 is no region where the strata of even a majority of 

 the rock systems of the earth are represented. Data 

 are gathered from various places and are finally put 

 together in their true order to make a complete and 

 accurate record. Unconformities occur which may 

 represent a loss of one or more systems of rocks, but 

 the beds in two different sections may complement 

 each other. What is missing in one section may be 

 present in the second and a complete section thus 

 gained. Beds are overturned or faulted and older beds 

 rest upon younger ones ; a stratum of rock may, 



