266 A TEXTBOOK OF GENERAL BOTANY 
accumulation of deposited material. In this way rocks are built 
up while the land is submerged, and are worn away when the 
land is exposed. 
Sometimes the same land has been elevated above the sea 
more than once. While the land is submerged, layers of rock 
are formed. When the land is exposed as the result of eleva- 
tion, the rock begins to 
be eroded, and if the area 
is exposed for a long pe- 
riod, may be removed to 
a considerable depth. If 
such an area is again sub- 
merged, its surface will 
be covered by new layers 
of rock. There will be a 
great difference in age be- 
tween the lowest of these 
new layers and the old 
layer on which it is de- 
posited. This is due to 
the fact that during the 
time when the area was 
exposed no new layers 
were formed, and, more- 
over, much of the former 
Fic. 243. Fossil leaf rock was removed. Owing 
to such occurrences as 
that just outlined, large gaps frequently occur in the geological 
records in a given series of rocks. By piecing together the geo- 
logical records from various parts of the earth, however, it is 
possible to get much information concerning the relative ages 
of different rock strata. 
Incompleteness of fossil records. While fossils have been formed 
in various ways, the great majority of them originated under 
water, and the discovery of most of them has been due to the emer- 
gence of land that was formerly submerged. When such fossils 

