DELVING INTO A PREHISTORIC RECORD 
ated with ground water containing lime or silica, their 
whole structure was preserved. In most cases, however, 
the nodules were formed by a crystallization around the 
plant material which was too slow to preserve the whole 
structure, and merely retained an impression of the leaf 
or similar plant form. 
The plants and insects found embedded in amber are 
also petrifacts, because their whole organism is preserved 
perfectly. Even the color, which is lost in other petri¬ 
facts, remains unchanged. The plant or insect is mummi¬ 
fied, rather than turned into mineral. Such a fossil is 
valuable, because it is extremely rare. 
In fossils that are casts, or impressions of plants, merely 
the external form has been retained. The impression is 
frequently colored black by the carbon film which is the 
only remnant of the organism itself. In a cast, the stem, 
seed, or root, as the case may be, has been covered by 
mud or sand and a mold formed around the plant rem¬ 
nant. The organic matter has decayed inside the mold 
and left a hollow. This in turn became filled with mud 
or sand, which hardened into shale or sandstone. As a 
result, a positive cast was formed inside a negative one, 
in the exact external form of the stem, seed, or root. 
In the case of leaves and flowers, and sometimes of 
seeds, spores, roots, and stems, we get but an impression, 
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