
HEREDITY AND EVOLUTION 2605 
sedimentary. Jgneows rocks are produced as the result of 
heat and do not contain fossils. Sedimentary rocks are formed 
by the transportation of small rock particles and their subse- 
quent deposition in another place; or they are the result of 
precipitation from solution, or of secretion by organisms, as 
in the case of limestones. Sedimentary rocks are the ones that 
yield. fossils. 
It is acommon observation 
that most dead plants and ani- 
mals are quickly destroyed by 
decay. They are not likely 
to be fossilized unless they 
are rather quickly covered 
by some protecting material. 
Also, oxygen must be largely 
excluded, as decay is depend- 
ent on oxidation. Such condi- 
tions are most usual in lakes, 
seas, or marshes. Organisms 
with hard parts are much more 
likely to be fossilized than 
are those without them. lor 
this reason the very primitive 
and soft plants and animals 
are rarely fossilized. | Fig. 242. Fossil leaves 
Age of fossils. In the for- 
mation of sedimentary rocks the oldest naturally occur at the 
bottom of the series and the youngest at the top. The most 
ancient fossils will be found in the oldest rocks, while the most 
recent fossils will be in the youngest rocks. In the past history 
of the earth, areas that were under the sea have risen and be- 
come dry land (Fig. 244), while some of the areas that were 
formerly dry land are now under the sea. Owing to erosion and 
the washing of materials from the land into the sea (Fig. 245) 
there is a general tendency for the surface of the dry land to 
be washed away, while that under the sea is built up by the 

