64 READINGS IN EVOLUTIOxN, 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 sjiales 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.— Ed.] 



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- 



