THE USES OF FOSSILS 399 
tory of the earth divided ito ages (Fig. 233). In some 
places, where unconformities exist, the line between two 
successive ages may be sharply drawn, because a por- 
tion of the record 
is missing. For in- 
stance, to make a 
comparison, in a 
house it would be dif- 
ficult, without pains- 
taking measurement, 
to say where the 
exact line of division 
between the right 
and left sides passes ; 
but if a section 
should be taken out 
of the middle, this 
division into two 
parts could be easily 
seen, and no one 
could doubt where 
the line of separation 
should fall. The 
same is true of rocks. 
Fig. 234. 
Diagram to illustrate basis of division of strata 
into ages. In Fig. A, ten beds (1-10) are ac- 
cumulated. The line between 6 and 7 marks 
the arbitrary division of rocks which have 
gathered without a break. In Fig. B, another 
section of the same beds near by, strata 1 to 
6 were deposited, then elevated, and eroded 
until the line w (unconformity) was reached. 
During this time, layer 7 was accumulating - 
elsewhere, but not on the land at B. So all 
of layer 6 and part of 5 were removed, and 
7 failed to be deposited. Hence the uncon- 
formity w marks a sharp contrast between 
the rocks of the upper age and those of the 
lower. Naturally, as in A, they grade into 
each other, and hence the divisional line is 
difficult to draw; but here in B it is easy. 
Sometimes a part has been removed or is wanting, and 
here the division is readily made (Fig. 254); but in 
other places, as for instance in New York, the divid- 
