THE PALEOZOIC ERA 225 



are probably of organic origin, and a fossil crustacean 

 has been recognized in rocks of this age in Montana. 

 But to the student of fossil botany all the Eozoic rocks 

 are a blank. 



The Palaeozoic Era embraces six Periods — the Cam- 

 brian, Ordovician, Silurian, Devonian, Carboniferous, 

 and Permian. The Cambrian, Ordovician, and Silurian 

 rocks represent the Older Palaeozoic Era, and the Dev- 

 onian, Carboniferous, and Permian rocks belong to the 

 Newer Palaeozoic. To cut a long story short, we say 

 at once that it is not until we come to the Newer Palae- 

 ozoic rocks that we find fossil plants that throw definite 

 light upon the plant-life of the past, and, curiously 

 enough, it is the vegetation of the Carboniferous Period 

 that is better known to the specialist than that of any 

 Period except the present. The Carboniferous rocks are 

 peculiarly rich in fossil plants. They occur in casts and 

 impressions, and also in petrified forms. In the latter, 

 which have become fossilized under peculiarly favourable 

 conditions, both hard and soft tissues have been pre- 

 served, and in microscopic sections the cellular structure 

 is readily apparent. Among these petrifactions are 

 masses of vegetation including fragments of stems, 

 leaves, roots, and seeds preserved in silica, found notably 

 in France; and also the "coal balls" discovered in 

 Yorkshire and Lancashire and in some places on the 

 Continent. These " coal balls " are found embedded in 

 coal seams. They occur as roundish masses of various 

 diameter, usually about the size of an average potato, 

 composed of carbonates of magnesia and lime. They 

 are of particular value to the investigator in that they 

 are petrifactions of masses of plant remains, and include 



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