CHAPTER XVI. 

 PLANTS OF PAST AGES. 



219. Antiquity of Plants. Only approximate estimates 

 can be made of the length of time which has elapsed since 

 the waters began to lay down the stratified rocks of the 

 Cambrian period, whose fossils contain the earliest evidence 

 of the life of the earth. An average of the estimates of 

 those best qualified to judge is between twenty-five million 

 and seventy-five million years. In all probability life 

 existed untold ages before this time in very simple forms 

 which were not of a character to leave their impress in the 

 rocks. If the great quantities of graphite existing in the 

 rocks of the Algonquin period are of plant origin, as is 

 probable, plant life must at that early time have already 

 attained an enormous quantitative development before 

 much complexity of structure had been evolved. 



220. Primitive Physical Conditions. The physical con- 

 ditions were probably such as to incite and sustain rapid 

 growth and multiplication. The atmosphere was charged 

 with aqueous vapor and carbon dioxide, a part of which 

 has since been locked up in limestone, coal, etc., and the 

 interior heat of the earth may have contributed to maintain 

 a uniformly high temperature. The oceans during the 

 Cambrian, Ordovician, and Silurian periods undoubtedly 

 abounded in Algae, but because of the uncertainty in the 

 preservation of their tissues as fossils, or of their leaving 

 impressions in the rocks, positive identification of Algae is 



