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PHYSICAL GEOLOGY 



secretions. Cavities lined with inward-pointing crystals are 

 geodes. Agates are also examples of secretions. 



Other changes accomplished by ground water. The 

 shells of animals, trunks of trees, etc., which are buried in 

 gathering sediments, may, as they decay, be carried away 

 bit by bit by ground waters and be replaced by the deposition 

 from solution of other mineral matter, often silica. The 



FIG. 114. Petrified logs in the petrified forest, near Adamana, Ariz. 



(Atwood.) 



minutest structure of the wood, for example, is thus pre- 

 served in stone, and the wood is said to be petrified (Fig. 114). 

 The process is called petrifaction. It has been of great im- 

 portance in preserving in fossil form a record of past life. 

 Replacement by ground waters has affected many kinds of 

 rocks on a vast scale. Many concretions are made by the 

 process of replacement. 



As already pointed out (p. 104), ground water may enter 

 into chemical combination with certain rock constituents. 

 Hydration is a chief factor in the decay of rocks. Mineral 

 matter in solution in percolating water may also form new 

 chemical combinations with constituents of the rocks through 

 which the water passes, and thus be locked up for long 

 periods of time. 



