of Life and Tim< 



the International Geological Congress. And 

 as these assemblages of rocks necessarily rep- 

 resent the time that elapsed while they were 

 being formed, the various divisions just named 

 are made to correspond to divisions of time 

 known, counting from the smallest to the 

 largest, as Age, Epoch, Period, and Era. This 

 may be shown as follows : 



Divisions of rocks. Divisions of time. 



Group Paleozoic Era. 



System Upper Silurian Period. 



Series Niagara Epoch. 



Stage or Formation . . . Medina Age. 



For example, the Medina sandstone, well known 

 as a building stone in western New York, is 

 the Medina stage of the Niagara series of 

 the Upper Silurian system of the Paleozoic 

 group ; and while all this may seem very tech- 

 nical and uninteresting, it is merely an aid to 

 the proper locating of specimens, and it is just 

 as necessary to exactly locate animals as it is to 

 locate cities. For example : To say that Roch- 

 ester is in the United States would mean very 

 little, but to say that Rochester is in Monroe 

 County, New York, would show just where it 



55 



