FERNS, FOSSILS AND FUEL 
water would rise again and the sea recede. The coarser 
sediment which had been washed down into the water 
formed sandstone; the finer mud, shale and slate. Lime¬ 
stone was made from the shells of water animals and from 
corals; ordinary limestone contains no plant fossils. 
Inundations were the primary agency in the making 
of such fossils. The plant materials were carried along 
in the flood waters and embedded in the mud, which 
hardened into shale, slate, or sandstone. Shale is the 
best material of the three in which to find fossils, because 
it is laminated, or formed in thin layers which are easily 
split apart. It is not so hard as slate, and its gray color 
makes it easier to see the plant impressions. The plant 
remains have been reduced by the terrific pressure of the 
rocks to a thin layer of carbon which shows very plainly 
on the gray shale background. Sandstone is too coarse 
to preserve many fossil impressions well. 
Excellent fossils are found also in volcanic tufa, which 
is the rock formed from volcanic ashes. Fossils embedded 
in this material are usually found in a place where there 
was once a lake with forests around it. Leaves and other 
plant organs drifted into the water and fine volcanic dust 
from some near-by volcano sifted over it. Finally the 
dust filled up and covered the lake entirely, forming in 
the process a sort of plant Pompeii. 
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