THE STATE GEOLOGIST. 



37 



"systems" which are accepted everywhere as standard units of 

 chronology; and the systems are further subdivided into ''periods" 

 and "epochs.'' It is important to observe the distinction between 

 the historical categories expressing time-relations, and the cor- 

 responding division of the solid rocks into systems, series and 

 groups. The stratigraphical column, that is to say, the entire 

 rock series, is divided by means of unconformity and character 

 of the fossils into "systems," as already observed, and these are 

 in turn divided into series, groups and formations. The corre- 

 spondence between this dual historical and stratigraphical class- 

 ification is exhibited bv the following: schedule : 



TIME. 



ROCKS. 



Eras 

 Ages 

 Periods 

 Epochs 



> Systems 



Series 

 Groups 



For convenience of reference we may also be permitted to insert 

 here a table showing the principal subdivisions of the strati- 

 graphical column : 



