CHAPTER FORTY-FIVE 



FOSSIL PLANTS 



When a leaf falls on soft mud, it may become imbedded 

 in it. Later the mud may be covered by other layers of sedi- 

 ment. When the mud dries, a perfect imprint of the outline 

 and veins may be left. As time goes on and the mud becomes 

 more deeply buried, it may harden into rock and retain the im- 

 print of the leaf as a record of a plant that lived when the rock 

 was merely soft mud. In this way leaves, fruits, seeds, stems, 

 and roots have left their imprints to testify, thousands and mil- 

 lions of years afterward, to their former existence. 



Plant remains also accumulate in deep water or in water con- 

 taining large amounts of mineral matter in solution. In such 

 places they may decay very slowly, and the material of which 

 they are composed may be gradually replaced by the mineral 

 substances in the water. Under these favorable conditions the 



T. D. A. Cockerell 



Fig. 307. Fossil oak leaf from the Tertiary shales at Florissant, Colorado, 

 and a modern oak leaf from the same region. 



495 



