THE USES OF FOSSILS 389 
In some rocks these remains are abundant, in others 
scarce or altogether absent; but whenever found, they 
tell us something of the earth as it existed at the time 
when the rock in which they occur was formed. Had 
there been preserved a goodly percentage of the ani- 
mals that have lived on the globe, we should now have 
preserved in the strata, a complete record of the ani- 
mal life of this planet; but for various reasons, our 
account is extremely imperfect and fragmentary. 
Conditions favoring the Preservation of Fossils. — As 
rocks are accumulated, organic remains are buried there 
in greater or less quantity. The most favorable place 
for such burial is in the sea; for here, in addition to 
the abundance of life, there is rapid formation of sedi- 
ments, which will cover the fossils and prevent destruc- 
tion. Moreover, the very presence of the water checks 
decay. In less degree, the sediments of lakes are favor- 
able for the entombment of plant and animal life. On 
the land, on the other hand, organisms dying in the air 
speedily decay, and are not usually buried. Therefore 
the record of life in the rocks is chiefly a record of 
salt and fresh water organisms. 
Upon the land there are a few places where fossils may be 
well preserved. In caves the chemical deposit of carbonate of 
lime often protects the remains of animals and plants that 
happen to fall there. Lava flows, and ash deposits too, cover 
organisms and protect them from destruction; and: swamps, 
