THE USES OF FOSSILS 401 
using stone implements (and some still use them); they were 
then in the stone age, or stage, while in Europe this stage was 
passed through many centuries since. 
To the divisions of the strata thus made, various 
names are given for the purpose of simplifying the 
study. These divisions are first imto Groups, which 
are again divided into Systems, and these into Series, 
which themselves are divided into Stages; or using 
the idea of time as a basis, the corresponding divisions 
are known respectively as Eras, Periods, Epochs, and 
Ages. 
The greater divisions into Groups or Eras, and the subdi- 
visions into Systems and Periods, are so large that they may be 
recognized in widely separated countries; but the smaller divi- 
sions, into Series or Epochs, are so local that they can be distin- 
guished only within limited areas. So the Series or Epochs of ° 
New York are given certain names, those of California others, 
those of India others still, and in each case the names applied are 
derived from some local feature of geography, usually some place 
where the rocks of that particular epoch are well developed. 
The names of the larger divisions are the same all over the 
world, and are everywhere recognized by the same general kinds 
of fossil organisms. 
Upon this basis the following geological time-scale 
is constructed, the oldest groups being placed at the 
bottom. For the minor subdivisions, names com- 
monly in use in the eastern part of _ country are 
introduced. 
2D 
