ALTERATIONS OF ROCKS 



287 



calrium carbonate of corals, shells, etc., may be replaced by silica. 

 The substitution may take place so that even the minutest details 

 of structure are preserved. Woody matter is, under proper condi- 

 tions, replaced by silica, forming petrified wood. 



The material of one crystal may be replaced by different mate- 

 rial, as the molecules of calcite by zinc carbonate. This gives a 

 l>scii(lonii>rf)h of zinc carbonate after calcite, the zinc carbonate 

 taking the form of calcite, instead of the form which it would take 

 if crystallizing under other circumstances. This sort of change may 

 affect the crystals in any sort of rock. 



Concretions. Another phase of the internal reconstruction of 

 sedimentary rocks is the assembling of matter of the same kind. 

 For instance, silica that was deposited in the form of siliceous shells 



279. l)i'|>Mt< nf ralritr ( t raviTtiiu 1 . stalactites, and stalagmites) in 

 Wyandotte C'uvi-, ln<l. (Hains.) 



and spicules of plants and animals, and disseminated through the 

 sediments as they were deposited, may be aggregated later into 

 nodules or concretions of chert or flint (Fig. 280). Similarly, con- 

 cretions of calcium carbonate or iron carbonate grow in silts or 

 muds. In many other cases, too, kind comes to kind. 



