398 
sion. 
ELEMENTARY GEOLOGY 
Still, in some cases, after a series of rocks was 
formed, with the enclosed fossils, these strata were 
I U 
Pleistocene 
Neocene 
3 
Eocene 
Cretaceous 
= Juratrias 
U2 
Carboniferous 
Devonian 
Silurian 
Ordovician 
Fic. 233. 
Cambrian 
ut 
Archean 
Diagram to illustrate superpo- 
sition of rocks of 
ages. 
(uw) also shown. 
the various 
Three unconformities 
raised above the sea, and after 
remaining as land for awhile, 
were again submerged, so that 
on these were deposited other 
strata with an unconformity 
between. During the gap, or 
the period represented by the 
unconformity, time enough had 
elapsed for a change in the 
conditions of life to occur. 
Therefore, the fossils of each 
set of strata are quite distinc- 
tive of their periods of forma- 
tion; and being carefully stud- 
ied, furnish a means of telling 
the age of rocks of the same 
To each of 
these groups of strata, names 
period elsewhere. 
are given, such for instance 
as the Devonian and Silurian ; 
and in other parts of the world 
all rocks with similar groups of fossils are recognized 
as belonging to the same periods. 
By careful studies and comparisons, made in different 
countries, a time-scale has been built up, and the his- 
