378 HISTORICAL GEOLOGY 



When a shell or other organic remain is buried in the mud or sand 

 of an ocean or lake bottom, in the dune sand of a desert, in volcanic 

 dust, in a. peat bog, or in the flood plain of a river, the record of its 

 existence may be preserved in a number of ways. 



(i) The Original Substance may be Preserved. — In recent sediments 

 the shells are often unchanged, even the nacreous luster being re- 

 tained. In the ice of Siberia mammoths have been found whose 

 flesh had been so perfectly preserved that it was eaten by dogs and 

 wolves and possibly by the natives themselves. Insects are found 

 in amber — the fossil gum of cone-bearing trees — in which they were 

 entrapped and covered. 



(2) Replacement. — The original substance may have been entirely 

 replaced by some other mineral, and shells, corals, and bones are often 

 found which, although bearing little external evidence of alteration, 

 are composed entirely of silica or some other mineral. 



As alkaline water is a salient of silica the petrifaction of wood (Fig 361) is 

 brought about when such water containing silica in solution is neutralized, since the 

 silica is then precipitated. If then a log buried in a bed of sand or volcanic ash 



Fig. 361. — Petrified log, Adamana, Arizona. 



is saturated with underground water that is slightly alkaline, the replacement of 

 the wood by the silica will be slowly brought about as the wood decays. As each 

 particle of wood is oxidized carbon dioxide will be formed, this acid (H 2 C0 3 ) will neu- 

 tralize the ;ilk;ili of the water and will cause the precipitation of the silica at the 

 point where the wood decayed. By some such slow process the wood may be replaced 

 particle by particle until the entire tree is converted into a solid cylinder of silica. 



Silica is not the only mineral which replaces the substance of shells, 

 bones, and other hard parts. Pyrite, iron oxide, lime carbonate, and 

 other minerals sometimes occur. 



