DELVING INTO A PREHISTORIC RECORD 
The whole plant structure has sometimes been pre¬ 
served in a special kind of limestone, where the lime has 
been deposited by mineral springs and has entirely 
encrusted the leaves and the body of the plant. We can 
see the same sort of thing happen today at mineral springs 
like those at Carlsbad, where a small branch placed in 
the spring for half an hour or so will become completely 
encrusted with stone. 
In coal-seams, which are themselves vegetable matter 
tremendously compressed, carbonized, and changed in 
form, it is often possible to see, with the aid of a micro¬ 
scope, certain organs of the plants, especially spores, also 
woody fibers, resin drops, the epidermis of plants, and 
the outlines of leaves. 
A coal-seam is associated with fossils in the shale above 
it and in the fire-clay below it. The fire-clay is the soil 
of the swamp in which the plants of the coal age grew; 
the shale is the sedimentary mud which sealed up the 
coal seams from above, killing the plants but preserving 
their remains. 
Sometimes plants, and insects also, are found embedded 
in amber. Amber is formed from drops of the resin of 
coniferous trees which have fallen upon the blue clay 
and hardened there. Occasionally, a drop would fall on 
an insect or part of a plant and solidify around it. When 
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