396 ELEMENTARY GEOLOGY 
the oldest fossils; the youngest would be found in 
the highest. 
While there is no such coherent record as this, there 
are many places where thousands of feet of rocks have 
been accumulated, one upon another. A careful study 
of these, then, will give us some life record for that 
part of the history which they represent. Elsewhere 
similar sections bear witness to other ages; and so, 
by piecing the fragments together, we have in each 
continent a fairly complete section from the oldest to 
the youngest rocks, and in them remains of animals 
from the very early ages to the present. 
It was this fact, based upon the law of superposition,' 
that gave to William Smith the clue upon which he 
laid the foundation of the present geological time-scale. 
Studied carefully, im many places and in many lands, 
fossils have given us the means of telling the relative 
age of strata; for thus we have been able to learn 
what kinds of animals characterized the different ages. 
To illustrate roughly, birds did not appear until a cer- 
tain age; and hence, if we find the skeleton of a bird 
in the rocks, we fairly conclude that the strata contain- 
ing it were deposited at some period later than the 
time of the first appearance of these creatures. Find- 
ing other fossils in the same bed, we may tell with 
certainty just what the age is, and just where in the 
1 That in a section of rocks, the oldest are the lowest. 
