CHAPTER III. 



THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE AND 

 THEIR TIME-VALUES. 



The Systems and Geological Revolutions. — • The systems, 

 although they are arbitrarily limited and classified, rep' 

 resent certain grand events in the history of the earth. 

 Without explaining how the series of stratified rocks came 

 to be divided into these particular ten systems, it may be said 

 that their retention as the great units of geological classifica- 

 tion and nomenclature is mainly due to the relatively sharp 

 boundaries which each system exhibits in its typical locality. 

 The systems thus serve as known and definite standards of com- 

 parison in the construction of the time-scale, as the dominance 

 of nations, or the dominance of dynasties, in each case serves 

 as a time-standard for the discussion of ancient human history. 

 As the period of each dynasty in ancient history is marked by 

 continuity in the successive steps of progress of the country, 

 of the acts of the people and of the forms of government, 

 and the change of dynasties is marked by a breaking of that 

 continuity, by revolutions and readjustment of affairs, so in 

 geological history the grand systems represent periods of con- 

 tinuity of deposition for the regions in which they were 

 formed, separated from one another by grand revolutions 

 which interrupted the regularity of deposition, and disturbed, 

 by folding, faulting, and sometimes by metamorphosing them, 

 the older strata upon which the succeeding strata rest uncon- 

 formably and constitute the beginnings of a new system. 



Geological Revolutions Local, Not Universal. — Geological rev- 

 olutions were not universal for the whole earth ; from which 

 it results that these typical systems and their classification 

 are not equally applicable to the geological formations of all 



39 



