FERNS, FOSSILS AND FUEL 
this happened, a perfect preservation of the body enclosed 
in the drop resulted. 
Fossil plants are of two varieties; they are petrifacts, 
or they are casts and impressions. When the entire plant 
structure has been preserved, we call it a petrifact. The 
petrified forests of Arizona, where everything has been 
turned to stone, are a large scale example of petrifacts. 
The wood has here been saturated with either lime or 
silica. What we see is the carbon stain which was left 
when the cellulose particles of the wood were replaced 
by particles of lime or silica. 
Petrifacts on a smaller scale are often found in the 
so-called coal balls. These are hard, dark lumps which 
appear in coal seams, sticking out like raisins in a cake. 
They are masses of original vegetation preserved in their 
original form by saturation with lime or silica. The 
vegetation around them has been compressed and changed 
in form, but the lime or silica has preserved a certain 
area from compression or decay. 
Concretions called roof-nodules are found in shale 
formations above the coal seams. They sometimes 
contain well-preserved pieces of stems or seeds, but more 
often merely impressions of leaves. They are composed 
of plant materials deposited in the mud which covered 
up the coal swamps. If the plant forms were soon satur- 
20 
