CHAPTER XII. 



GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 

 OP PLANTS. 



»o»;o« 



1. The forces now operating in producing changes of the 

 surface of the earth were acting — and of course with similar 

 results — during a preceding (geological) period, which con- 

 tinued a very great length of time. The denudations of the 

 elevated portions of the earth's surface, and deposition of the 

 transported material, caused a succession of layers of mud to 

 be formed. Occasionally remains of any plants (and animals) 

 that may then have been living, especially the harder parts in 

 their structure, became incased in this mud. These layers of 

 mud became converted into rock, in the layers of which the 

 organic remains or their imprints, called fossils, are contained. 

 They afford to-day the record — as yet but partially examined 

 — whose reading gives the past, or geological, history of our 

 earth. For convenience of study, this period of development 

 is divided by geologists into Times, called, beginning with the 

 earliest, Archaean (Gr. archein, to be first) ; Paleozoic (Gr. 

 palaios, ancient; zoos, living), including the Cambrian, Lower 

 Silurian, Upper Silurian, Devonian, and Carboniferous eras ; 

 Mesozoic (Gr. mesas, middle), formerly called the Reptilian 

 Age, including the Triassic, Jurassic, and Cretaceous eras; 

 and Cenozoic (Gr. kainos, recent), including the Mammalian 

 Era (divided into the Tertiary and Quaternary periods). No fos- 

 sil plants have as yet been found in the Archaean rocks, though 

 there is reason to believe that a flora existed at that time. One 

 proof of this is found in the extensive deposits of iron ore. The 

 compounds of iron, widely deposited in the soil, are decom- 

 posed in the presence of organic matter and then dissolved 



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