Past History 



able substance, and it is hard to believe it possible that 

 we can expect to find fossil plants. Nor do we, as a 

 matter of fact, find the actual substance of which the 

 plants were composed preserved for us, but we discover 

 that these ancient plants have left traces of their exist- 

 ence in the rocks, and the botanist skilled in decipher- 

 ing such traces can read for us their story in much the 

 same way as the historian can picture for us the past of 

 a nation from a study of historical relics. 



What are these traces ? When the sinking of the 

 land, to which we have already referred, took place, 

 many leaves, twigs, branches, stems, and seeds must 

 have been embedded in the mud or sand. By the time 

 this mud or sand had solidified, forming a slaty shale or 

 a sandstone, the actual substance of the plant fragments 

 would have completely disappeared, but impressions of 

 their outward forms would be left on the newly formed 

 rocks. Some of the hollow impressions would be filled 

 up again by later deposits of mud or sand, and then we 

 should have what is called a " cast " of the original plant 

 fragment ; or, as sometimes happened, the stem or, 

 rather, part of the stem of a plant may have been 

 buried in a material containing some mineral in solution. 

 As the substance of the plant decayed it was replaced, 

 particle by particle, by the mineral matter, which subse- 

 quently hardened, so that an almost perfect model of 

 the original stem fragment, sometimes including both 

 internal and external features, was formed. But these 

 petrifactions, as they are called, are not so common as 

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