390 ELEMENTARY GEOLOGY 
being places from which the action of air is partly excluded, also 
serve as repositories of fossils. 
In swampy places, the rapid destruction of organic remains is 
prevented by the preservative effect of certain vegetable acids 
produced by the partial decay of the swamp plants. As a result 
of this, coal beds, with perfect impressions of delicate ferns, have 
been preserved for long ages. Another way in which land organ- 
isms may be protected from destruction, is by being drifted into 
lake or sea, and there buried beneath the gathering sediments. 
Even where the conditions are favorable for animal 
preservation, as a rule only the most durable parts 
remain.’ The shells of animals, and the teeth, bones, 
and other more indestructible parts, are the commonest 
of animal fossils. Those creatures that are destitute 
of the harder substances have little chance of preser- 
vation ; and hence whole races of animals have lived 
and passed away without leaving us even a trace of 
their existence. The most complete records have been 
left by the shell-bearing animals of the sea. Some 
beds of rock are almost entirely made of their remains, 
and throughout the strata they are abundant as fossils. 
Even when animals or plants have escaped decay after death, 
and become entombed in the strata, the danger of destruction is 
not over. The rocks in which they are enclosed may be so 
affected by metamorphism that no trace of the fossils is left. 
And again, in some rocks the action of percolating water is able 
1 There are exceptions to this. For instance, in the National Museum at 
Washington, there is the distinct impression of a delicate jelly-fish upon the 
surface of a slab of rock, formed ages since. 
