236 STSTEMA TIC BOTANY. 



be first); Palaeozoic (Gr. palaios, ancieut; zoa, life), 

 includiug the Silurian, Devonian, and Carboniferous Ages; 

 Mesozoic (Gr. mesos, middle), including the Reptilian 

 Age ; and Cenozoic (Gr. kainos, recent), includiug the 

 Mammalian Age and the Age of Man. 



200. No fossil plants have as yet been found in the 

 Archaean rocks, though there is reason to believe that a 

 Flora existed at that time. There are extensive deposits 

 of iron-ore, and the presence of organic matter is con- 

 sidered indispensable to the formation of Ferric Oxide 

 and Ferrous Carbonate, which, according to Le Conte, 

 takes place thus: The Peroxide of Iron (Ferric Oxide), 

 which is quite generally distributed through the soil, is 

 insoluble ; but in contact with organic matter becomes 

 deoxidized and reduced to Protoxide (Ferrous Oxide), 

 which unites with Carbonic Dioxide (one of the products 

 of the decomposition of organic matter), and forms Carbon- 

 ate of Iron. This Ferrous Carbonate is soluble in the 

 water which contains an excess of Carbonic Dioxide, and 

 is, therefore, gradually washed out of the soil, and in solu- 

 tion appears at the surface again at springs, etc. Here the 

 Carbonic Dioxide is replaced by Oxygen, and the iron is 

 deposited in its original form of Ferric Oxide. Or if the 

 iron-waters accumulate, and a deposit is made in the 

 presence of the reducing agent (organic matter), the Fer- 

 rous Carbonate is not reoxidized, but deposited in that 

 form. The other proof of an Archtean Flora is the occur- 

 rence of beds of Graphite, for " graphite is only the 

 extreme term of the metamorphism of coal." 



201. In the Silurian Age the lower forms of animals 

 were very abundant ; fully ten thousand fossil species have 

 already been described ; this is conclusive proof that plants 



