64 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
Class 3. Casts and impressions.—Very frequently the animal or 
plant has been buried in mud or has lain on a soft mud flat only 
long enough to have left its impress in the plastic material. Sub- 
sequently the entire organism has decayed and been dissolved away, 
and its place has been taken by a mineral deposit. Thus only the 
external appearance has been preserved, as would be the case in 
making plaster-of-paris casts. Sometimes traceries of soft-bodied 
animals have been left upon forming slate or coal that are almost as 
accurate in detail as a lithograph. 
Perhaps the most remarkable fossils known are those found by 
Professor Charles D. Walcott in the marine oily shales of British 
Columbia. A large number of soft-bodied invertebrates of Cambrian 
age have been found so wonderfully preserved that not only are 
the external features revealed, but sometimes even the details of 
the internal organs may be seen through the transparent integu- 
ment. : 
Some authorities include among fossils such traces of extinct life 
as footprints, utensils and tools of extinct man, and even the 
vestiges of archaic sea beaches. Perhaps this is stretching the 
definition of the term ‘‘fossil” too far.—Eb.] 
ON THE CONDITIONS NECESSARY FOR FOSSILIZATION 
‘Examination and study of the rocks of the earth reveal the fact 
that fossils or the remains of animals and plants are found in certain 
kinds of rocks only. They are not found in lava, because lava 
comes from volcanoes and rifts in the earth’s crust, as a red-hot, 
viscous liquid, which cools to form a hard rock. No animal or plant 
caught in a lava stream will leave any trace. Furthermore, fossils 
are not found in granite, nor in ores of metals, nor in certain other of 
the common rocks. Many rocks are, like lava, of igneous origin; 
others, like granite, although not originally in the melted condition, 
have been so heated subsequent to their formation, that any traces of 
animal or plant remains in them have been obliterated. Fossils are 
found almost exclusively in rocks which have been formed by the slow 
deposition in water of sand, clay, mud, or lime. The sediment which 
is carried into a lake or ocean by the streams opening into it sinks 
slowly to the bottom of the lake or ocean and forms there a layer 
which gradually hardens under pressure to become rock. This is called 
sedimentary rock, or stratified rock, because it is composed of sedi- 
